Benzene

Endpoint summary

Description of key information

After repeated dose exposure via oral or inhalation routes, benzene causes adverse effects on the haematopoietic system of animals and humans.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Link to relevant study records

Reference

Endpoint:

sub-chronic toxicity: oral

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: GLP compliant, near guideline study, published as NIH publication, some limitations in design but fully adequate for evaluation

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 408 (Repeated Dose 90-Day Oral Toxicity Study in Rodents)

Deviations:

yes

Remarks:

no clinical chemistry analysis, no ophthalmological examination, no neurobehaviour, no organ weights.

GLP compliance:

yes

Remarks:

assumed - audit of 2 year data documented

Limit test:

no

Species:

rat

Strain:

Fischer 344

Sex:

male/female

Details on test animals or test system and environmental conditions:

TEST ANIMALS
- Source: Charles River Breeding Laboratories (Portage, MI)
- Age at study initiation: 6 weeks
- Weight at study initiation: mean weights per group males 110-137 g ; females 75-101 g
- Housing: 5 per sex per cage in polycarbonate cages
- Diet: Purina Lab Chow 5001 - pellets (Ralston-Purina Co., St. Louis, NJ) ad libitum
- Water: ad libitum
- Acclimation period: 15 days

ENVIRONMENTAL CONDITIONS
- Temperature: 22±1°C
- Humidity: 40-65%
- Air changes: 15 per h
- Photoperiod: 12 h dark / 12 h light

IN-LIFE DATES: From: 14 October 1978 To: 13 February 1979

Route of administration:

oral: gavage

Vehicle:

corn oil

Details on oral exposure:

PREPARATION OF DOSING SOLUTIONS: A weighed amount of benzene was mixed with the appropriate amount of corn oil and stirred for 5 minutes. Rats were dosed at a rate of 5 mL/kg. Benzene in corn oil was found to be stable at 25º C for at least 7 days. Dose mixtures were used within 2 weeks of preparation.

Analytical verification of doses or concentrations:

yes

Details on analytical verification of doses or concentrations:

All benzene/corn oil mixtures analyzed by gas chromatography and were within ±10% of the target concentrations.

Duration of treatment / exposure:

120 days (17 weeks). Sub-group of animals from 0, 200, and 600 mg/kg groups killed after 60 days (8-9 weeks).

Frequency of treatment:

Once per day, 5 days/week.

Remarks:

Doses / Concentrations:
0, 25, 50, 100, 200, 400, 600 mg/kg bw/day
Basis:
other: nominal in corn oil

No. of animals per sex per dose:

10/sex/group for 0, 25, 50, 100 and 400 mg/kg; 15/sex/group for 0, 200 and 600 mg/kg

Control animals:

yes, concurrent vehicle

Details on study design:

Post-exposure period: none

Observations and examinations performed and frequency:

CAGE SIDE OBSERVATIONS: Yes
- Time schedule: twice daily

DETAILED CLINICAL OBSERVATIONS: No data

BODY WEIGHT: Yes
- Time schedule for examinations: once per week

FOOD CONSUMPTION: Yes
- Time schedule: once per week

OPHTHALMOSCOPIC EXAMINATION: No

HAEMATOLOGY: Yes
- Time schedule / number of animals for collection of blood: animals killed at day 0 and day 60 and on 5 animals/group at terminal kill and on any animals killed in a moribund condition.
- Method of collection: from the orbital sinus
- Anaesthetic used for blood collection: No data
- Animals fasted: No data
- Parameters examined: haemoglobin, haematocrit, white blood cell count, red blood cell count, mean corpuscular volume, reticulocyte count, coagulation time.

CLINICAL CHEMISTRY: No

URINALYSIS: No

NEUROBEHAVIOURAL EXAMINATION: No

Sacrifice and pathology:

GROSS PATHOLOGY: Yes. Examinations on all animals except those with excessive autolysis or cannibalized.

HISTOPATHOLOGY: Yes. The following tissues were examined histologically in the predosing vehicle control, study vehicle control, and interim-kill animals and the 600 mg/kg animals at terminal kill: mandibular lymph node, salivary glands, femur, thyroid gland, parathyroid, small intestine, colon, liver, prostate/testes or ovaries/uterus, lungs and mainstem bronchi, mammary gland, heart, oesophagus, stomach, brain, thymus, trachea, pancreas, spleen, kidneys, adrenal glands, urinary bladder, pituitary gland; in addition, spleens were examined in all dose groups. Special histology studies performed on animals killed at days 0 and 60 and on 5 animals/group at 0, 200, and 600 mg/kg at terminal kill and on any animals killed in a moribund condition.

Statistics:

Tests of significance included pairwise comparisons of high dose and low dose groups with vehicle controls and tests for overall dose-response trends. Haematology data was initially screened for outliers and as the same animals were examined across time, a repeated measures analysis of variance (Winer, 1971) method was used to investigate temporal and dose-related variation.

Clinical signs:

not specified

Mortality:

not specified

Body weight and weight changes:

effects observed, treatment-related

Food consumption and compound intake (if feeding study):

not specified

Food efficiency:

not examined

Water consumption and compound intake (if drinking water study):

not examined

Ophthalmological findings:

not examined

Haematological findings:

effects observed, treatment-related

Clinical biochemistry findings:

not examined

Urinalysis findings:

not examined

Behaviour (functional findings):

not examined

Organ weight findings including organ / body weight ratios:

not examined

Gross pathological findings:

not specified

Histopathological findings: non-neoplastic:

effects observed, treatment-related

Histopathological findings: neoplastic:

not examined

Details on results:

BODY WEIGHT AND WEIGHT GAIN: Final mean bodyweights, relative to vehicle controls, were depressed: 4, 2, 7, 14, 20 and 22% for males and 6, 9, 6, 16, 15 and 20 % for females at dose levels of 25, 50, 100, 200, 400 and 600 mg/kg respectively.

HAEMATOLOGY: A dose-related leukopenia was observed for both male and female rats. Day 60 mean WBC counts were 6.4, 2.5 and 1.7 (males) and 4.3, 2.3 and 1.7 (females) at 0, 200 and 600 mg/kg respectively. Day 60 mean LYM counts were 5.8, 2.0 and 1.3 (males) and 3.8, 1.9 and 1.5 (females) for 0, 200 and 600 mg/kg respectively. Day 120 mean WBC counts were 5.0, 7.0, 5.7, 6.0, 5.2, 3.5 and 3.1 (males) and 8.1, 6.2, 4.9, 5.0, 4.8, 3.4 and 3.8 (females) for 0, 25, 50, 100, 200, 400 and 600 mg/kg respectively. Day 120 mean LYM counts were 4.1, 5.1, 3.9, 3.9, 3.4, 2.3, and 2.4 (males) and 6.6, 4.7, 3.9, 3.6, 3.7, 2.7 and 2.8 (females) for 0, 25, 50, 100, 200, 400 and 600 mg/kg respectively.

HISTOPATHOLOGY: NON-NEOPLASTIC: Lymphoid depletion in the B-cell of the spleen was observed in 3/5 male and 4/5 female rats at 200 mg/kg and 5/5 male and 5/5 female rats at 600 mg/kg at 60 days and in 1/10 male and 10/10 females at 600 mg/kg at 120 days. Increased extramedullary haematopoiesis was observed in the spleen of 4/5 male and 3/5 female rats that received 600 mg/kg for 120 days.

Dose descriptor:

NOAEL

Effect level:

100 mg/kg bw/day (nominal)

Sex:

male

Basis for effect level:

other: Based on effects observed at 200 mg/Kg bw/day dose level.

Dose descriptor:

LOAEL

Effect level:

200 mg/kg bw/day (nominal)

Sex:

male

Basis for effect level:

other: decreased final body weight, white blood cell and lymphocyte counts and lymphoid depletion of B-cells in the spleen at 200 mg/kg and above

Dose descriptor:

LOAEL

Effect level:

25 mg/kg bw/day (nominal)

Sex:

female

Basis for effect level:

other: reduction in white blood cell and lymphocyte counts at 25 mg/kg (lowest dose tested)

Critical effects observed:

not specified

Conclusions:

Repeat oral administration of benzene to rats is associated with adverse effects in the haematopoietic system. NOAEL for males was 200 mg/kg. No NOAEL was established for females. LOAEL for females was 25 mg/kg/day (lowest dose tested).

