Respiratory infections in coalminers exposed to nitrogen oxides. Final report on Health Effects Institute research contract no.84-11

Coalminers working underground may be exposed chronically to low levels of nitric oxide (NO) and nitrogen dioxide (NO2) from diesel engine emissions and from the use of explosives for blasting. The aims of this study were to establish whether long-term exposures to low concentrations of these gases at nine British coalmines had been associated with increased susceptibility to respiratory infections, and if so, to estimate relative risks for different levels of exposure. The nine mines concerned had been involved since 1954 in a prospective epidemiological study of coalminers’ health.Median levels of nitrogen oxides in 4,933 pairs of full-shift samples taken at the mines during the years 1976-82 were 0.2 ppm NO and 0.03 ppm NO2; 10% of the concentrations exceeded 1.1 ppm and 0.08 ppm respectively. Multiple regression estimates of concentrations associated with different underground locations, types of work and mining conditions at each mine, and detailed records of miners’ attendance at work at similar locations, were used to estimate individuals’ underground exposures to nitrogen oxides for up to 16 years, back to the time when mechanical mining methods were established at the collieries, in the 1960s. Also available for study were records of the men’s exposures to respirable mine dusts and information from five-yearly medical surveys about their smoking habits, respiratory symptoms, and questionnaire-elicited reports of sickness absences attributed inter alia to respiratory infections.The reliability of the latter reports was examined in a sample of 471 of the men by comparing the answers to the questionnaire with physicians’ diagnoses on certified sickness absence records. Miners’ references to bronchitis, influenza or colds as the cause of prolonged sickness absence during three years preceding the surveys did, in general, reflect real spells of absence from work, lasting at least seven days, that had been diagnosed by doctors as due to respiratory nfections. But only about 20% of the men whose colliery records indicated that there had been such an absence acknowledged them at survey as due to a “”chest illness””. Most of the under-reporting was of absences certified as due to influenza or colds or “”upper respiratory tract infection””, and this was not related to the men’s ages or smoking habits.The main analyses referred to 5,408 reports of colds, influenza or bronchitis at a total of 40,071 interviews involving nearly 20,000 miners. Changes in 11,000 of these miners’ responses between consecutive medical surveys were studied in relation to exposures that occurred in the corresponding approximately five-year intervals. Responses from 7,463 of the men were examined in relation to estimates of exposures to nitrogen oxides cumulated over periods averaging seven years, and extending to 15 years or more for 750 of them. Reports of infections were related positively to age and cigarette smoking, to earlier reports of similar illnesses, to evidence of chronic bronchitic symptoms, low lung function, pneumoconiosis, and relatively long periods of underground, as opposed to surface work. The latter contrast was particularly clear for work involving shotfiring and, to a lesser extent, for diesel driving. However, there were no consistent associations between the chosen indicators of respiratory infections and estimates of occupational exposures to nitric oxide or nitrogen dioxide in any of the sub-sets of data studied, including those from non-smokers and from men whose responses indicated enhanced susceptibility to respiratory infections.We conclude that long-term exposures to the levels of nitrogen oxides found in British coalmines during the 1970s do not detectably increase the chances that miners will absent themselves from work because of chest infections. The range of concentrations of nitrogen oxides to which the miners had been exposed occupationally were similar to those recorded in some cities polluted by motor vehicle exhaust emissions. The results suggest that similar concentrations of these gases in urban environments are unlikely to cause complaints in manual workers that would be diagnosed by family doctors as respiratory infections. “”