Air Pollution Linked to Mixed, Small Health Effects

Researchers looking into the possible health effects of air pollution in the United Kingdom found "no clear evidence" that air pollution raises the risk for heart attack and stroke, according to an article published online June 4 in Heart. However, small-diameter particulate matter appears to increase the risk for death from arrhythmias, atrial fibrillation, and blood clot in the lungs.

Ai Milojevic, PhD, from the Department of Social and Environmental Health Research, London School of Hygiene & Tropical Medicine, United Kingdom, and colleagues analyzed data from 3 databases that covered hospital admissions for acute coronary syndrome or myocardial infarction (MI) to all 230 hospitals in England and Wales during 2003-2008, routine statistics on all admissions to National Health Service hospitals, and mortality data.

They compared those data with data on average exposures to ambient air pollution captured by monitoring stations closest to patients' geographic locations. Pollution types assessed included particulate matter (PM₁₀ and PM_2.5), carbon monoxide, nitrogen dioxide, sulphur dioxide, and ozone.

During the study period, the investigators identified 452,343 geographically coded MI events in the cardiovascular hospitalization database. Among those, nitrogen oxide exposure showed the strongest link to MI, with a 2.4% increased risk (95% confidence interval [CI], 0.3% - 4.5%) between the 10th and 90th percentiles of exposure. Sulfur dioxide showed the next strongest link, with a 1.7% increased risk (95% CI, −0.1% to 3.4%) between the 10th and 90th percentiles of exposure, but the difference was not statistically significant. The effects were stronger for non-ST-elevated MI compared with ST-elevated MI and were even greater for fatal non-ST-elevated MI. Patients older than 70 years carried a greater risk.

As for emergency hospital admissions, only nitrogen oxide was associated with greater risk for admission for any of the causes studied. Specifically, nitrogen oxide was associated with a 1.7% (95% CI, 0.9% - 2.6%) increased risk for cardiovascular disease, 2.9% (95% CI, 0.6% - 5.2%) increased risk for arrhythmias, 2.8% (95% CI, 0.3% - 5.4%) increased risk for atrial fibrillation, and 4.4% (95% CI, 2.0% - 6.8%) increased risk for heart failure.

Particulate matter "showed little evidence of increased risk of admissions, and indeed, in many cases the risks were negative. Ozone was associated with reduced admissions" for cardiovascular disease, non-MI cardiovascular disease, and ischemic heart disease.

Small-Diameter Particulates May Up Mortality

For mortality outcomes, an increase in exposure from the 10th percentile to the 90th percentile of small-diameter particulate matter was associated with a 21% (95% CI, 3.9% - 40.8%) increased risk for arrhythmias, 21% (95% CI, 3.9% - 41%) increased risk for atrial fibrillation, and 20.5% (95% CI, 3.5% - 39.7%) increased risk for pulmonary embolism.

The researchers use the day of each health event as cases, and other days of the month as controls.

"This study found no clear evidence for pollution effects on [ST-elevated MIs] and stroke, which ultimately represent thrombogenic processes, though it did for pulmonary embolism. The strongest associations with air pollution were observed with selected non-MI outcomes," the researchers conclude.

Reductions in Air Pollution May Mask Health Effects

In an accompanying editorial, Anoop S.V. Shah, MbChB, and David E. Newby, BSBM, cardiologists from the Royal Infirmary of Edinburgh, United Kingdom, note that published research puts the global burden of ambient pollution as responsible for 3.2 million deaths globally, primarily through cardiovascular events.

However, considerable variations exist in the literature as to the overall effect of ambient air pollution, they add, "due to various factors including heterogeneity in the underlying population studied, accurate measurement of the pollutants, analytical variation in terms of controlling for confounding and choice of lag (time interval between exposure and outcome) used for each pollutant."

The current findings regarding nitrogen oxide and particulate matter are in contrast with previous studies, but this discrepancy may be because of relatively low numbers of particulate matter monitors in the United Kingdom.

"Indeed, the authors' own previous data suggested an association between incident myocardial infarction and ambient pollutant concentrations in the hours prior to the event," they add.

"[T]he current study demonstrates the challenges of trying to integrate diverse and complex datasets with remote pollution monitoring across national datasets using daily air pollutant averages," they write.

They conclude, "The current lack of consistent associations with contemporary UK data may suggest that as the fog begins to clear, the adverse health effects of air pollution are starting to have less of an impact and are more difficult to delineate."