Executive summary:

Groups of 10 or 15 rats/sex were administered 0, 25, 50, 100, 200, 400 or 600 mg/kg benzene in corn oil by gavage, 5 days/ week for 17 weeks. Five rats/sex were killed on days 0 and 60 from the 0, 200, and 600 mg/kg groups, remaining surviving animals were killed on day 120. Clinical observations, bodyweights, food consumption, haematological analyses and histopathological examinations were performed. No compound-related deaths occurred. Final mean body weights (relative to those of the vehicle controls) were depressed 14%-22% for male and female rats that received ≥200 mg/kg benzene. A dose-related leukopenia and lymphocytopenia was observed in males at ≥200 mg/kg and in females at ≥25 mg/kg. In the spleen, lymphoid depletion of B-cells was observed in both sexes at ≥200 mg/kg benzene and increased extramedullary haematopoiesis was observed 600 mg/kg.

NOAEL for males was 100 mg/kg/day. LOAEL for males was 200 mg/kg and for females was 25 mg/kg/day.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

LOAEL

25 mg/kg bw/day

Study duration:

chronic

Species:

rat

Quality of whole database:

Adequate information is available to characterise the repeated oral toxicity of benzene in animals.

Repeated dose toxicity: inhalation - systemic effects

Link to relevant study records

Reference

Endpoint:

repeated dose toxicity: inhalation, other

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Literature 1980-2018

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Qualifier:

no guideline required

Principles of method if other than guideline:

Quality review of epidemiological (genotoxicity and haematoxicity) studies for Occupational Exposure Limit Derivation.

GLP compliance:

no

Remarks:

Not applicable

Species:

other: human

Key result

Dose descriptor:

NOAEC

Effect level:

0.59 ppm

Based on:

test mat.

Basis for effect level:

haematology

Remarks on result:

other: arithmetic average

Key result

Dose descriptor:

LOAEC

Effect level:

2 ppm

Based on:

test mat.

Basis for effect level:

haematology

Remarks on result:

other: aggregate

Key result

Dose descriptor:

other: OEL

Effect level:

0.25 ppm

Based on:

test mat.

Remarks on result:

other: NOAEC modified by assessment factor of 2

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

2.19 ppm

System:

haematopoietic

Organ:

blood

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

yes

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

2.33 ppm

System:

haematopoietic

Organ:

bone marrow

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

yes

Results

 

Quality scoring results for genotoxic and haematological studies

Among the group of 31 haematology and 56 genotoxicity study populations each had a top score of 20 (of a possible 24), which in both cases was due to the (Qu et al., 2003) study. Both haematotoxicity and genotoxicity studies showed wide ranges (8–20 and 6–20, respectively) indicating marked differences in study quality for each body of literature.

Ties in scores for the haematotoxicity studies resulted in initial stratification of 11 studies in the top tertile (score range 14.5–20), 9 studies in the second tertile (score range 11–14), and 16 studies at or above the median score of 12.5. Similarly, for genotoxicity studies, 21 studies are in the first tertile (score range 13.5–20), 17 studies form the second tertile (score range 11–13), and 29 studies are at or above the median score of 12.5.

 

LOAECs and NOAECs for high quality studies

Haematology

Derivation of LOAECs

The highest quality studies (i.e. first tertile) that generated a more certain LOAEC were: Qu et al., 2003 (2.26 ppm, neutrophils), Schnatter et al., 2010, (7.8 ppm, neutrophils), Ward et al., 1996,(7.2 ppm, total leukocytes), Lan et al., 2004, (2.2 ppm, various cell types), Rothman et al., 1996, (7.6 ppm, lymphocytes), and Zhang et al., 2016 (2.1 ppm, leukocytes). From these values, a bimodal distribution results, in which there are two clusters of studies: three studies that suggest a LOAEC near 2 ppm and three studies that suggest a LOAEC near 7−8 ppm.

Sensitivity analyses supported a LOAEC around 2 ppm: Looking at the LOAECs in which all studies at or above the median quality score are considered as high quality, the LOAECs are similar, with only (Bogadi-Šare et al., 2003) at 8 ppm added to the above list from the first tertile. Thus, there are four studies suggesting a LOAEC of 7−8 ppm and three studies suggesting a LOAEC near 2 ppm. This alternative definition of high quality is a sensitivity analysis that supports the top tertile result. For the highest quality (top tertile) studies that generated a less certain LOAEC, values were: (Swaen et al., 2010) (0.75 ppm); and (Koh et al., 2015) (2.6 ppm). Inclusion of these studies is another sensitivity analysis that would lend more weight to a LOAEC in the range of 2 ppm, rather than the second cluster at 7−8 ppm. Various sensitivity analyses incorporating all less certain LOAECs above the median did not change this conclusion.

 

Derivation of NOAECs

For first tertile studies, the more certain NOAECs are 0.25 ppm (Swaen et al., 2010), 2.9 ppm (Schnatter et al., 2010), 2.2 ppm (Ward et al., 1996), 0.19 ppm (Collins et al., 1991), 0.21 ppm (Koh et al., 2015), and 1.7 ppm (Pesatori et al., 2009). Thus, there are three studies that suggest a NOAEC near 2−3 ppm, and three studies that suggest a NOAEC near 0.2−0.25 ppm. When studies that scored above the median and that show a more certain NOAEC are included, the NOAECs are 0.55 ppm (Collins et al., 1997), 0.81 ppm (Khuder et al., 1999), and 0.33 ppm (Tsai et al., 2004). Collectively, all studies above the median with more definitive NOAECs show four studies near 0.2−0.3 ppm, two studies near 0.6−0.8 ppm, and three studies near 2−3 ppm.

Examining studies above the median is justified and increases the number of studies although quality is somewhat lower (i.e. average quality is 16.25 versus 14.93 for NOAECs, and identical (16.67) for LOAECs).

Based upon frequencies, a LOAEC of 7−8 ppm, and a NOAEC of 2−3 ppm is one defensible conclusion from the analysis above. The NOAECs of 2−3 ppm are of a similar magnitude to three LOAECs from high quality studies, which introduces the problem of overlapping NOAECs with LOAECs. An alternative strategy would be to select the higher quality study(/ies) with more certain LOAECs that do not overlap NOAECs from high quality studies. Thus, there are three studies (Qu et al., 2003; Lan et al., 2004; Zhang et al., 2016) that show LOAECs near 2 ppm. If the LOAEC selected is near 2 ppm, a lower NOAEC should be selected. The two studies with the highest NOAEC yet still below 2 ppm are Collins et 1997 (0.55 ppm) and Khuder et al., 1999 (0.81 ppm). More studies show NOAECs of 0.2−0.3 ppm (Collins et al., 1991; Koh et al., 2015; Swaen et al., 2010; Tsai et al., 2004). Collins et al., 1991; Swaen et al., 2010; Tsai et al., 2004, all studied exposures < 0.5 ppm, so that a NOAEC was not achievable for those studies. Collectively, the results are not in conflict with a 0.5 ppm NOAEC, which is four times lower than the LOAEC (see Table 5). All sensitivity analyses (using top tertile studies, above median studies, and lower certainty LOAECs and NOAECs) in Table 5 result in LOAECs between 1.98 and 2.19 ppm, and NOAECs of 0.58 and 0.59 ppm. Thus, the result based on first tertile studies (a LOAEC of 2.19 ppm and a NOAEC of 0.59 ppm) is a conservative yet coherent interpretation of this information and is the preferred approach or base case.

 

Genotoxicity

Factory workers

Of the 21 studies in the top tertile, ten studies were among factory workers, five among fuel handlers and six among workers exposed to traffic and ambient air. In factory workers, the five studies with more certain LOAECs were (Qu et al., 2003) (LOAEC=3.07 ppm), (Xing et al., 2010)(LOAEC>1.6 ppm), (Zhang et al., 2012) (LOAEC>2.64 ppm), (Zhang et al., 2007) (LOAEC=13.6 ppm) and (Zhang et al., 2014) (LOAEC=2 ppm). The top tertile study generating a less certain LOAEC (>0.56 ppm) was (Kim et al., 2004a) due to the presence of PAH co-exposures.

