Registration Dossier

Administrative data

Description of key information

Key value for chemical safety assessment

Acute toxicity: via oral route

Endpoint conclusion
Endpoint conclusion:
no study available

Acute toxicity: via inhalation route

Link to relevant study records
Reference
Endpoint:
acute toxicity: inhalation
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Qualifier:
according to guideline
Guideline:
other: OECD Guideline 413 (Subchronic Inhalation Toxicity: 90-Day)
Deviations:
yes
Remarks:
yes limited pathology and blood work undertaken
GLP compliance:
no
Test type:
other: 90-d
Limit test:
no
Species:
mouse
Strain:
Swiss Webster
Sex:
male/female
Route of administration:
inhalation: gas
Type of inhalation exposure:
whole body
Vehicle:
air
Duration of exposure:
ca. 4 h
Concentrations:
0, 5000, 50000 or 500000 ppm
No. of animals per sex per dose:
15/sex/gp
Control animals:
yes
Sex:
male/female
Dose descriptor:
LC50
Effect level:
> 500 000 ppm
Exp. duration:
4 h

OBSERVATIONS:

Clinical signs of toxicity:

All animals survived to the scheduled necropsy.

 

Bodyweight and bodyweight gain:

Treatment related decreases in body weight were observed in high dose group animals, with a depression of 77 and 63% in body weight gain in males and females respectively. This depression in weight gain was statistically significant (p<0.025 and p<0.01, respectively).

 

Haematology & clinical chemistry:

No treatment related changes were observed in any of the parameter measured.

 

Urinalysis:

None undertaken

 

Sacrifice and Gross Pathology:

No treatment related changes were observed in any of the parameter measured.

 

Organ weights: 

No discussion of organ weight data.

Conclusions:
Swiss Webster mice (15/sex/gp) were exposed to N2Oviawhole body inhalation at concentrations of 0, 5000, 50000 or 500000 ppm [0, 0.5, 5, 50%] for 4/h/d, 5d/wk over 14 wks. At necropsy limited histopathology and haematology / biochemistry parameters were measured. 
 
All animals survived to the scheduled necropsy. The study failed to demonstrate exposure related haematopoietic changes. There was no change in the white blood cell count nor was granulocytopenia or thrombocytopenia observed. The lack of effect suggests that either the strain of mouse was insensitive to N2O, or more likely that continuous exposure is necessary to induce leucocytopenia, as previous demonstrated following continuous exposure to N2O at high concentrations (20-80%). Furthermore, no treatment related changes in organ weights, biochemical or histopathological parameters were observed. 
 
Treatment related decreases in body weight were observed in high dose group animals, with a depression of 77 and 63% in body weight gain in males and females respectively. This depression in weight gain was statistically significant (p<0.025 and p<0.01, respectively).
 
The objective of the study was to demonstrate the maximum tolerated concentration of N2O, which was deemed to be 50% (500000 ppm). In terms of establishing a NOAEL, based on the results of this study and the data presented, 50000 ppm (5%) was deemed to be the NOAEL, based on statistically significant reductions in body weight gains observed at the LOAEL (500000 ppm [50%]).
Executive summary:

Swiss Webster mice (15/sex/gp) were exposed to N2Oviawhole body inhalation at concentrations of 0, 5000, 50000 or 500000 ppm [0, 0.5, 5, 50%] for 4/h/d, 5d/wk over 14 wks. At necropsy limited histopathology and haematology / biochemistry parameters were measured. 

 

All animals survived to the scheduled necropsy. The study failed to demonstrate exposure related haematopoietic changes. There was no change in the white blood cell count nor was granulocytopenia or thrombocytopenia observed. The lack of effect suggests that either the strain of mouse was insensitive to N2O, or more likely that continuous exposure is necessary to induce leucocytopenia, as previous demonstrated following continuous exposure to N2O at high concentrations (20-80%). Furthermore, no treatment related changes in organ weights, biochemical or histopathological parameters were observed. 

 

Treatment related decreases in body weight were observed in high dose group animals, with a depression of 77 and 63% in body weight gain in males and females respectively. This depression in weight gain was statistically significant (p<0.025 and p<0.01, respectively).

