There is widespread concern over the health effects of oxidant air pollutants. The state of California and the Health Effects Institute (HEI) (a nonprofit research institute funded jointly by the U.S. Environmental Protection Agency [USEPA] and combustion engine manufacturers) nominated ozone for evaluation in long-term animal studies. The NTP study designs were a result of a series of meetings at the NIEHS with scientists from NIEHS, USEPA, and HEI, as well as experts from academic institutions working in the area of air pollutants. Male and female F344/N rats and B6C3F1 mice were exposed to ozone by inhalation for 4 weeks, 2 years, or for 124 weeks (rats) or 130 weeks (mice). The oxygen used to generate the ozone was greater than 99.9% pure. Additional groups of male F344/N rats were administered injections of 4-(N-methyl-Nnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) (~99% pure) 3 times per week for 20 weeks and exposed to ozone by inhalation for 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium.
Four-week ozone study in rats
Groups of five male and five female F344/N rats were exposed to 0, 0.5, or 1.0 ppm ozone by inhalation 6 hours per day, 5 days per week, for a total of 20 days. All rats survived to the end of the study. The final mean body weights and mean body weight gains of 0.5 ppm males and females and of 1.0 ppm females were similar to those of the controls. The final mean body weight of 1.0 ppm males was 7% lower than that of the controls. Clinical findings included hypoactivity in 1.0 ppm males and females and ruffled fur in exposed groups of males.
Male and female rats exposed to 0.5 or 1.0 ppm developed multifocal lesions of the lung, which consisted of infiltration of granulocytes and macrophages with extension of the bronchial epithelium into the alveolar ducts. Female rats exposed to ozone developed minimal squamous metaplasia of the laryngeal epithelium at the base of the epiglottis.
Absolute and relative lung weights of all exposed groups of males and females were greater than those of the controls, and absolute and relative thymus weights of all exposed groups were generally lower than those of the controls.
Four-week ozone study in mice
Groups of five male and five female B6C3F1 mice were exposed to 0, 0.5, or 1.0 ppm ozone by inhalation 6 hours per day, 5 days per week, for a total of 20 days. All mice survived to the end of the study. The final mean body weights and body weight gains of all exposed groups of mice were less than those of the controls. Hypoactivity was observed in 1.0 ppm mice.
Male and female mice exposed to 0.5 or 1.0 ppm ozone developed patchy, multifocal lesions of the lung, which consisted of infiltration of granulocytes and macrophages with extension of the bronchial epithelium into the alveolar ducts.
The relative lung weight of 1.0 ppm males was significantly greater than that of the controls. There were no other statistically significant differences in absolute or relative organ weights in males or females.
Two-year ozone study in rats
The 2-year study was designed to include the present USEPA standard (0.12 ppm), the maximum concentration believed compatible with long-term survival (1.0 ppm), and an intermediate concentration (0.5 ppm). Groups of 50 male and 50 female F344/N rats were exposed to 0, 0.12, 0.5, or 1.0 ppm ozone by inhalation for 6 hours per day, 5 days per week, for 105 weeks.
Survival, body weights, and clinical findings
Survival of exposed groups of rats was similar to that of the controls at the end of the study. The mean body weights of 0.12 and 0.5 ppm males and females were similar to those of the controls throughout the study. The mean body weights of 1.0 ppm males and females were slightly lower than those of the controls throughout the study. Hypoactivity was observed in male and female rats exposed to ozone.
Pathology findings
Increased incidences of ozone-induced metaplasia occurred in the nose and lung of rats exposed to 0.5 or 1.0 ppm ozone. The lesions in the nose were characterized by an increase in the number of goblet cells in the respiratory epithelium with mild squamous metaplasia of the cuboidal epithelium on the lateral wall. The increase in the number of goblet cells was found primarily in level I and II epithelium occurring along the lateral wall and on the maxilloturbinates and nasoturbinates. The metaplasia in the lung was a patchy multifocal lesion consisting of extension of the bronchial epithelium into the alveoli of the centriacinar region. This may represent more an extension of the bronchial epithelium into the pulmonary parenchyma than an actual transition of one epithelial cell type into another. There were increased incidences of squamous metaplasia at the base of the epiglottis characterized by one or more layers of flattened epithelial cells where low cuboidal cells are normally found.