 

Fuel workers

Three studies (Carere et al., 1995; Pandey et al., 2008 and Rekhadevi et al., 2010) in the top tertile were associated with a more certain LOAEC and none with a less certain LOAEC. The three studies showed similar LOAECs of 2 ppm, 2 ppm, and > 1 ppm, respectively. A NOAEC in the Carere study for micronuclei is 0.47 ppm and in the Pandey study∼0.9 ppm. The quality scores of the first tertile fuel studies (14.5) are lower than those from the factory

setting (17.25).

 

Traffic/ambient air

There were only two studies (Leopardi et al., NOAEC=0.003 ppm; Maffei et al., LOAEC=0.008 ppm) in the top tertile which produced a more certain LOAEC or NOAEC. Violante et al. (15.5) has a less certain NOAEC of 0.005 ppm and Angelini (14.5) has a less certain LOAEC of 0.006 ppm. Since the exposure concentrations present in the traffic/ambient air studies are lower than other NOAECs based on fuel and factory studies, this group of studies does not add meaningful information to the NOAEC analysis.

Since the single top tertile study that showed a more certain LOAEC is of lower quality (13.5) than studies from the factory and fuel sectors (average=16.07), this group of studies also does not add meaningful information to the LOAEC analysis. Thus, these studies are not subsequently considered.

 

Derivation of LOAECs

The highest quality studies (i.e. first tertile) that generated a more certain LOAEC originated from the factory and fuel study scenarios. There were five such studies from the factory scenario: Qu et al. (LOAEC=3.07 ppm), Xing et al. (LOAEC>1.6 ppm), Zhang et al. (2012) (LOAEC>2.64 ppm), Zhang et al., 2007(LOAEC=13.6 ppm), and Zhang 2014 (LOAEC=2 ppm).

Zhang et al., 2007 studied mainly higher exposures, and can therefore be excluded. The four remaining high-quality factory studies result in an average LOAEC of 2.33 ppm. This is the best supported LOAEC (leading case) since it is a weighted average of the highest quality studies, with an average quality score of 17.25. When the three additional studies from the fuel scenario: Carere et al. (2 ppm), Rekhadavi et al. (1 ppm), and Pandey et al. (2 ppm) are added, the resulting LOAEC is 2.04 ppm, which can be regarded as the sensitivity analysis based on the next highest quality studies.

If high quality is defined more inclusively as studies above the median, adding the one additional study from the factory setting with a more certain LOAEC (Eastmond et al., 1.29 ppm) with the other first tertile more certain factory studies, results in an average LOAEC of 2.12 ppm. The average quality score in this sensitivity analysis decreases to 16.3 (from 17.25), but still supports a LOAEC of approximately 2 ppm. There were no additional studies from the fuel nor ambient scenarios which generated more certain LOAECs above the median score of 12.5. All high certainty LOAECs above the median score from the factory and fuel sector combined, result in a LOAEC of 1.95 ppm (average score – 14.85). Although average quality score has decreased, this also supports an aggregate LOAEC of∼2ppm.

Consideration of the Less certain LOAECs included Kim et al., 2004a, >0.56 ppm, potential confounding by PAH exposure; average LOAEC for all factory studies in the first tertile was 1.97 ppm, quality score of 17.10); Factory studies with a less certain LOAEC (Bogadi-Sare et al., 2003 LOAEC=13 ppm, Holz et al., 1995, LOAEC=0.6–1 ppm). The LOAECs from Bogardi-Sare and Holz differ by more than two orders of magnitude, thus sensitivity analyses are not warranted.

The leading case LOAEC of 2.33 ppm is supported by the leading sensitivity analyses which account for more studies with a lower quality score and suggest slightly lower LOAECs near 2 ppm. Interpreted with due regard to quality, in aggregate the literature supports a LOAEC of 2 ppm.

Derivation of NOAECs. Three studies from the factory scenario that suggest NOAECs: Bogadi-Sare et al. 1997a (8 ppm), Zhang et al., 2011 (4.95 ppm) and Basso et al., 2011

(0.029 ppm). These studies differ by more than two orders of magnitude and as such, do not offer a good “base case” on which to justify a NOAEC. We face the problem of a NOAEC that is higher than the LOAEC. Despite the difficulty in isolating an effect of benzene in impure fuel and (especially) ambient studies, they are the best avenue at present for estimating a NOAEC for genotoxicity. In the fuel scenario, two studies scored in the first tertile and were characterized by more certain NOAECs: Carere et al. (1995) (0.47 ppm) and Pandey et al. (2008) (0.9 ppm). Combining these gives an average NOAEC of 0.69 ppm for genotoxicity. There are three other studies: Fracasso et al. (2010) (0.012 ppm), Pitarque et al. (1996) (0.3 ppm) and Göethel et al. (2014) (0.6 ppm) from the fuel sector that score above the median with more certain NOAECs. Using this set of studies as a sensitivity analysis a NOAEC of 0.45 ppm results. These analyses suggest that a NOAEC of 0.5 ppm is justified.

 

OEL derivation

Based on haematology studies

Method 1: (Use of the LOAEC)

POINT OF DEPARTURE FOR HAEMATOLOGICAL EFFECTS:

2.19 ppm (Based on three studies with a more certain LOAEC that are high quality (top tertile quality score). This is a conservative interpretation that does not consider that four other high-quality studies showed LOAECs of 7−8 ppm. 

POTENTIAL ASSESSMENT FACTORS:

• Dose-response (LOAEC to NOAEC). 2.19 ppm is the lowest level of exposure among three high quality studies with more certain LOAECs. Most other high-quality studies show a higher LOAEC. In addition, there are other high-quality studies (viz. Schnatter et al., 2010; Ward et al., 1996; Pesatori et al., 2009) (Pesatori et al., 2009; Schnatter et al., 2010; Ward et al., 1996) which report NOAECs for exposure levels similar to 2 ppm. Given this degree of potential overlap in LOAECs and NOAECs and the conservative selection of 2.19 ppm, the factor should be lower than the usual value of 3. A value of 2 is recommended.

• Intraspecies. A factor lower than 3 is recommended when a reasonably large human study is used in which a range of sensitivities are already present and extrapolations from the study data are to other occupational populations. In aggregate, the LOAEC studies considered included >2700 benzene exposed individuals. In addition, it can be seen that the lowest LOAECs ((Qu et al., 2003, (Zhang et al., 2016)) are those based on Chinese workers, who may be a more sensitive population. Thus, a value of 2 is recommended, although a value of 1 would not be unreasonable since the aggregate value is from studies showing the lowest LOAECs and the studies cover diverse populations, already including potentially sensitive sub-populations.

OEL=2.19 ppm / 4 (=2×2)=0.55 ppm METHOD 1

 

Method 2: (Use of NOAECs)

Method 2 is derived from the NOAECs of four studies of high quality.

NOAECs that are near or above the LOAEC from above are not considered, thus the Schnatter et al., 2010 (Schnatter et al., 2010) study (NOAEC 2.9 ppm) Ward et al( 1996), ( NOAEC 2.2 ppm) are excluded. A NOAEC is usually preferred to a LOAEC, provided that the lack of effect can be observed in a clear and precise way.

