Low Air Pollution Levels Can Still Be Lethal

LONDON, July 31 -- Air polluted with even declining levels of black smoke and sulphur dioxide can still increases the risk of premature death, according to British health data.

Although air pollutants fell here over 16 years, a risk of premature death remained during the decline, especially for respiratory mortality, Paul Elliott, M.D., Ph.D., of Imperial College London, and colleagues, reported online in Thorax.

The findings came from data monitoring air quality in different electoral wards in Great Britain, from 1966 to 1998. During this period, mean concentrations of black smoke fell from 74.9 ?/m³ in 1966 to 1970 to 13.3 ?/m³ in 1990 to 1994. Sulphur dioxide concentrations declined from 41.4 parts per billion to 12.2 ppb.

Despite this decline, black smoke and sulphur dioxide concentrations were linked to subsequent all-cause and cause-specific mortality using models adjusted for social deprivation and urban/rural classification.

In a study that included 5,301 adults, ages 30 and older, deaths were studied for four successive four-year "mortality periods" from 1982 to 1986 through 1994 to 1998.

The effects were stronger for respiratory illness than for other causes of mortality (cardiovascular deaths, for example) for the most recent exposure periods (shorter latency times) and for the most recent mortality period even though pollutant concentrations were lower, the researcher reported.

In a pooled analysis across the four mortality periods, the adjusted excess relative risk for respiratory mortality was 3.6% per 10 mg/m³ black smoke (95% CI 2.6% to 4.5%) and 13.2% per 10 ppb sulphur dioxide (CI 11.5% to 14.9%), the researchers found.

The risks were greatest for respiratory mortality in the most recent four-year period. From 1994 to 1998, the comparable figures were 19.3% (CI 5.1% to 35.7%) and 21.7% (CI 2.9% to 38.5%), respectively.

The findings held even after adjusting for social deprivation. From 1966 to 1986, concentrations of the pollutants were significantly higher in the more deprived wards, after which there were no significant differences, the researchers found.

There were also no differences in concentrations between urban and rural areas, except for sulphur dioxide in the earlier exposure periods when concentrations in urban areas were slightly higher than in rural areas.

Unadjusted excess risks were positive and significant for both pollutants (P<0.05), but declined with increasing duration of exposure beyond four years, the researchers said.

For both pollutants adjustment reduced estimated risks, especially for lung cancer, for which associations were no longer significant at 0 to 4 years.

Other than for lung cancer, the effects on mortality were greater for the latest exposure period than for longer latencies and for up to four year periods than for the 16-year latencies. For lung cancer, the absences of excess risk for the short exposure period may reflect the long latency period of this disease, the researchers said.

These results are broadly consistent with those from other recent studies that used individual or semi-individual designs, the researchers wrote.

As an example, for sulphur dioxide, the largest to date is the American Cancer Society study across 151 U.S. cities. Re-analysis of the data gave significant excess risks for all-cause mortality of 5% per 10 ppb, 6% for cardiorespiratory mortality and 5% for all other causes compared with 4.2%, 5.2% and 3.5% in the British study (all mortality periods, four-year exposure window).

Study limitations included the lack of access to individual data for level of exposure. The researchers also lacked individual data on potential confounders, including risk factors such as smoking and diet.

Also, because monitoring tended to focus on areas with suspected air pollution, the study wards may have been more polluted than average. Furthermore, the study populations were not fixed over time.

These findings confirm the enduring legacy of air pollution on health, the authors said, and point to "continuing public health risks even at the relatively lower levels of black smoke and sulphur dioxide that now occur."

The results, they said, have importance for public health policies through regulation and control of air pollution.