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The substance was found to cause methaemoglobin formation of > 40%  at doses of 100 and 250 mg/kg bw in cats. Therefore, the LD50 values for rats underestimate the hazard for human health and the substance is classified as harmful. 
Oral: LD50 > 3200 mg/kg bw (BASF: XIX/429, 1970), rat. Mortality occurred at 3200 mg/kg bw, but not at 1600 mg/kg bw.
Inhalation: LC0 0.22 mg/l No mortalities occurred. (BASF: XIX/429, 1970), inhalation hazard test, rat
Dermal: LD50 > 7940 mg/kg bw (ECB-IUCLID, 2000), rabbit; LD 50 > 5000 mg/kg bw (International Programme on Chemical Safety (IPCS), rat
Methaemoglobinaemia in the cat: In animals treated once with 250 and 100 mg/kg bw, respectively, per gavage, formation of methaemoglobin was observed (BASF XIX/429, 1970).

Key value for chemical safety assessment

Additional information

Acute oral toxicity

In the BASF study (XIX/429, 1970) the acute oral toxicity of the test substance was determined after a single oral administration to rats. The study was conducted similar to the OECD-Guideline 401. The test substance was mixed with aqueous traganth suspension and administered once orally via gavage to 10 rats of each sex at doses of 3200, 1600, and 200 mg/kg body weight. The physical conditions and deaths were monitored throughout the whole observation period of 14 days. One male and two female rats of the high dose group died 24 hours after test substance administration. Clinical signs in this dose group were slight atony, abdominal position, stagger and tremor 90 min after application. These symptoms were also seen on day 1 after treatment. On the following 2 days the surviving animals showed still slight atony. Thereafter no clinical signs were observed. In the mid and low dose groups no symptoms were seen. Sacrificed animals showed no abnormalities, dead animals showed stomachs filled with substance. The oral LD50 was determined to be > 3200 mg/kg body weight.

 

The results of the acute oral toxicity are supported by the findings of the study performed by Vernot et al. (1977), conducted according to the method of Smythe et al. (1962) after single administration of the test substance to rats. The test substance was suspended in corn oil and administered once orally via gavage to male Sprague Dawley rats (body weight 200-300 g). No further details of the study were reported in the publication. The oral LD50 was determined to be 2830 mg/kg body weight.

 

Further data on acute oral toxicity is available only as secondary source (ECB-IUCLID, 2000; and Commission of the European Communities DG XI, 1993):

 

The acute oral toxicity of the test substance was tested in rats (Commission of the European Communities DG XI, 1993) and a LD50 was determined to be 5376 mg/kg bw for males and 3963 mg/kg bw for females, respectively. No further details were given.

 

The acute oral toxicity of the test substance was tested in Albino Sprague Dawley rats of both sexes (5 each dose group) by gavage of 20 % test material in corn oil at doses of 2510, 3160, 3980, and 5010 mg/kg bw (ECB-IUCLID, 2000). During the subsequent observation period the animals were observed for clinical signs and deaths. Mortality occurred but was not further specified. Further findings were reduced appetite and activity (lasting 1-2 days in survivors), increasing weakness, and collaps. Autopsy of decedents revealed haemorrhgic areas of the lungs, slight liver discolouration, and acute gastrointestinal inflammation. Viscera of surviving animals appeared normal. The LD50 was found to be 3100 mg/kg bw. No further details were given.

 

The acute oral toxicity of the test substance was tested in rats (ECB-IUCLID, 2000) and a LD50 was determined to be 3200 mg/kg bw. No further details were given.

 

The acute oral toxicity of the test substance was tested in rats (ECB-IUCLID, 2000) and a LD50 was determined to be 336 mg/kg bw. No further details were given.

 

Acute inhalation toxicity

In the BASF study (XIX/429, 1970) the acute inhalation toxicity of the test substance was determined in an inhalation hazard test (IHT) in the rat, principally performed as described in the OECD Guideline 403 and demonstrated the toxicity of an atmosphere saturated with vapors of the volatile components of test substance at the temperature chosen for vapor generation (20°C). Dust of the test substance was brought into the chamber via air and 12 rats of each sex were exposed to 0.22 mg/l of the test material in a whole body inhalation system for 8 hours. No animal died and no abnormal clinical signs were detected during the subsequent 7-day observation period. The autopsy did not reveal any compound related gross organ changes. The inhalative LC0 was determined to be 0.22 mg/l.

