Correspondence

Ozone Exposure and Mortality

N Engl J Med 2009; 360:2786-2789June 25, 2009

Article

To the Editor:

Jerrett et al. (March 12 issue)1 report a significant increase in death from respiratory causes in association with an increase in ozone concentration. Local and regional anthropogenic pollutants are the primary sources of air pollution associated with tropospheric ozone in industrialized cities. However, there is growing concern regarding the export and long-distance transport of pollution from industrialized centers to remote and pristine environments. Increasing levels of pollution in Southeast Asia have the potential for a dramatic impact on the health outcomes in populations who reside and work in remote regions such as the Himalayas.2,3 Surface measurements in the Mount Everest region (Figure 1Figure 1Ozone Concentrations According to Elevation in the Solukhumbu Valley.) have indicated the presence of ozone concentrations that exceed 140 ppb (during an 8-hour exposure); these concentrations are the result of both long-range transport of tropospheric pollutants from Southeast Asia and the descent of ozone-rich stratospheric air.4,5 With global levels of background ozone increasing, unique communities in high-altitude areas, such as those in Nepal, are unexpectedly exposed to ozone concentrations that are similar to, if not higher than, those reported in industrialized cities.

John L. Semple, M.D.
G.W.K. Moore, Ph.D.
University of Toronto, Toronto, ON M5S 1B2, Canada
john.semple@wchospital.ca

5 References

1. 1

   Jerrett M, Burnett RT, Pope CA III, et al. Long-term ozone exposure and mortality. N Engl J Med 2009;360:1085-1095
   Full Text | Web of Science | Medline

2. 2

   Ramanathan V, Ramana MV, Roberts G, et al. Warming trends in Asia amplified by brown cloud solar absorption. Nature 2007;448:575-578
   CrossRef | Web of Science | Medline

3. 3

   Bonasoni P, Laj P, Angelini F, et al. The ABC-Pyramid Atmospheric Research Observatory in Himalaya for aerosol, ozone and halocarbon measurements. Sci Total Environ 2008;391:252-261
   CrossRef | Web of Science | Medline

4. 4

   Moore GWK, Semple JL. A Tibetan Taylor Cap and a halo of stratospheric ozone over the Himalaya. Geophys Res Lett 2005;32:L21810-L21810
   CrossRef | Web of Science

5. 5

   Semple JL, Moore GWK. First observations of surface ozone concentration from the summit region of Mount Everest. Geophys Res Lett 2008;35:L20818-L20818
   CrossRef | Web of Science

To the Editor:

Jerrett et al. report an increase in the risk of death from respiratory causes associated with ozone. We want to draw attention to the inherent uncertainties in their approach to estimates of exposures. The authors estimated ozone exposure by first averaging concentrations over 6 months and then for all active air-pollution monitors within each metropolitan area. A single value was then used to represent the ozone exposure for all populations. By simply averaging ozone concentrations over space and time, this approach assumes that where and how the study cohorts and high and low ozone levels are distributed do not matter. Uncertainties associated with this oversimplified estimation need to be addressed.

Jerrett and colleagues chose the peak 1-hour concentration to describe ozone exposure. Although the peak is important, lower but more frequent concentrations may make a larger contribution to the risk of death, since there is no threshold below which ozone poses no threat. Diurnal variation in ozone levels is also controlled by precursor emissions. There is a higher afternoon maximum concentration and a lower nighttime minimum concentration of ozone in polluted areas than in less polluted areas.1 Thus, the single peak 1-hour value may not represent the cumulative exposure throughout the day.

Daniel Q. Tong, Ph.D.
North Carolina State University, Raleigh, NC 27695
qtong@ncsu.edu

Shaocai Yu, Ph.D.
Science and Technology Corporation, Hampton, VA 23666

Haidong Kan, Ph.D.
Fudan University, Shanghai 200032, China

1 References

1. 1

   Chameides WL, Saylor RD, Cowling EB. Ozone pollution in the rural United States and the new NAAQS. Science 1997;276:916-917
   CrossRef | Web of Science | Medline

Author/Editor Response

Ozone concentrations in the troposphere have doubled globally since preindustrial times, with annual averages in the range of 30 ppb in remote areas, as compared with estimates of 10 to 15 ppb in preindustrial times.1 A major source of this doubling is anthropogenic combustion of fossil fuel. With a positive radiative forcing of 0.35 W per square meter, ozone is also a short-lived greenhouse gas, and with adverse feedbacks from climate change, ozone pollution will probably worsen in many regions.2,3 We agree with Semple and Moore that increasing levels of ozone in remote areas are a problem for local health and global climate change.

Tong and colleagues question our use of daily 1-hour maximum values for estimating exposure to ozone, and they argue that lower but more persistent levels of exposure over longer daily periods may pose a greater risk to health. Although longer averaging times could theoretically affect our results, the question would be more relevant to comparisons between rural settings and urban settings, where contrasts in diurnal ozone levels are usually greater. Our results are based on the subjects who lived in metropolitan statistical areas that by definition exclude rural areas.

Estimates of exposure with the use of different daily averaging times are highly correlated across cities and consequently are unlikely to produce results that are different from those reported. To evaluate this question, we computed the average of the second and third quarterly averages for all the monitors within each metropolitan statistical area using the daily maximum 1-hour averages and daily maximum 8-hour averages (the averaging time for the current National Ambient Air Quality Standard for ozone). We made these calculations for the 239 metropolitan statistical areas with available data in the United States during the year 2000. The correlation between the averages for the metropolitan statistical areas based on the maximum 1-hour and maximum 8-hour averages across metropolitan statistical areas was 0.99. A regression of the maximum 8-hour average values on the maximum 1-hour averages resulted in a slope of 0.89 with a statistically nonsignificant intercept of −0.76 ppb (Figure 1Figure 1Comparison of Maximum Average Ozone Levels.) or a slope of 0.88 with no intercept. Therefore, estimates of exposure based on a longer averaging time would have resulted in a similar association between ozone and mortality. We will nonetheless examine other averaging times in future research and thank Tong and colleagues for their suggestion.

Michael Jerrett, Ph.D.
University of California, Berkeley, CA 94720
jerrett@berkeley.edu

Kazuhiko Ito, Ph.D.
New York University School of Medicine, New York, NY 10016

Richard T. Burnett, Ph.D.
Health Canada, Ottawa, ON K1A 0K9, Canada

3 References

1. 1

   Finlayson-Pitts BJ, Pitts JN Jr. Tropospheric air pollution: ozone, airborne toxics, polycyclic aromatic hydrocarbons, and particles. Science 1997;276:1045-1052
   CrossRef | Web of Science | Medline

2. 2

   Intergovernmental Panel on Climate Change (IPCC). Climate change 2007: the physical science basis: contribution of Working Group I to the fourth assessment report on the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
   

3. 3

   Steiner AL, Tonse S, Cohen RC, Goldstein AH, Harley RA. Influence of future climate and emissions on regional air quality in California. J Geophys Res 2006;111:D18303-D18303
   CrossRef | Web of Science