Carbon monoxide

Carcinogenicity

Key value for chemical safety assessment

Carcinogenicity: via inhalation route

Link to relevant study records

Reference

Endpoint:

carcinogenicity: inhalation

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Non-GLP

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 451 (Carcinogenicity Studies)

Deviations:

yes

Remarks:

single dose level

Principles of method if other than guideline:

Female rats exposed to carbon monoxide for 20hrs/day, 5 days/wk for 72 weeks.

GLP compliance:

no

Species:

rat

Strain:

Wistar

Sex:

female

Route of administration:

inhalation: gas

Type of inhalation exposure (if applicable):

whole body

Vehicle:

air

Duration of treatment / exposure:

72 wks (5d/wk)

Frequency of treatment:

10 h/d

Post exposure period:

none

Remarks:

Doses / Concentrations:
0, 200 ppm
Basis:

No. of animals per sex per dose:

control gp: 20
treatment gp: 56

Control animals:

yes, concurrent vehicle

Dose descriptor:

NOAEC

Effect level:

> 200 ppm

Remarks on result:

other: Effect type: carcinogenicity (migrated information)

Dose descriptor:

LOAEC

Effect level:

< 200 ppm

Sex:

female

Basis for effect level:

other: cardiovascular effects

Remarks on result:

other: Effect type: toxicity (migrated information)

Results: In total 3 animals exposed to CO (at weeks 20, 60 and 68 weeks) and 1 control animal (68 weeks) were withdrawn from the study due to illness.

There were no observed differences in weight gain between CO exposed and control animals, with group mean weights of 275±4g and 270±6g (p=0.544) obtained respectively. The only difference in organ weights was seen in cardiac weights (see below)  

Effects on the cardiovascular system

Organ	Control (n=23)	CO Exposed (n=43)
Right ventricle (mg)	112 ± 5.2	134 ± 3.2*
Left ventricle (mg)	561 ± 14.7	642 ± 12.4*
Right ventricle/bodyweight (mg/g)	0.42 ± 0.02	0.48 ± 0.01*
Left ventricle/bodyweight (mg/g)	2.09 ± 0.04	2.33 ± 0.03*

± SEM *p≤0.001 vs. Control group  

Chronic CO exposure for 72 weeks induced a 20% (p=0.001) increase in right ventricular weight and a 14% increase in left ventricular + interventricular septum (p=0.001) increase in right ventricular weight and a 14% increase in left ventricular + interventricular septum (p<0.001). This same trend was observed after only 6 months of exposure, where analysis of some CO exposed animals showed an increase in left ventricular + interventricular septum compared to the controls (556 ± 33mg (n=4) vs. 510±14mg (n=2) respectively.

No macroscopic changes in the heart were observed in either of the two groups. Furthermore, no histopathological changes like oedema, inflammation or signs of scarring were observed in the right or left ventricular wall.

No structural signs of hypertension were observed in pulmonary arteries. In addition, no differences in the proportion of small muscularised arteries to non-muscularised arteries were observed in CO exposed and control animals (47.9±2.3% vs. 47.6±1.9% [p=0.936] respectively).

Signs of atherosclerotic lesions in the femoral and three sections of the distal part of the thoracic aorta were absent in the CO exposed group. Of note of the 15 control animals examined showed plaque-like lesions in the femoral artery with thickening of the lamina interna. No abnormality was observed in the thoracic aorta in the control group.

Effects on the respiratory system

No morphological signs of smoking associated pathology (i.e. emphysema, inflammation, bronchial/peribronical thickening, fibrosis, pulmonary hypertension) were observed in CO exposed animals. No distinct morphological differences between CO exposed and control animal were observed. No ultrasound differences in alveolar epithelial cells or alveolar septas between the two groups were observed with the use of electron microscopy.

The thickness of the fused basal laminas of the alveolar epithelial ad endothelial cells of the blood-air barrier did not differ significantly between CO exposed and control animals (89.21±2.6nm vs. 85.22±2.32nm [p=0.252] respectively). Chronic CO exposure was not associated with any significant morphological changes in the pulmonary neuroendocrine system.

The number of single PNEC in the airway was slightly higher in CO exposed animals compared to control animals, but this did not achieve statistical significance (1.9±0.2 cells/cm²vs. 1.7±0.3 cells/cm²[p=0.579] respectively). Similarly, no statistically significant difference was found between the CO exposed group and control group regarding the size of NEBs located in the alveolobronchiolar (1.7±0.3 cells/cm²vs. 1.8±0.6 cells/cm²[p=0.837], respectively) and bronchial (1.9±0.1 cells/cm2vs. 2.1±0.3 cells/cm2[p=0.530), respectively). No difference between CO exposed and control animals was seen regarding the size of aNEBs (5.6±0.6 cells vs. 5.2±0.7 cells [p=0.696], respectively) or bNEBs (7.0±0.5 cells vs. 7.1±0.2 cells [p=0.902], respectively).