POINT OF DEPARTURE FOR HAEMATOLOGICAL EFFECTS:

NOAECs from four high quality studies (i.e. top tertile) are used as the basis for a weighted NOAEC of 0.58 ppm. These studies are: Collins et al., 1991; Koh et al., 2015; Pesatori et al., 2009; and Swaen et al., 2010. (Collins et al., 1997 and 1991; Khuder et al., 1999; Koh et al., 2015; Pesatori et al., 2009; Swaen et al., 2010; Tsai et al., 2004), in aggregate >11,700 benzene exposed individuals. Three studies (Bogadi-Šare et al., 2003; Schnatter et al., 2010; Ward et al., 1996) that report NOAECs of 2−3 ppm are not included, because they overlap with the chosen LOAEC value. The arithmetic average of these NOAECs is 0.59 ppm

POTENTIAL ASSESSMENT FACTORS:

• Dose response. A factor of 1 is suggested because the point of departure is derived from a NOAEC. • Intra-species factor. A factor of 1 is suggested because a larger aggregate human population is used (>11,700 benzene exposed individuals) than in METHOD 1 (>2700 benzene exposed individuals). Given that Method 1 (based on LOAECs) provides an OEL of 0.55 ppm (8 h TWA) and that Method 2 (based on NOAECs) provides an OEL of 0.59 ppm (8 hTWA) both methods would support an OEL of 0.5 ppm (8 h TWA).

 

The data supporting this position however are derived from worker studies examining effects in peripheral blood, while the target organ for benzene toxicity is bone marrow. An additional factor of two is proposed for possible subclinical effects in the bone marrow. We adopted ECHA RAC’s view that the associated uncertainty is relatively small and that an assessment factor of 2 would be appropriate (ECHA, 2018a, b). Although there is limited scientific experimental information available on this topic, and only in the rodent, French et al. (2015) and Ferris et al (1996) show that the bone marrow may (French et al 2015) or may not (Ferris et al 1996) be more sensitive than peripheral blood. The mouse micronucleus test results however cannot be simply translated to humans because of large differences in splenic function, such as removal of MN erythrocytes (Schlegel and MacGregor, 1983).

For workers, measured events in T-cells afford global assessments of in vivo mutagenicity but are not specific for bone marrow effects, while MN detected in reticulocytes are produced in reticulocyte precursors in the bone marrow (Albertini and Kaden, 2020). In humans only a small difference between MN in peripheral blood lymphocytes and reticulocytes was observed for radioiodine therapy (Stopper et al., 2005).

This available information overall would imply a small assessment factor. Therefore, an additional factor of two is proposed for possible subclinical effects in the bone marrow until additional research clarifies the sensitivity of peripheral blood versus bone marrow effects. This additional factor would support an OEL of 0.25 ppm (8 h TWA).

 

Based on genotoxicity studies

Method 1: (Use of the LOAEC)

POINT OF DEPARTURE FOR GENOTOXIC EFFECTS: >2.33 ppm.

This preferred approach is based on four studies (Table 6) in the factory setting with a more certain LOAEC that are high quality (top tertile). A fifth study (Zhang et al., 2007) which showed a higher LOAEC of 13.6 ppm was not considered. This preferred derivation is supported by additional sensitivity analyses summarized previously which consider the fuel sector as well as the factory sector, and the alternative definition of “high quality” using studies above the median rather than the top tertile.

 

POTENTIAL ASSESSMENT FACTORS:

• Dose-response (LOAEC to NOAEC).>2.33 ppm is the lowest level of exposure among four high quality (top tertile). Subsequently, a NOAEC of 0.69 ppm was calculated (see below). Other NOAECs which were near or greater than the LOAEC were not considered. In addition, the preferred LOAEC is noted as greater than 2.33, thus 2.33 should be regarded as the minimum preferred value. Given the degree of potential overlap in LOAECs and NOAECs, and the fact that there is some uncertainty in the inequality >2.33 ppm, the factor should be lower than the usual value of 3. A value of 2 is recommended. 

• Intraspecies. A factor lower than 3 is recommended when a reasonably large human study is used in which a range of sensitivities are already present and extrapolations from the study data are to other occupational populations. In aggregate, the LOAEC studies considered included >2700 benzene exposed individuals. In addition, all the LOAECs are based on Chinese workers, who may be a more sensitive population. Thus, a value of 2 is recommended. A value of 1 could also be considered since a possibly more sensitive population generates the LOAEC, thus, sensitive sub-populations may have already been accounted for in the selection of this LOAEC.

OEL=2.33 ppm / 4 (=2×2)=0.58 ppm METHOD 1

 

Method 2: (Use of NOAECs)

Method 2 is derived from the NOAECs of two studies of high quality in the fuel sector since studies in the factory sector showed higher NOAECs when compared to the preferred LOAEC. NOAECs that are

near or above the LOAEC from above are not considered, thus this could be considered a conservative approach.

POINT OF DEPARTURE FOR GENOTOXIC EFFECTS:

NOAECs from two high quality studies are used as the basis for a weighted NOAEC of 0.69 ppm. Studies of Zhang et al., 2011 (NOAEC=4.95) and Bogadi-Šare et al., 2003 (NOAEC=8) were not considered, thus the value of 0.69 may be conservative. On the other hand, only two studies are used to calculate the aggregate NOAEC, which could balance the conservative nature of the selection of studies that were included. Concordance with method 1, arguably based on stronger data (average quality score of LOAEC studies=17.25, average quality score of NOAEC studies=14.5) would also justify an intra-species factor of 1.

OEL=0.69 ppm. METHOD 2.

 

Given that the haematology data suggest an OEL of 0.5 ppm, the genotoxicity based OELs of 0.58 ppm (Method 1), and 0.69 ppm (Method 2) it can be agreed that both datasets would support an OEL of 0.5 ppm (8 h TWA). 

As was the case for haematotoxicity, the data supporting this position are mainly derived from worker studies examining effects in peripheral blood (except for (Xing et al., 2010). An additional factor of two is proposed for possible subclinical effects in the bone marrow until additional research clarifies the sensitivity of peripheral blood versus bone marrow effects. This additional factor would support an OEL of 0.25 ppm (8 h TWA) for both haematotoxicity and genotoxicity endpoints.

Conclusions:

The data presented by Schnatter et al 2020 define a benzene LOAEC of 2 ppm (8 h TWA) and a NOAEC of 0.5 ppm (8 h TWA). However, the use of peripheral blood measures of bone marrow effects introduces some scientific uncertainty, thus until the issue of bone marrow sensitivity compared to that of peripheral blood is resolved an extra assessment factor of two is applied. An OEL of 0.25 ppm (8 h TWA) for benzene is the best estimate based on available human data.

Executive summary:

This paper derives an occupational exposure limit for benzene using quality assessed data. Seventy-seven genotoxicity and thirty six haematotoxicity studies in workers were scored for study quality with an adapted tool based on that of Vlaanderen et al., 2008 (Environ Health. Perspect. 116 1700−5). These endpoints were selected as they are the most sensitive and relevant to the proposed mode of action (MOA) and protecting against these will protect against benzene carcinogenicity. Lowest and No- Adverse Effect Concentrations (LOAECs and NOAECs) were derived from the highest quality studies (i.e. those ranked in the top tertile or top half) and further assessed as being “more certain” or “less certain”. Several sensitivity analyses were conducted to assess whether alternative “high quality” constructs affected conclusions. The lowest haematotoxicity LOAECs showed effects near 2 ppm (8 h TWA), and no effects at 0.59 ppm. For genotoxicity, studies also showed effects near 2 ppm and showed no effects at about 0.69 ppm. Several sensitivity analyses supported these observations. These data define a benzene LOAEC of 2 ppm (8 h TWA) and a NOAEC of 0.5 ppm (8 h TWA). Allowing for possible subclinical effects in bone marrow not apparent in studies of peripheral blood endpoints, an OEL of 0.25 ppm (8 h TWA) is proposed.

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEC

1.6 mg/m³

Study duration:

chronic

Experimental exposure time per week (hours/week):

40

Species:

other: human (epidemiological findings)

Quality of whole database:

Adequate information is available to characterise the repeated inhalation toxicity of benzene in animals and humans.

System:

haematopoietic

Organ:

bone marrow

Repeated dose toxicity: inhalation - local effects

Link to relevant study records

Reference

Reference 1

Endpoint:

chronic toxicity: inhalation

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: GLP-status unknown, non-guideline, animal experimental study, published in peer-reviewed literature, notable limitations in design and reporting but contributing to a weight of evidence

Qualifier:

no guideline followed

Principles of method if other than guideline:

Eight to twelve-week old male and female C57B1/6 BNL mice were exposed to air or benzene vapour in air. At various times during and after the exposure period, five to ten mice were removed from both the treated and control groups and the blood, bone marrow and spleens removed and examined for evidence of haematotoxicity.