 

Acute dermal toxicity

Data on acute dermal toxicity of the test substance is available only as secondary source (ECB-IUCLID, 2000; Commission of the European Communities DG XI, 1993; International Programme on Chemical Safety (IPCS)):

 

In one study, the test material was semiocclusively applied to New Zealand White rabbits for 24 hours (ECB-IUCLID, 2000). The test material was prepared as 40 % mixture in corn oil and applied to intact, clipped skin at doses of 5010 mg/kg bw (1 rabbit), and 7940 mg/kg bw (2 rabbits), respectively, for 24 hours. Following exposure, the animals were observed for 14 days. Reduced appetite and activity was observed for 2-3 days. No mortalities occured. Viscera of the animals apeared normal at sacrifice. The dermal LD50 was determined to be > 7940 mg/kg body weight. No further details were given.

 

The second study investigated the acute toxicity in rats treated dermally with the test substance (Commission of the European Communities DG XI, 1993). The LD 50 was set to be > 2000 mg/kg bw. No further details were given.

 

In the third study a limit test was conducted to assay the acute dermal toxicity of the test substance in rats (International Programme on Chemical Safety (IPCS)). The LD 50 was set to be > 5000 mg/kg bw. No further details were given.

 

Methaemoglobinaemia

An acute oral toxicity study was even performed with cats (BASF XIX/429, 1970): 4 male and 2 female cats were administered a single dose of the test material as 2 and 10 % aqueous suspension with traganth in doses of 50, 100, 250 mg/kg bw via gavage. During the postexposure period of 7 days the animals were examined for toxic symptoms, and haemoglobin in blood was spectrophotometrically determined by the method of Evelyn and Malloy. Necropsy was conducted of dead cats. At 250 mg/kg bw cyanosis, lateral position, atony, convulsions, increased respiration, and yellow staining of the iris and ear tips was observed. One cat died after 2 days. At 100 mg/kg bw cyanosis was observed. One animal showed increased respiration at 50 mg/kg bw. The methaemoglobin formation after exposure was up to 57% (4 h after gavage) at 250 mg/kg bw, up to 44% (2 h after gavage) at 100 mg/kg bw, and up to 6% (2 h after gavage) at 50 mg/kg bw. The methaemoglobin amounts were measured every two hours. After 24 hours all methaemoglobin levels normalised again. Necropsy of the dead animal revealed an urinary bladder tightly filled with yellow-green-brownish urine (colour of substance). Necropsy of the other animals was not done. In conclusion, this substance induces methaemoglobin formation.

In comparison with other methaemoglobin causing substances such as aniline, the potency of o-nitrophenol is not as strong.

Justification for classification or non-classification

In contrast to other methaemoglobin inducing substances, o-nitrophenol contains a phenolic hydroxy group that rapidly undergoes conjugation with sulfonate or glucuronic acid. Sulfotransferases and UDP-glucuronosyltransferases are expressed in epithelial tissues lining lung and intestine and in skin. Detoxification via phase-II xenobiotic metabolism is consistent with the high intraperitoneal toxicity in mice and the low oral and dermal toxicity in rats and rabbits. Also in the study with cats, methaemoglobin levels returned to normal within 24h after dosing. Based on the doses inducing methaemoglobin formation in cats, o-nitrophenol has a moderate potency to induce methaemoglobin.

Based on the mortality in rats observed at 3200 mg/kg bw and the methaemoglobin content of > 40% in cats at > 100 mg/bw, the substance is considered as harmul. All routes of exposure are relevant as the substance is expected to penetrate the skin and be inhaled.

Dangerous Substance Directive (67/548/EEC)

The available studies are considered reliable and suitable for classification purposes under 67/548/EEC. As a result the substance is considered to be classified for acute oral, dermal and inhalation toxicity (Xn; R20/21/22) under Directive 67/548/EEC, as amended for the 28th time in Directive 2001/59/EC.

Classification, Labelling, and Packaging Regulation (EC) No. 1272/2008

The available experimental test data are reliable and suitable for classification purposes under Regulation 1272/2008. As a result the substance is considered to be classified for acute oral, dermal and inhalation toxicity (Category 4) under Regulation (EC) No. 1272/2008, as amended for the second time in Directive (EC 286/2011).

Since an acute toxicity class can be derived, no additional classification for acute specific target organ toxicity based on methaemoglobin formation is considered necessary.