There were no increases in the incidences of alveolar/bronchiolar adenoma or carcinoma in either males or females exposed to ozone.
Lifetime ozone study in rats
For this study, rats were exposed to 0.5 and 1.0 ppm ozone for an additional 6 months to determine the effect of extended exposure on neoplasm incidence. Groups of 50 male and 50 female F344/N rats were exposed to 0, 0.5, or 1.0 ppm ozone by inhalation for 6 hours per day, 5 days per week, for 125 weeks.
Survival, body weights, and clinical findings
Survival rates of exposed rats were similar to those of the controls. The mean body weights of 0.5 ppm males and females were similar to those of the controls throughout the study. The mean body weights of 1.0 ppm males and females were slightly lower than those of the controls for the first two years of the study. Hypoactivity was observed in exposed groups of males and females.
Pathology findings
Increased incidences of metaplasia occurred in the nose, larynx, and lung of rats exposed to 0.5 or 1.0 ppm ozone. The lung lesions were multifocal, centriacinar and were characterized by the presence of cuboidal epithelium (ciliated and nonciliated) along the alveolar ducts where type I epithelium is normally present. Inflammation (histiocytic infiltration) and interstitial fibrosis were observed in the lung of exposed males and females, and hyperplasia was observed in the nose of exposed male and female groups. There were no ozone-related increased incidences of neoplasms.
Two-year ozone/NNK study in male rats
An intermediate concentration of 0.5 ppm ozone was combined with exposure to two levels of a known carcinogen (0.1 and 1.0 mg NNK/kg body weight) in order to determine if ozone promotes the carcinogenic process or acts as a cocarcinogen. Groups of 48 male F344/N rats were exposed to 0 or 0.5 ppm ozone by inhalation, 6 hours per day, 5 days per week for 105 weeks. During the first 20 weeks of the study, these rats were subcutaneously injected with 0, 0.1, or 1.0 mg NNK per kg body weight in trioctanoin three times weekly.
Survival and body weights
Two-year survival rates of male rats were similar in all groups. Final mean body weights of all males exposed to NNK alone or NNK and ozone were similar to that of the controls, with the exception of rats exposed to 1.0 mg NNK/kg body weight and 0.5 ppm ozone. Hypoactivity was observed in males exposed to NNK and ozone, in those exposed to NNK without ozone, and in those exposed to ozone only.
Pathology findings
Alveolar epithelial metaplasia and interstitial fibrosis occurred in all groups of rats exposed to ozone or to NNK and ozone, but not in those exposed to NNK without ozone. Increased incidences of hyperplasia occurred in groups of rats exposed to NNK or to ozone and NNK. Incidences of hyperplasia were similar among groups of rats exposed to NNK only. An increased incidence of alveolar/bronchiolar adenoma or carcinoma (combined) occurred in rats administered 1.0 mg/kg NNK, with or without ozone. The administration of ozone did not affect the occurrence of pulmonary neoplasms or nonneoplastic lesions in rats administered NNK.
Two-year ozone study in mice
The 2-year study was designed to include the present USEPA standard (0.12 ppm), the maximum concentration believed compatible with long-term survival (1.0 ppm), and an intermediate concentration (0.5 ppm). Groups of 50 male and 50 female B6C3F1 mice were exposed to 0, 0.12, 0.5, or 1.0 ppm ozone by inhalation for 6 hours per day, 5 days per week, for 105 weeks.