GLP compliance:

not specified

Limit test:

no

Species:

mouse

Strain:

C57BL

Sex:

male/female

Route of administration:

inhalation: vapour

Type of inhalation exposure:

whole body

Vehicle:

other: air

Details on inhalation exposure:

GENERATION OF TEST ATMOSPHERE / CHAMBER DESCRIPTION
- Exposure apparatus: details on construction and design reported elsewhere
- Generation of vapour: No details reported

Analytical verification of doses or concentrations:

not specified

Duration of treatment / exposure:

2-16 weeks

Frequency of treatment:

6 h/day, 5 days/week or 3 consecutive days/week

Remarks:

Doses / Concentrations:
0, 10, 25, 100, 300, 400 ppm (0, 32, 80, 320, 960, 1280 mg/m3)
Basis:
other: target concentration - (6h/d, 5d/wk for 2-16 wks)

Remarks:

Doses / Concentrations:
0, 300 ppm (960 mg/m3)
Basis:
other: target concentration - (6h/day, 3 d/week for 8 weeks)

No. of animals per sex per dose:

No specific detail. At 300 ppm for 16 weeks 88 control and 89 exposed females

Control animals:

yes, concurrent no treatment

Details on study design:

Post-exposure period: 16 weeks

Dose descriptor:

NOAEC

Effect level:

10 ppm

Sex:

male/female

Basis for effect level:

other: lymphocytopenia after 10 expousres at 25 ppm

Dose descriptor:

NOAEC

Effect level:

32 mg/m³ air (nominal)

Sex:

male/female

Basis for effect level:

other: lymphocytopenia after 10 expousres at 80 mg/m3

Critical effects observed:

not specified

Benzene (100 ppm or higher for 6 h/day, 5 days/week for 2 weeks) produced a reduction in bone marrow cellularity and the number of pluripotent stem cells in the bone marrow. There was also as increase in the fraction of stem cells in DNA synthesis. At 25 ppm lymphocyte concentration in peripheral blood was decreased. Exposure to 300 ppm 6 h/day, 5 days/ week for 2, 4, 8, and 16 weeks produced a diminution in the stem cell levels in bone marrow. Stem cells returned to control levels 2 weeks after the end of benzene exposure for 2 and 4 weeks, 16 weeks after exposure for 8 weeks, and to 92% of controls 25 weeks after 16 weeks of exposure. Blood lymphocytes showed a more rapid return to the control level.

Conclusions:

Repeated inhalation exposure to benzene at 6h/day, 5 days/week produces haematotoxicity in mice. The NOAEC was 10 ppm (32 mg/m3).

Executive summary:

Haematotoxicity was examined in male and female C57Bl/6 BNL mice exposed to benzene vapour at concentrations of 0, 10, 25, 100, 300 or 400 ppm benzene for 2 -16 weeks. At various times during and after the exposure period, five to ten mice were removed from both the treated and control groups and the blood, bone marrow and spleens removed and examined.

At ≥100 ppm for 10 exposures benzene produced a reduction in bone marrow cellularity and the number of pluripotent stem cells in the bone marrow. The fraction of stem cells in DNA synthesis was also increased. At 25 ppm lymphocyte concentration was decreased. 16 weeks of exposure to 300 ppm benzene, 6 h/day, 5 days/week, produced a diminution in the haemopoietic stem cells with incomplete recovery 16 weeks after termination of exposure although full recovery was seen with shorter exposure durations (2, 4 or 8 weeks). There was a more rapid return of blood lymphocytes to control levels.

The NOAEC for haematotoxicity was 10 ppm (32 mg/m³).

Reference 2

Endpoint:

repeated dose toxicity: inhalation, other

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Literature 1980-2018

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Qualifier:

no guideline required

Principles of method if other than guideline:

Quality review of epidemiological (genotoxicity and haematoxicity) studies for Occupational Exposure Limit Derivation.

GLP compliance:

no

Remarks:

Not applicable

Species:

other: human

Key result

Dose descriptor:

NOAEC

Effect level:

0.59 ppm

Based on:

test mat.

Basis for effect level:

haematology

Remarks on result:

other: arithmetic average

Key result

Dose descriptor:

LOAEC

Effect level:

2 ppm

Based on:

test mat.

Basis for effect level:

haematology

Remarks on result:

other: aggregate

Key result

Dose descriptor:

other: OEL

Effect level:

0.25 ppm

Based on:

test mat.

Remarks on result:

other: NOAEC modified by assessment factor of 2

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

2.19 ppm

System:

haematopoietic

Organ:

blood

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

yes

Key result

Critical effects observed:

yes

Lowest effective dose / conc.:

2.33 ppm

System:

haematopoietic

Organ:

bone marrow

Treatment related:

yes

Dose response relationship:

yes

Relevant for humans:

yes

Results

 

Quality scoring results for genotoxic and haematological studies

Among the group of 31 haematology and 56 genotoxicity study populations each had a top score of 20 (of a possible 24), which in both cases was due to the (Qu et al., 2003) study. Both haematotoxicity and genotoxicity studies showed wide ranges (8–20 and 6–20, respectively) indicating marked differences in study quality for each body of literature.

Ties in scores for the haematotoxicity studies resulted in initial stratification of 11 studies in the top tertile (score range 14.5–20), 9 studies in the second tertile (score range 11–14), and 16 studies at or above the median score of 12.5. Similarly, for genotoxicity studies, 21 studies are in the first tertile (score range 13.5–20), 17 studies form the second tertile (score range 11–13), and 29 studies are at or above the median score of 12.5.

 

LOAECs and NOAECs for high quality studies

Haematology

Derivation of LOAECs

The highest quality studies (i.e. first tertile) that generated a more certain LOAEC were: Qu et al., 2003 (2.26 ppm, neutrophils), Schnatter et al., 2010, (7.8 ppm, neutrophils), Ward et al., 1996,(7.2 ppm, total leukocytes), Lan et al., 2004, (2.2 ppm, various cell types), Rothman et al., 1996, (7.6 ppm, lymphocytes), and Zhang et al., 2016 (2.1 ppm, leukocytes). From these values, a bimodal distribution results, in which there are two clusters of studies: three studies that suggest a LOAEC near 2 ppm and three studies that suggest a LOAEC near 7−8 ppm.

Sensitivity analyses supported a LOAEC around 2 ppm: Looking at the LOAECs in which all studies at or above the median quality score are considered as high quality, the LOAECs are similar, with only (Bogadi-Šare et al., 2003) at 8 ppm added to the above list from the first tertile. Thus, there are four studies suggesting a LOAEC of 7−8 ppm and three studies suggesting a LOAEC near 2 ppm. This alternative definition of high quality is a sensitivity analysis that supports the top tertile result. For the highest quality (top tertile) studies that generated a less certain LOAEC, values were: (Swaen et al., 2010) (0.75 ppm); and (Koh et al., 2015) (2.6 ppm). Inclusion of these studies is another sensitivity analysis that would lend more weight to a LOAEC in the range of 2 ppm, rather than the second cluster at 7−8 ppm. Various sensitivity analyses incorporating all less certain LOAECs above the median did not change this conclusion.

 

Derivation of NOAECs

For first tertile studies, the more certain NOAECs are 0.25 ppm (Swaen et al., 2010), 2.9 ppm (Schnatter et al., 2010), 2.2 ppm (Ward et al., 1996), 0.19 ppm (Collins et al., 1991), 0.21 ppm (Koh et al., 2015), and 1.7 ppm (Pesatori et al., 2009). Thus, there are three studies that suggest a NOAEC near 2−3 ppm, and three studies that suggest a NOAEC near 0.2−0.25 ppm. When studies that scored above the median and that show a more certain NOAEC are included, the NOAECs are 0.55 ppm (Collins et al., 1997), 0.81 ppm (Khuder et al., 1999), and 0.33 ppm (Tsai et al., 2004). Collectively, all studies above the median with more definitive NOAECs show four studies near 0.2−0.3 ppm, two studies near 0.6−0.8 ppm, and three studies near 2−3 ppm.

