a protective shield when high up in the atmosphere, ozone near the earth's surface can be a fatal pollutant. Three independent research reviews -- commissioned by the us Environmental Protection Agency (epa) and published in the July issue of the journal Epidemiology (July, Vol 16, No 4) -- draw "remarkably consistent" conclusions linking daily levels of ozone pollution to an increased risk of death. Accompanying editorial commentaries concur, although pointing out the studies are limited by the nature of the data and the statistical methods used. Ozone forms near the earth's surface when the Sun's ultraviolet rays fall on oxides of nitrogen and volatile organic hydrocarbons emitted by cars, power plants and industrial sources.
All three papers analysed previous research data. By combining results from many studies, important patterns can emerge that are not apparent in the individual studies. All the studies had the same goal: to assess whether death rates increase on days with high levels of ozone pollution (and if so, how much).
Michelle L Bell and colleagues of Yale University 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. For each 10 parts per billion (ppb) increase in daily ozone level, the total death rate for that day and the following two increased by 0.87 per cent.
Another analysis by Jonathan I Levy and colleagues of Harvard School of Public Health found similar results: death rate increased by 0.86 per cent for every 10 ppb spurt in ozone levels. Most of the ozone-related increase occurred during the summer months. However, the link was weaker in areas where most homes were centrally air-conditioned.
The analytic approach taken by 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 worldwide confirmed a significant relation between ozone and mortality in all but five regions.
In an editorial commentary, Steven N Goodman of the us -based Johns Hopkins School of Medicine highlighted the fact that three independent studies reached consistent conclusions, despite using different methods and operating under differing assumptions. A commentary by David V Bates of University of British Columbia says the new studies were timely, because epa was planning a review of federal ozone standards in 2005. Added to previous research, the new studies "point to the urgent need to reduce public exposures to ambient ozone by all possible means," Bates concluded.
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