 

The objective of the study was to demonstrate the maximum tolerated concentration of N2O, which was deemed to be 50% (500000 ppm). In terms of establishing a NOAEL, based on the results of this study and the data presented, 50000 ppm (5%) was deemed to be the NOAEL, based on statistically significant reductions in body weight gains observed at the LOAEL (500000 ppm [50%]).

Endpoint conclusion
Dose descriptor:
LC50
Value:
900 061 mg/m³

Acute toxicity: via dermal route

Endpoint conclusion
Endpoint conclusion:
no study available

Additional information

N2O is an approved medical product in accordance with applicable medical regulations. It has been used in clinical anaesthetic practice for more than 150 yr, and its longevity should be considered within the context of all the major advances in anaesthetic practice over that time. No acute toxicity effects to humans have been reported throughout its use.*

As N2O is a gas the oral and dermal route of exposure are not considered relevant.

 

From the available acute inhalatory toxicity data conducted with N2O one study is available to address the concern of acute toxicity. The Hoechst (1993) study would appear to address the points required by REACh for the acute inhalation toxicity endpoint, with this data referenced by all agencies that have summarised the available data on N2O. In this study the 4 h LC50of >250 ppm in rats (equivalent to >450 mg/m3) was reported. On closer examination, the source of N2O used in this study was contaminated with methyl nitrate. The result therefore from this study is considered unreliable and cannot be used to address this toxicity endpoint.

 

Looking at the repeat dose toxicity studies conducted on N2O, continuous exposure to rats in the initial days of treatment (Rice et al1983) at doses up to 500000 ppm for 4 h/day did not result in deaths, initially or 14 days post treatment the first exposure, with all animals surviving to the scheduled necropsy. These data can be used to address the acute toxicity endpoint. Whilst an actual LC50value is lacking the data repeat dose data demonstrate a lack of mortality or morbidity at high concentrations, which would lead to an LC50value of >500000 ppm. 

 

The mechanism of toxicological action of N2O isviadepression of the respiratory system. Lung effects during high concentrations of N2O result in oxygen in the lungs being rapidly used up, with the resultant anoxia increased respiratory effort causes rapid depletion of CO2in tissues. Absence of CO2and depression of the medullary centres by N2O quickly leads to respiratory failure, with cerebral function failing to recover from cerebral damage caused by prolonged anoxia. Therefore, the limited dose of 250 ppm in the 4 h acute inhalation, whilst meeting the requirements of REACh is inconsistent with the both the knowledge of how N2O exerts its toxicological action and also from the available sub-acute, sub-chronic and chronic toxicity testing where doses are in excess of 3-orders of magnitude greater than the LC50value reported in the Hoechst (1993) study.

 

The data from the Rice et alstudy (1983), whilst not meeting the testing requirements of REACh for acute inhalation toxicity are deemed sufficient to conclude that acute exposure to N2O is insufficient for classification.

 

[1]Conversion of ppm to mg/m3= ppm * MWT / 24.45. [MWT of N2O = 44.013]

 

[Conversion of ppm to mg/m3= ppm * MWT1/ 24.45]

 

1. MWT of N2O = 44.013

References

* Sanders, RD et al (2008) Biologic effects of Nitrous Oxide. Anesthesiology 109: 707 -722


Justification for selection of acute toxicity – oral endpoint
In accordance with ANNEX VII column 2 of the REACH regulation: this study does not need to be conducted as the substance is a gas.

Justification for selection of acute toxicity – inhalation endpoint
The Rice study, whilst deemed to be a sub-chronic study, a 4h exposure/day was used. No morbidity or mortality was observed initally, with the all animals suriving 14d post the initial exposure. This is therefore deemed an appropriate study to address this endpoint in the absence of specific data.

Justification for selection of acute toxicity – dermal endpoint
In accordance with ANNEX VII column 2 of the REACH regulation: this study does not need to be conducted as the substance is a gas.

Justification for classification or non-classification

Acute oral toxicity:waiver requested as N2O is a gas the oral route of exposure is not relevant.

 

Acute toxicity inhalation:insufficient for classification.

 

Acute toxicity dermal:waiver as N2O is a gas the dermal route of exposure is not relevant.

STOT SE H336: May cause drowsiness or dizziness based on the anaesthetic properties.