Examining studies above the median is justified and increases the number of studies although quality is somewhat lower (i.e. average quality is 16.25 versus 14.93 for NOAECs, and identical (16.67) for LOAECs).

Based upon frequencies, a LOAEC of 7−8 ppm, and a NOAEC of 2−3 ppm is one defensible conclusion from the analysis above. The NOAECs of 2−3 ppm are of a similar magnitude to three LOAECs from high quality studies, which introduces the problem of overlapping NOAECs with LOAECs. An alternative strategy would be to select the higher quality study(/ies) with more certain LOAECs that do not overlap NOAECs from high quality studies. Thus, there are three studies (Qu et al., 2003; Lan et al., 2004; Zhang et al., 2016) that show LOAECs near 2 ppm. If the LOAEC selected is near 2 ppm, a lower NOAEC should be selected. The two studies with the highest NOAEC yet still below 2 ppm are Collins et 1997 (0.55 ppm) and Khuder et al., 1999 (0.81 ppm). More studies show NOAECs of 0.2−0.3 ppm (Collins et al., 1991; Koh et al., 2015; Swaen et al., 2010; Tsai et al., 2004). Collins et al., 1991; Swaen et al., 2010; Tsai et al., 2004, all studied exposures < 0.5 ppm, so that a NOAEC was not achievable for those studies. Collectively, the results are not in conflict with a 0.5 ppm NOAEC, which is four times lower than the LOAEC (see Table 5). All sensitivity analyses (using top tertile studies, above median studies, and lower certainty LOAECs and NOAECs) in Table 5 result in LOAECs between 1.98 and 2.19 ppm, and NOAECs of 0.58 and 0.59 ppm. Thus, the result based on first tertile studies (a LOAEC of 2.19 ppm and a NOAEC of 0.59 ppm) is a conservative yet coherent interpretation of this information and is the preferred approach or base case.

 

Genotoxicity

Factory workers

Of the 21 studies in the top tertile, ten studies were among factory workers, five among fuel handlers and six among workers exposed to traffic and ambient air. In factory workers, the five studies with more certain LOAECs were (Qu et al., 2003) (LOAEC=3.07 ppm), (Xing et al., 2010)(LOAEC>1.6 ppm), (Zhang et al., 2012) (LOAEC>2.64 ppm), (Zhang et al., 2007) (LOAEC=13.6 ppm) and (Zhang et al., 2014) (LOAEC=2 ppm). The top tertile study generating a less certain LOAEC (>0.56 ppm) was (Kim et al., 2004a) due to the presence of PAH co-exposures.

 

Fuel workers

Three studies (Carere et al., 1995; Pandey et al., 2008 and Rekhadevi et al., 2010) in the top tertile were associated with a more certain LOAEC and none with a less certain LOAEC. The three studies showed similar LOAECs of 2 ppm, 2 ppm, and > 1 ppm, respectively. A NOAEC in the Carere study for micronuclei is 0.47 ppm and in the Pandey study∼0.9 ppm. The quality scores of the first tertile fuel studies (14.5) are lower than those from the factory

setting (17.25).

 

Traffic/ambient air

There were only two studies (Leopardi et al., NOAEC=0.003 ppm; Maffei et al., LOAEC=0.008 ppm) in the top tertile which produced a more certain LOAEC or NOAEC. Violante et al. (15.5) has a less certain NOAEC of 0.005 ppm and Angelini (14.5) has a less certain LOAEC of 0.006 ppm. Since the exposure concentrations present in the traffic/ambient air studies are lower than other NOAECs based on fuel and factory studies, this group of studies does not add meaningful information to the NOAEC analysis.

Since the single top tertile study that showed a more certain LOAEC is of lower quality (13.5) than studies from the factory and fuel sectors (average=16.07), this group of studies also does not add meaningful information to the LOAEC analysis. Thus, these studies are not subsequently considered.

 

Derivation of LOAECs

The highest quality studies (i.e. first tertile) that generated a more certain LOAEC originated from the factory and fuel study scenarios. There were five such studies from the factory scenario: Qu et al. (LOAEC=3.07 ppm), Xing et al. (LOAEC>1.6 ppm), Zhang et al. (2012) (LOAEC>2.64 ppm), Zhang et al., 2007(LOAEC=13.6 ppm), and Zhang 2014 (LOAEC=2 ppm).

Zhang et al., 2007 studied mainly higher exposures, and can therefore be excluded. The four remaining high-quality factory studies result in an average LOAEC of 2.33 ppm. This is the best supported LOAEC (leading case) since it is a weighted average of the highest quality studies, with an average quality score of 17.25. When the three additional studies from the fuel scenario: Carere et al. (2 ppm), Rekhadavi et al. (1 ppm), and Pandey et al. (2 ppm) are added, the resulting LOAEC is 2.04 ppm, which can be regarded as the sensitivity analysis based on the next highest quality studies.

If high quality is defined more inclusively as studies above the median, adding the one additional study from the factory setting with a more certain LOAEC (Eastmond et al., 1.29 ppm) with the other first tertile more certain factory studies, results in an average LOAEC of 2.12 ppm. The average quality score in this sensitivity analysis decreases to 16.3 (from 17.25), but still supports a LOAEC of approximately 2 ppm. There were no additional studies from the fuel nor ambient scenarios which generated more certain LOAECs above the median score of 12.5. All high certainty LOAECs above the median score from the factory and fuel sector combined, result in a LOAEC of 1.95 ppm (average score – 14.85). Although average quality score has decreased, this also supports an aggregate LOAEC of∼2ppm.

Consideration of the Less certain LOAECs included Kim et al., 2004a, >0.56 ppm, potential confounding by PAH exposure; average LOAEC for all factory studies in the first tertile was 1.97 ppm, quality score of 17.10); Factory studies with a less certain LOAEC (Bogadi-Sare et al., 2003 LOAEC=13 ppm, Holz et al., 1995, LOAEC=0.6–1 ppm). The LOAECs from Bogardi-Sare and Holz differ by more than two orders of magnitude, thus sensitivity analyses are not warranted.

The leading case LOAEC of 2.33 ppm is supported by the leading sensitivity analyses which account for more studies with a lower quality score and suggest slightly lower LOAECs near 2 ppm. Interpreted with due regard to quality, in aggregate the literature supports a LOAEC of 2 ppm.

Derivation of NOAECs. Three studies from the factory scenario that suggest NOAECs: Bogadi-Sare et al. 1997a (8 ppm), Zhang et al., 2011 (4.95 ppm) and Basso et al., 2011

(0.029 ppm). These studies differ by more than two orders of magnitude and as such, do not offer a good “base case” on which to justify a NOAEC. We face the problem of a NOAEC that is higher than the LOAEC. Despite the difficulty in isolating an effect of benzene in impure fuel and (especially) ambient studies, they are the best avenue at present for estimating a NOAEC for genotoxicity. In the fuel scenario, two studies scored in the first tertile and were characterized by more certain NOAECs: Carere et al. (1995) (0.47 ppm) and Pandey et al. (2008) (0.9 ppm). Combining these gives an average NOAEC of 0.69 ppm for genotoxicity. There are three other studies: Fracasso et al. (2010) (0.012 ppm), Pitarque et al. (1996) (0.3 ppm) and Göethel et al. (2014) (0.6 ppm) from the fuel sector that score above the median with more certain NOAECs. Using this set of studies as a sensitivity analysis a NOAEC of 0.45 ppm results. These analyses suggest that a NOAEC of 0.5 ppm is justified.

 

OEL derivation

Based on haematology studies

Method 1: (Use of the LOAEC)

POINT OF DEPARTURE FOR HAEMATOLOGICAL EFFECTS:

2.19 ppm (Based on three studies with a more certain LOAEC that are high quality (top tertile quality score). This is a conservative interpretation that does not consider that four other high-quality studies showed LOAECs of 7−8 ppm. 