Survival, body weights, and clinical findings
Survival rates of exposed mice were generally similar to those of the controls; the 2-year survival rate of 1.0 ppm females was greater than that of the controls. The mean body weights of 0.12 and 0.5 ppm males were similar to that of the controls throughout the study; the mean body weights of 1.0 ppm males and of all exposed groups of females were generally lower than those of the controls throughout the study. Hypoactivity was observed in male and female mice exposed to ozone.
Pathology findings
Increased incidences of metaplasia occurred in the nose and lung of mice exposed to 0.5 or 1.0 ppm ozone. The metaplasia in the nose consisted of increased thickening and extension of the squamous epithelium in the anterior portion of the nasal passage. The metaplasia in the lung consisted of extension of the bronchial epithelium into the alveoli of the centriacinar region. There were increased incidences of hyperplasia in the nose characterized by thickening of the noncuboidal (transitional) epithelium. There were increased incidences of hyperplasia in the epiglottis of female mice, a change that was characterized by a minimal increase in the thickness of the epithelium.
Incidences of alveolar/bronchiolar adenoma or carcinoma (combined) were marginally increased in 0.5 and 1.0 ppm males (0 ppm, 14/50; 0.12 ppm, 13/50; 0.5 ppm, 18/50; 1.0 ppm, 19/50) and were increased in 1.0 ppm females (6/50, 7/50, 9/49, 16/50).
Lifetime ozone study in mice
For this study, mice were exposed to 0.5 and 1.0 ppm ozone for 30 months to determine the effect of extended exposure on neoplasm incidence. Groups of 50 male and 50 female B6C3F1 mice were exposed to 0, 0.5, or 1.0 ppm ozone by inhalation for 6 hours per day, 5 days per week, for 130 weeks.
Survival and body weights
Survival rates of exposed mice were similar to those of the controls. The mean body weights of 0.5 ppm males and females were similar to those of the controls throughout the study. The mean body weights of 1.0 ppm males and females were generally lower than those of the controls throughout the study. Hypoactivity was observed in male and female mice exposed to ozone.
Pathology findings
The incidences of alveolar/bronchiolar adenoma and carcinoma (combined) were marginally increased in exposed males (0 ppm, 16/49; 0.5 ppm, 22/49; 1.0 ppm, 21/50) and in exposed females (6/50, 8/49, 12/50).
Increased incidences of metaplasia occurred in the nose, larynx, and lung of exposed groups of males and females, and the incidences of hyperplasia were increased in the larynx and nose of exposed mice. The morphology of the lesions was similar to that seen in the 2-year study. There were no ozone-related increases in alveolar epithelial hyperplasia.
Genetic toxicology
Ozone was mutagenic in Salmonella typhimurium strain TA102, with and without S9 metabolic activation.
Conclusions
Under the conditions of these 2-year and lifetime inhalation studies, there was no evidence of carcinogenic activity of ozone in male or female F344/N rats exposed to 0.12, 0.5, or 1.0 ppm. There was equivocal evidence of carcinogenic activity of ozone in male B6C3F1 mice based on increased incidences of alveolar/bronchiolar adenoma or carcinoma. There was some evidence of carcinogenic activity of ozone in female B6C3F1 mice based on increased incidences of alveolar/bronchiolar adenoma or carcinoma.
There was no evidence that exposure to 0.5 ppm ozone enhanced the incidence of NNK-induced pulmonary neoplasms in male rats.
Exposure of male and female rats to ozone for 2 years or 125 weeks was associated with goblet cell hyperplasia and squamous metaplasia in the nose, squamous metaplasia in the larynx, and metaplasia (extension of bronchial epithelium into the centriacinar alveolar ducts) and interstitial fibrosis in the lung. Exposure of male and female mice to ozone for 2 years or 130 weeks was associated with hyperplasia and squamous metaplasia in the nose and inflammation (histiocytic infiltration) and metaplasia (extension of bronchial epithelium into the centriacinar alveolar ducts) of the lung.