Tumourgenesis

	Control (n=25)	CO Exposed (n=49)
Pituitary gland -adenoma	4	6
Mammary gland - fibroadenoma	0	3
Ovary -Adenocarcinoma -Haemangioma	0 1	1 0
Uterus -leiomyoma	0	1
Salivary gland -Adenocarcinoma	1	0
Haematopoietic system - leukaemia	0	1
Liver -Metastasis from adenocarcinoma of the uterus	0	1
Lung -Adenocarcinoma -Tumourlets/NE hyperplasia	0 1	1 1
Total number of animals with tumours	7 (28%)	14 (29%)

 

Tumours of the anterior pituitary gland were the most frequent neoplasia, observed in 12 and 16% of CO exposed and control animals respectively. These were all classified as benign adenomas, with a neuroendocrine morphology without atypia and with a low rate of mitosis.

Mammary gland tumours were observed only in rats exposed to CO. However the level of animals exhibiting mammary tumours (6%) is consistent with published data which report that not only do nulliparous rats have a higher incidence of mammary tumours (Nagaokaet al) but rats (in particular females) frequently develop various tumours. In the case of females, the majority of these are in the mammary glands (Gross and Dreyfuss).

The proportion of total number of animals with tumours did not differ significantly between groups (28% vs. 29%,p=0.959). This data is consistent with published data where tumour incidences in Sprague Dawley and Long Evans female rats have been reported as 22% in both cases (Gross and Dreyfuss).

Only one of the CO exposed animals died spontaneously during the exposure period, suffering from leukaemia. In addition, two CO exposed rats were sacrificed after 15 and 17 months, respectively, and one control rat after 17 months due to signs of illness. Examination of the animals revealed a large mammary tumour, an adenocarcinoma of the ovary and a pituitary adenoma, respectively.

COHb levelsDuring the study, levels of COHb and Hb were measured. In CO exposed animals COHb levels ranged from 11 to 14.7%. COHB levels of control animals wereincreased in the CO exposed animals compared to control animals from week 12 until the end of the study.

 

Time (weeks)	COHb (%)				Hb (g/dL)
	Control		CO Exposed		Control		CO Exposed
	n		n		n		n
0	-	-	-	-	8	13.2±0.2	8	13.7±0.2
2	41	0.1	2	11.0±1.0	-	-	-	-
12	-	-	-	-	8	13.4±0.3	16	14.5±0.6*
24	2	0.1±0.1	4	12.6±0.7	2	11.8±0.2	4	15.0±0.9
72	22	0.3±0.1	43	14.7±0.3*	22	13.1±0.2	43	14.7±0.1*

Data presented as means± SEM *p<0.02 vs. Control Group

References: Gross, L. Yolande, Y. (1979). Spontaneous tumors in Sprague-Dawley and Long-Evans rats in F1 hybrids: Carcinogenic effect of total-body x-irradiation. Proc. Natl. Acad. Sci., vol. 76 (11), pp 5910 -5913

Nagaoka, T., Takegawa, T., Takeuchi, M., Maekawa, A. (2000). Effects of reproduction on spontaneous development of endometrial adenocarcinomas and mammary tumors in Donryu rats. Jpn. J. Cancer Res., vol. 91. pp 375 -382.

Conclusions:

No definitive NOAEC for toxicity could be identified. Therefore, the LOAEC is 200 ppm (based on evidence of cardiac hypertrophy). No exposure related carcinogenic effects were observed

Executive summary:

Carbon monoxide (CO) is a dangerous poison in high concentrations, but the long-term effects of low doses of CO, as in the gaseous component of tobacco smoke, are not well known. The aims of our study were to evaluate the long-term effects of inhaled CO on the respiratory and cardiovascular system at doses corresponding to tobacco smoking and its effect on tumourigenesis and pulmonary neuroendocrine (NE) cells. Female Wistar rats were exposed to either CO (200 ppm) for 20 h/day (n = 51) or air (n = 26) for 72 weeks. Carboxyhaemoglobin was 14.7 ± 0.3% in CO exposed animals and 0.3 ± 0.1% in controls. In the lungs, no signs of pathology similar to that associated with cigarette smoking were observed, and no differences in number of pulmonary NE cells were observed between the groups. Chronic CO inhalation induced a 20% weight increase of the right ventricle (p = 0.001) and a 14% weight increase of the left ventricle and interventricular septum (p < 0.001). Histological examination of the myocardium did not reveal any signs of scarring. In the aorta and femoral artery, no signs of atherosclerosis were observed in CO exposed rats. No exposure related carcinogenic effects were observed. Spontaneous tumours were identified in 29% of CO exposed animals and in 28% of the controls. Our results suggest that low dose CO exposure is probably not responsible for the respiratory pathology associated with tobacco smoking. The effects on the cardiovascular system seem to involve myocardial hypertrophy, but not atherogenesis.