POTENTIAL ASSESSMENT FACTORS:

• Dose-response (LOAEC to NOAEC). 2.19 ppm is the lowest level of exposure among three high quality studies with more certain LOAECs. Most other high-quality studies show a higher LOAEC. In addition, there are other high-quality studies (viz. Schnatter et al., 2010; Ward et al., 1996; Pesatori et al., 2009) (Pesatori et al., 2009; Schnatter et al., 2010; Ward et al., 1996) which report NOAECs for exposure levels similar to 2 ppm. Given this degree of potential overlap in LOAECs and NOAECs and the conservative selection of 2.19 ppm, the factor should be lower than the usual value of 3. A value of 2 is recommended.

• Intraspecies. A factor lower than 3 is recommended when a reasonably large human study is used in which a range of sensitivities are already present and extrapolations from the study data are to other occupational populations. In aggregate, the LOAEC studies considered included >2700 benzene exposed individuals. In addition, it can be seen that the lowest LOAECs ((Qu et al., 2003, (Zhang et al., 2016)) are those based on Chinese workers, who may be a more sensitive population. Thus, a value of 2 is recommended, although a value of 1 would not be unreasonable since the aggregate value is from studies showing the lowest LOAECs and the studies cover diverse populations, already including potentially sensitive sub-populations.

OEL=2.19 ppm / 4 (=2×2)=0.55 ppm METHOD 1

 

Method 2: (Use of NOAECs)

Method 2 is derived from the NOAECs of four studies of high quality.

NOAECs that are near or above the LOAEC from above are not considered, thus the Schnatter et al., 2010 (Schnatter et al., 2010) study (NOAEC 2.9 ppm) Ward et al( 1996), ( NOAEC 2.2 ppm) are excluded. A NOAEC is usually preferred to a LOAEC, provided that the lack of effect can be observed in a clear and precise way.

POINT OF DEPARTURE FOR HAEMATOLOGICAL EFFECTS:

NOAECs from four high quality studies (i.e. top tertile) are used as the basis for a weighted NOAEC of 0.58 ppm. These studies are: Collins et al., 1991; Koh et al., 2015; Pesatori et al., 2009; and Swaen et al., 2010. (Collins et al., 1997 and 1991; Khuder et al., 1999; Koh et al., 2015; Pesatori et al., 2009; Swaen et al., 2010; Tsai et al., 2004), in aggregate >11,700 benzene exposed individuals. Three studies (Bogadi-Šare et al., 2003; Schnatter et al., 2010; Ward et al., 1996) that report NOAECs of 2−3 ppm are not included, because they overlap with the chosen LOAEC value. The arithmetic average of these NOAECs is 0.59 ppm

POTENTIAL ASSESSMENT FACTORS:

• Dose response. A factor of 1 is suggested because the point of departure is derived from a NOAEC. • Intra-species factor. A factor of 1 is suggested because a larger aggregate human population is used (>11,700 benzene exposed individuals) than in METHOD 1 (>2700 benzene exposed individuals). Given that Method 1 (based on LOAECs) provides an OEL of 0.55 ppm (8 h TWA) and that Method 2 (based on NOAECs) provides an OEL of 0.59 ppm (8 hTWA) both methods would support an OEL of 0.5 ppm (8 h TWA).

 

The data supporting this position however are derived from worker studies examining effects in peripheral blood, while the target organ for benzene toxicity is bone marrow. An additional factor of two is proposed for possible subclinical effects in the bone marrow. We adopted ECHA RAC’s view that the associated uncertainty is relatively small and that an assessment factor of 2 would be appropriate (ECHA, 2018a, b). Although there is limited scientific experimental information available on this topic, and only in the rodent, French et al. (2015) and Ferris et al (1996) show that the bone marrow may (French et al 2015) or may not (Ferris et al 1996) be more sensitive than peripheral blood. The mouse micronucleus test results however cannot be simply translated to humans because of large differences in splenic function, such as removal of MN erythrocytes (Schlegel and MacGregor, 1983).

For workers, measured events in T-cells afford global assessments of in vivo mutagenicity but are not specific for bone marrow effects, while MN detected in reticulocytes are produced in reticulocyte precursors in the bone marrow (Albertini and Kaden, 2020). In humans only a small difference between MN in peripheral blood lymphocytes and reticulocytes was observed for radioiodine therapy (Stopper et al., 2005).

This available information overall would imply a small assessment factor. Therefore, an additional factor of two is proposed for possible subclinical effects in the bone marrow until additional research clarifies the sensitivity of peripheral blood versus bone marrow effects. This additional factor would support an OEL of 0.25 ppm (8 h TWA).

 

Based on genotoxicity studies

Method 1: (Use of the LOAEC)

POINT OF DEPARTURE FOR GENOTOXIC EFFECTS: >2.33 ppm.

This preferred approach is based on four studies (Table 6) in the factory setting with a more certain LOAEC that are high quality (top tertile). A fifth study (Zhang et al., 2007) which showed a higher LOAEC of 13.6 ppm was not considered. This preferred derivation is supported by additional sensitivity analyses summarized previously which consider the fuel sector as well as the factory sector, and the alternative definition of “high quality” using studies above the median rather than the top tertile.

 

POTENTIAL ASSESSMENT FACTORS:

• Dose-response (LOAEC to NOAEC).>2.33 ppm is the lowest level of exposure among four high quality (top tertile). Subsequently, a NOAEC of 0.69 ppm was calculated (see below). Other NOAECs which were near or greater than the LOAEC were not considered. In addition, the preferred LOAEC is noted as greater than 2.33, thus 2.33 should be regarded as the minimum preferred value. Given the degree of potential overlap in LOAECs and NOAECs, and the fact that there is some uncertainty in the inequality >2.33 ppm, the factor should be lower than the usual value of 3. A value of 2 is recommended. 

• Intraspecies. A factor lower than 3 is recommended when a reasonably large human study is used in which a range of sensitivities are already present and extrapolations from the study data are to other occupational populations. In aggregate, the LOAEC studies considered included >2700 benzene exposed individuals. In addition, all the LOAECs are based on Chinese workers, who may be a more sensitive population. Thus, a value of 2 is recommended. A value of 1 could also be considered since a possibly more sensitive population generates the LOAEC, thus, sensitive sub-populations may have already been accounted for in the selection of this LOAEC.

OEL=2.33 ppm / 4 (=2×2)=0.58 ppm METHOD 1

 

Method 2: (Use of NOAECs)

Method 2 is derived from the NOAECs of two studies of high quality in the fuel sector since studies in the factory sector showed higher NOAECs when compared to the preferred LOAEC. NOAECs that are

near or above the LOAEC from above are not considered, thus this could be considered a conservative approach.

POINT OF DEPARTURE FOR GENOTOXIC EFFECTS:

NOAECs from two high quality studies are used as the basis for a weighted NOAEC of 0.69 ppm. Studies of Zhang et al., 2011 (NOAEC=4.95) and Bogadi-Šare et al., 2003 (NOAEC=8) were not considered, thus the value of 0.69 may be conservative. On the other hand, only two studies are used to calculate the aggregate NOAEC, which could balance the conservative nature of the selection of studies that were included. Concordance with method 1, arguably based on stronger data (average quality score of LOAEC studies=17.25, average quality score of NOAEC studies=14.5) would also justify an intra-species factor of 1.

OEL=0.69 ppm. METHOD 2.

 

Given that the haematology data suggest an OEL of 0.5 ppm, the genotoxicity based OELs of 0.58 ppm (Method 1), and 0.69 ppm (Method 2) it can be agreed that both datasets would support an OEL of 0.5 ppm (8 h TWA). 

As was the case for haematotoxicity, the data supporting this position are mainly derived from worker studies examining effects in peripheral blood (except for (Xing et al., 2010). An additional factor of two is proposed for possible subclinical effects in the bone marrow until additional research clarifies the sensitivity of peripheral blood versus bone marrow effects. This additional factor would support an OEL of 0.25 ppm (8 h TWA) for both haematotoxicity and genotoxicity endpoints.

