Air pollution conclusively linked
to ischemic heart disease

Traffic exhaust ramps up the dangers for everyone;
smokers are especially vulnerable

By Owen Dyer

It's no secret that exposure to air pollution shortens life and brings added risk of respiratory diseases like pneumonia or chronic obstructive pulmonary disease (COPD). But traditionally research has overlooked the possibility of other adverse health effects.

Two new studies suggest that the problem goes beyond simple respiratory disease and that pollution can have health effects as broad -- though not as deep -- as smoking cigarettes.

Research in the journal Circulation used data from the 1.2 million subjects enrolled in the American Cancer Society's Cancer Prevention Study II. Over 300,000 people were considered in the final analysis, which controlled for confounding factors such as smoking, diet, socioeconomic status and occupational exposure to pollution. The researchers set out to measure what health effects were seen when the cities where the subjects lived experienced an increase in pollution by fine particulate matter.

They found that a 10 micrograms/metre³ increase in particles smaller than 25 micrometres in diameter caused a definite rise in mortality from cardiovascular diseases, particularly ischemic heart disease. When pollution was elevated by this degree, non-smokers' daily risk of dying from a heart attack rose 18%. Unexpectedly, non-smokers' risk of dying from chronic obstructive pulmonary disease or from all respiratory diseases actually fell in times of elevated pollution, by 14% in the case of COPD.

The picture was considerably bleaker for former and current smokers. Current smokers ran nearly twice the average risk of dying from ischemic heart disease during times of high pollution, and no less than 10 times the normal risk of dying from COPD or allied conditions. This grim finding led the researchers to conclude that smoking and pollution may combine to create a nefarious health effect greater than the sum of its parts.

Lung Link?
Heart disease remains the leading killer, but few issues could be more controversial than the association between air pollution and lung cancer. Yet lung cancer rates have always been slightly higher in cities than in rural areas, and many pollutants are known carcinogens. A Norwegian study published in the journal Thorax sought to compare lung cancer rates among 16,209 men to rates of observed pollution near their homes between 1974 and 1998. The men were all aged 40-49 at baseline.

Two pollutants were measured, nitrogen oxide (NOx) and sulphur dioxide (SO2). Neither of these pollutants is likely to be carcinogenic alone, but nitrogen oxide is a good indicator of traffic-related pollution, while sulphur dioxide is a reliable marker for heating-related pollution.

Over the 14-year study period, 418 of the men developed lung cancer. That is an unusually high figure because an unusually high proportion of the subjects were current smokers (56%) or former smokers (25%).

The first and most obvious result of the study was further proof, if any were needed, of the strong association between smoking and lung cancer. Participants who smoked over 20 cigarettes a day were 35 times as likely to develop the disease as non-smokers. Another phenomenon observed in previous studies was the strong link between low education and lung cancer. When other factors were controlled, the disease was twice as likely to afflict those with 10 or fewer years of education than those with 12 years or more. As for pollution, there was a small but statistically robust increased risk of lung cancer associated with elevated nitrogen oxide levels. The added risk was about 8% for each extra 10 micrograms/metre³ of nitrogen oxide. This seems to hold true for both smokers and non-smokers, although lung cancer was so rare in the non-smoking cohort that the effect was harder to measure. Nitrogen oxide pollution in the early years of the study appeared to be most relevant to risk, suggesting the potential for a long-term delayed effect.