Endpoint conclusion

Endpoint conclusion:

no adverse effect observed

Dose descriptor:

LOAEC

229 mg/m³

Study duration:

chronic

Species:

rat

Justification for classification or non-classification

There was no evidence of either genotoxicity or carcinogenicity in studies with carbon monoxide; therefore carbon monoxide has not been classified as carcinogenic (or genotoxic).

Additional information

Animal toxicology data for carbon monoxide are limited and current occupational exposure limits appear to have been established using human data. The NIOSH limit of 35 ppm is based on the risk of cardiovascular effects (NIOSH, 1992), the ACGIH limit of 25 ppm is based on the risk of elevated carboxyhaemoglobin levels (ACGIH, 1994) and the OSHA limit of 50 ppm on the lack of clinical signs of toxicity and elevated carboxyhaemoglobin levels (OSHA). The evaluation published in Environmental Health Criteria (EHC, 1999) also recommends exposure limits which are based on human carboxyhaemoglobin levels. A detailed evaluation of the rationale used by NIOSH and ACGIH to establish occupational exposure limits has not been made because the documents are not yet available, but it appears that animal data have only been used to identify hazards and have not been used for risk assessment.

Under these circumstances conducting any new toxicity studies in animals seems of limited value. With respect to REACH requirements there are data gaps for carcinogenicity studies. A chronic inhalation study in female rats has been found in the literature (Sørhauget al., 2006). Whilst there are deficiencies within the study design, this study should address the concern over carcinogenicity and therefore replace the need for a rat study. In this study rats were exposed to CO for 20 h/d for 72 wks. In the lungs, no signs of adverse pathology were observed. The NOAEL_toxicitywas not identified, with cardiovascular effects reported (20% weight increase in right ventricle (p≤0.001 and a 14% weight increase of the left ventricle and interventricular septum (p≤0.001). No exposure related carcinogenic effects were observed.

 

It is difficult to see how the use of this NOAEL would be used for the risk assessment. The NOAEL for carcinogenic effects reported in the Sørhauget alstudy was 200 ppm, if human risk assessment was based on this study a safety factor of 100 would be applied giving a DNEL of only 2 ppm. This is obviously much lower that the values quoted above and provides additional evidence that animal toxicity data have not been considered.

 

The carbon monoxide levels inside motor vehicles are generally around 9-25 ppm and street workers can be exposed to 10-50 ppm (EHC, 1999)¹. Lowering the occupational limit to < 1 ppm would clearly give an exposure which is much lower than that to which a significant proportion of the human is already exposed and that would be illogical. Ideally consideration should be given to assessing the carcinogenic evidence of carbon monoxide in humans from any available human epidemiology data and to determine if scientifically robust data on safe levels of exposure can be determined. These data could then form the basis for preparing a risk assessment based on human rather than animal data. However, no such epidemiology data is available which specifically assess the carcinogenic potential of CO.

 

Whilst it is important to recognise that only female rats were treated in this study, there are few experimental studies that have been reported in relevant laboratory animals (and are considered robust) that examined sub-chronic or chronic exposure to CO. Unfortunately, as male animals were not treated in this study, the effects on testes and epididymides following exposure to CO could not be further examined. However, despite the incompleteness of the reported data, it is important to stress the only adverse effect observed was limited to cardiac hypertrophy.

 

References:

ACGIH, 1994. Threshold limit values for chemical substances and physical agents and biological exposure indices for 1994-1995. Cincinnati, OH: American Conference of Governmental Industrial Hygienists.

 

EHC, 1999. Environmental Health Criteria, 213, International Programme for Chemical Safety, WHO.

 

NIOSH, 1992.Recommendations for occupational safety and health: Compendium of policy documents and statements. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication No. 92-100.

 

OSHA. Occupational safety and health guideline for carbon monoxide, United States Department of Labor, Occupational Safety and Health Administration.
 

Sørhaug, S., Steinshamn, S., Nilsen, O.G. & Waldum, H.L. (2006).Chronic inhalation of carbonmonoxide: Effects on the respiratory and cardiovascular system at doses corresponding to tobacco smoking.Toxicology.228 (2-3), pp 280-290

 

Footnotes:

1. These concentrationswere probably measured before reductions were made to vehicle emissions and more recent data should be sought if this type of comparison is to be used.

Carcinogenicity: via inhalation route (target organ): cardiovascular / hematological: heart