Soaring temperatures and high ozone levels work together to boost death risks from heart disease and stroke, researchers report. Global warming—which brings more heat and more ozone—may further increase the number of people who die of cardiovascular events.

"Temperature and ozone are strong factors in cardiovascular mortality during June to September in the United States," noted the study's lead author, Cizao Ren, PhD, from the Department of Epidemiology in the School of Medicine at the University of California, Irvine. "Temperature and air pollution combine to affect the health of large populations," he added.

Dr. Ren expects the problem will get worse as the earth becomes hotter. "The increases in both temperature and air pollution will have a strong affect on health," he said.

The Study

His team based its findings on data on nearly 100 million people living in 95 different areas across the United States. The data was collected from June to September, the hottest time of year.

Four million deaths from heart attacks or strokes occurred during the study period. Dr. Ren's team compared death rates against changes in temperature during one day.

Study Findings

The higher the ozone level, the greater the risk for cardiovascular death attributable to high temperatures, Dr. Ren's team concluded.

Ozone levels ranged from an average of 36.74 parts per billion to 142.85 parts per billion, while daily temperatures ranged from 68 to around 107 degrees Fahrenheit.

When the ozone level was at its lowest, a 10-degree increase in temperature was associated with about a 1% increase in deaths from heart disease and stroke.

However, when the ozone level was at its highest, there was a more than 8% increase in deaths from heart disease and stroke per 10degree increase in temperature, Dr. Ren's group found.


Ozone is a pollutant strongly linked to weather conditions, particularly the amount of ultraviolet light in the atmosphere.

Ozone is generated by a reaction between airborne nitrogen oxides, volatile organic compounds, and oxygen in sunlight.

Exposure to high levels of ozone can affect the airways and the autonomic nervous system, making people more susceptible to the effects of temperature changes, the team explained.

Expert Agrees

"This paper reinforces what we know—that both temperature and ozone affect health, even to the extent that they affect mortality," said George Thurston, an associate professor of environmental medicine at New York University.

Global warming will increase both temperatures and pollution, Thurston added, because higher temperatures are conducive to the production of ozone. "This will be a growing problem," he said.

For the general public, the study raises questions about pollution and climate change, Thurston noted.

"The health effects may be even worse than thought," he said. There are health benefits to reducing climate change.

Cutting back on the use of fossil fuels will help, Thurston pointed out. "Reducing fossil fuel combustion will reduce climate change and pollution," he said.

"We have seen the problem, and it's fossil fuel combustion. Now, all we have to do is come up with an alternative," Thurston concluded.

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