Dinitrogen oxide

Endpoint summary

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 air

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.