Ozone Amplifies Summer Heat's Effect on Cardiovascular Mortality


IRVINE, Calif., Nov. 21 -- Ozone appears to be a key to the link between high temperatures and the increased risk of death from heart disease or stroke, according to investigators here.
Action Points
Explain to patients who ask that this study found that high ozone levels amplify the harmful cardiovascular effects of hot weather.
High outdoor ozone levels from air pollutants make hot weather an even more potent force for cardiovascular mortality, Cizao Ren, Ph.D., of the University of California at Irvine, and colleagues reported online in Occupational and Environmental Medicine.
A 10-degree increase in temperature on the same day was associated with an increase in cardiovascular mortality of 1.17% at the lowest ozone concentration and 8.31% for the highest concentration of ozone in all measured areas, the researchers said.
With rising temperatures as a result of global warming, the impact of increasing ozone air pollution on human health is likely to increase, they concluded.
Ozone is strongly tied to weather conditions, particularly the amount of ultraviolet light in the atmosphere. It is generated by a complex photochemical reaction between nitrogen oxides, volatile organic compounds, and oxygen in sunlight, the researchers wrote.
Both ambient ozone and temperature have been separately associated with human health, Dr. Ren said. To explore whether ozone enhances the association between maximum temperature and cardiovascular mortality in the U.S., the researchers obtained data from the U.S. National Morbidity, Mortality, and Air Pollution Study (NMMAPS) website.
The data contained time series information for health outcomes, air pollution, and weather conditions from January 1987 through December 2000. The study included 95 large communities in the mainland with a total population of nearly 100 million and four million cardiovascular deaths (heart attack or stroke) during the study period.
Using two time-series Poisson regression models, the researchers examined the joint effects of summer temperatures (June through September) and ozone levels on cardiovascular mortality.
The communities were divided into seven regions, ranging from the northeast, the industrial midwest, and the upper midwest to the northwest, the southeast, the southwest, and southern California.
Maximum hourly ozone concentrations ranged from 36.74 parts per billion to 142.85 ppb. Mean maximum temperatures ranged from about 69 degrees F to more than 100 degrees F.
The results indicated that ozone positively modified the temperature-cardiovascular associations across the different regions. In general, the higher the ozone concentration, the higher the risk of cardiovascular death attributable to high temperatures, the researchers said.
In general, ozone concentrations were positively and significantly correlated with temperatures in the northeast, industrial midwest, northwest, southeast, and southern California, but no significant modifications were observed in the upper midwest and the southwest. The latter finding may have resulted from smaller numbers of persons in these areas, the researchers suggested.
Because regional classification resulted in a small number of communities especially in certain regions, the researchers redivided the communities using latitude as a cut-off and confirmed the ozone-temperature effect for all these communities.
Previous studies have shown that heat waves are associated with human morbidity and mortality. Because high temperature is often associated with high ozone concentrations, the estimated effects of heat waves may be partly attributable to the ozone effect, the researchers said.
High temperature can aggravate many pre-existing health conditions and is a well-known cause of heat-related mortality. In addition, the researchers said, exposure to ozone may directly affect airways through inhalation and involve modulation of the autonomic nervous system, making people more vulnerable to the effects of temperature variability, the researchers said.
Because the temperature-ozone-mortality risk may vary with areas, multisite studies are needed, Dr. Ren and colleagues wrote.
Secondly, the study estimated the effect on only a single day, so that longer periods need to be examined. Finally, they said, it is important to estimate interactive effects between temperature and multiple pollutants such as ozone, particulate matter, and nitrogen dioxide.
Ozone, a secondary pollutant, is formed by reactions in sunlight with volatile organic compounds, coming from petroleum, solvents, road transport, and industrial processes. It is therefore important to provide warnings to the public on days with both high temperature and high ozone levels, the researchers said.
No financial conflicts were declared. This study was partly funded by an Australia Research Council Discover Grant.Primary source: Occupational and Environmental MedicineSource reference: Ren C, et al "Ozone modifies associations between temperature and cardiovascular mortality: analysis of the NMMAPS data"Occup Environ Med 2007; DOI: 10.1136/oem.2007.033878.