Three Studies Link Daily Ozone Levels to Mortality Rates

Newswise — Three independent research reviews —commissioned by the U.S. Environmental Protection Agency and published in the July issue of Epidemiology—draw "remarkably consistent" conclusions linking daily levels of ozone pollution to an increased risk of death.

Accompanying editorial commentaries point out that, although the studies are limited by the nature of the data and the statistical methods used, the results provide the strongest evidence to date relating ambient ozone levels to daily mortality.

All three papers were meta-analyses, or statistical reviews of previous research data. By combining results from many studies, important patterns can emerge that are not apparent in the individual studies. Although the researchers worked independently, using a broad range of studies and applying varying statistical methods,, all studies had the same goal: to assess whether death rates increase on days with high levels of ozone pollution -- and if so, how much.

Dr. Michelle L. Bell of Yale University and colleagues found a significant relationship between ozone and short-term mortality rates using two different datasets. The effect was particularly strong for cardiovascular and respiratory causes of death, in elderly people, and for same-day ozone levels.

For each 10 parts per billion (ppb) increase in daily ozone level, the total death rate for that day and the two following days increased by 0.87%. This relationship remained consistent after adjustment for other factors, including levels of particulate pollutants.

The analysis by Dr. Jonathan I. Levy and colleagues of Harvard School of Public Health found a very similar increase of 0.86% per 10 ppb. Most of the ozone-related increase occurred during the summer months. In this study, the relationship between ozone and mortality appeared significantly weaker in areas with high rates of home central air conditioning.

The analytic approach taken by Dr. Kazuhiko Ito and colleagues of New York University suggested a smaller overall effect of ozone on death rates, but confirmed that the main effect occurred during the warm months. Analysis of data from 23 geographic locations around the world confirmed a significant relation between ozone and mortality in all but 5 regions. The relationship was strongest in Brisbane, Australia, and Mexico City—cities where ozone levels remain fairly constant year-round.

Ambient ozone pollution is different from the higher-level "ozone layer" that shields the Earth from harmful ultraviolet radiation. At ground level, ozone—created when pollutants interact with sunlight—is a major component of smog. Previous studies have found increased rates of health problems on days with high levels of ozone pollution, especially in children and older adults with asthma and other respiratory diseases.

In an editorial commentary, Dr. Steven N. Goodman of Johns Hopkins School of Medicine highlights the fact that three independent research groups reached such consistent conclusions, despite using different methods and operating under differing assumptions. Dr. Goodman also points out some weaknesses of the statistical methods used, especially the use of pooled data from many single-site studies. A 14-year follow-up study of air quality from 95 U.S. cities suggests that ozone's effect on death rates may be smaller, though still significant. Very large, multi-site studies would be needed to provide the data needed for more definitive conclusions on the health risks of ozone.

A commentary by Dr. David V. Bates of University of British Columbia points out that the new studies are timely, because the EPA is planning a review of federal ozone standards in 2005. Added to previous research showing harmful health effects of ozone pollution, the new studies "point to the urgent need to reduce public exposures to ambient ozone by all possible means," Dr. Bates concludes.