Conclusions:

The data presented by Schnatter et al 2020 define a benzene LOAEC of 2 ppm (8 h TWA) and a NOAEC of 0.5 ppm (8 h TWA). However, the use of peripheral blood measures of bone marrow effects introduces some scientific uncertainty, thus until the issue of bone marrow sensitivity compared to that of peripheral blood is resolved an extra assessment factor of two is applied. An OEL of 0.25 ppm (8 h TWA) for benzene is the best estimate based on available human data.

Executive summary:

This paper derives an occupational exposure limit for benzene using quality assessed data. Seventy-seven genotoxicity and thirty six haematotoxicity studies in workers were scored for study quality with an adapted tool based on that of Vlaanderen et al., 2008 (Environ Health. Perspect. 116 1700−5). These endpoints were selected as they are the most sensitive and relevant to the proposed mode of action (MOA) and protecting against these will protect against benzene carcinogenicity. Lowest and No- Adverse Effect Concentrations (LOAECs and NOAECs) were derived from the highest quality studies (i.e. those ranked in the top tertile or top half) and further assessed as being “more certain” or “less certain”. Several sensitivity analyses were conducted to assess whether alternative “high quality” constructs affected conclusions. The lowest haematotoxicity LOAECs showed effects near 2 ppm (8 h TWA), and no effects at 0.59 ppm. For genotoxicity, studies also showed effects near 2 ppm and showed no effects at about 0.69 ppm. Several sensitivity analyses supported these observations. These data define a benzene LOAEC of 2 ppm (8 h TWA) and a NOAEC of 0.5 ppm (8 h TWA). Allowing for possible subclinical effects in bone marrow not apparent in studies of peripheral blood endpoints, an OEL of 0.25 ppm (8 h TWA) is proposed.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Repeated dose toxicity: dermal - systemic effects

Endpoint conclusion

Endpoint conclusion:

no study available

Repeated dose toxicity: dermal - local effects

Endpoint conclusion

Endpoint conclusion:

no study available

Additional information

Non-human data

Oral

Benzene toxicity following sub-chronic and chronic oral (gavage) exposure was investigated in studies using F344/N rats and B6C3F1 mice conducted as part of the National Toxicology Program (NTP, 1986). Animals were dosed 5 days per week for 17 or up to 103 weeks. The most significant findings in rats were a dose-related leukopenia and lymphocytopenia observed in males at ≥ 200 mg/kg and in females at ≥ 25 mg/kg in the 17 week study and in all groups dosed for one and two years. No NOAEL could be determined in the 2-year study. The LOAEL was 50 mg/kg/day for male rats and 25 mg/kg/day for female rats, i.e. the lowest doses administered.

In mice, tremors were observed intermittently at 400 and 600 mg/kg throughout the 17 week study. White blood cell and lymphocyte counts were decreased in males at 50 mg/kg bw or more and in females white blood cells were decreased at 600 mg/kg and lymphocytes were decreased at 400 mg/kg or more. In the chronic toxicity study weight gain reductions occurred in male and female mice at 100 mg/kg. Haematotoxic effects were limited to lymphocytopenia and associated leukocytopenia in all dose groups (males from 3 to 18 months, female mice from 12 to 18 months). Benzene increased the frequency of micronucleated normochromatic peripheral erythrocytes in male and female mice of all dose groups, males were more sensitive than females. Haematopoietic hyperplasia in the bone marrow and splenic haematopoiesis was observed in all dosed mice groups. The LOAEL was 25 mg/kg bw/day for male and female mice. A NOAEL was, therefore, not achieved.

Inhalation

In the rat, the key study is considered to be that of Ward et al, 1985. Animals were exposed to concentrations of 0, 1, 10, 30 or 300 ppm (0, 3.2, 9.6, 960 mg/m³) benzene vapour, 6 h/day, 5 days/week, for 13 weeks. Decreased blood lymphocyte counts, relative increase in neutrophil percentages and slightly decreased femoral marrow cellularity were the only significant treatment-related parameters noted in animals exposed to 300 ppm. The NOAEC for toxicity at 28 and 90 days was 30 ppm (96 mg/m³) for both male and female rats.

In mice haematotoxic effects following repeated inhalation exposure to benzene include: decreases in haematocrit, total haemoglobin, erythrocyte count, leukocyte count, platelet count, myeloid/erythroid ratios, and percentage of lymphocytes at 300 ppm (960 mg/m³) (Ward et al, 1985); depressed bone marrow and splenic Multipotential Haematopoietic Stem cells (CFU-S) and Granulocyte/Macrophage Progenitor cells (GM-CFU-C) at benzene concentrations ≥103 ppm (Green et al, 1981a, b); bone marrow erythroid progenitor cell numbers were depressed 1 day after exposure to concentrations ≥ 10 ppm (32 mg/m³) (Dempster and Snyder, 1990); significant depression in femoral lipopolysaccharide (LPS) -induced B-colony-forming ability and splenic phytohaemagglutinin (PHA) -induced blastogenesis at 31 ppm (Rozen et al, 1984); a reduction in bone marrow cellularity and the number of pluripotent stem cells in the bone marrow at ≥ 100 ppm for 10 exposures (Cronkite et al, 1985).

On the basis of these studies the LOAEC for haematotoxicity in mice is 10 ppm (32 mg/m³). A NOAEC could not be defined.

Dermal

No published data are available

 

Human data

In a review by Schnatter et al 2020, repeat dose studies in workers (seventy-seven genotoxicity and thirty-six haematotoxicity) were scored for study quality with an adapted tool based on that of Vlaanderen et al., 2008 (Environ Health. Perspect. 116 1700−5). These endpoints were selected as they are the most sensitive and relevant to the proposed mode of action (MOA) and protecting against these will protect against benzene carcinogenicity. Lowest and No- Adverse Effect Concentrations (LOAECs and NOAECs) were derived from the highest quality studies (i.e. those ranked in the top tertile or top half) and further assessed as being “more certain” or “less certain”. Several sensitivity analyses were conducted to assess whether alternative “high quality” constructs affected conclusions. The lowest haematotoxicity LOAECs showed effects near 2 ppm (8 h TWA), and no effects at 0.59 ppm. For genotoxicity, studies also showed effects near 2 ppm and showed no effects at about 0.69 ppm. Several sensitivity analyses supported these observations. These data define a benzene LOAEC of 2 ppm (8 h TWA) and a NOAEC of 0.5 ppm (8 h TWA). Allowing for possible subclinical effects in bone marrow not apparent in studies of peripheral blood endpoints, an OEL of 0.25 ppm (8 h TWA) is proposed.

References

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Collins JJ, Ireland BK, Easterday PA, Nair RS and Braun J (1997). Evaluation of lymphopenia among workers with low-level benzene exposure and the utility of routine data collection. J Occup Environ Med 39, 232-237.

Cronkite EP, Drew RT, Inoue T and Bullis JE (1985). Benzene hematotoxicity and leukemogenesis. Am. J. Ind. Med. 7, 447-456.

Dempster AM, Snyder CA. (1990). Short term benzene exposure provides a growth advantage for granulopoietic progenitor cells over erythroid progenitor cells. Arch Toxicol 64(7):539-544.

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Rozen MG, Snyder CA, Albert RE. (1984). Depression in B- and T-lymphocyte mitogen-induced blastogenesis in mice exposed to low concentrations of benzene. Toxicology Letters 20, 343-349.

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Justification for selection of repeated dose toxicity via oral route - systemic effects endpoint:
Sub-chronic and chronic studies indicate that benzene causes adverse effects on the haematopoietic system of rats and mice following repeated oral exposure.

Justification for selection of repeated dose toxicity inhalation - systemic effects endpoint: Human data show haematological changes with a NOAEC of 0.5 ppm (1.6 mg/m³).

Repeated dose toxicity: via oral route - systemic effects (target organ) cardiovascular /haematopoietic system: bone marrow  

   

Repeated dose toxicity: inhalation - systemic effects (target organ) cardiovascular /haematopoietic system: bone marrow

Justification for classification or non-classification

After repeated dose exposure via oral or inhalation routes, benzene causes adverse effects on the haematopoietic system of animals and in humans. Consequently, benzene is classified as STOT RE, Cat 1 (H372) according to Regulation (EC) No 1272/2008 of the European Parliament.