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updated 3/22/2011 2:04:33 PM ET 2011-03-22T18:04:33

Rising temperatures and pollution levels may act together to worsen heart health, a new study suggests.

The results show high temperatures in the summer months in a U.S. city are associated with a decrease in heart-rate variability, or how regular the time between heartbeats is, which acts as a measure of how well the heart is working. Previous studies have linked low heart-rate variability to an increased risk of death following a heart attack.

Temperature was more likely to affect cardiovascular function when ozone levels were high, the researchers say.

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The findings are particularly concerning in light of the changes global warming is predicted to bring.

"Given that global warming is likely to increase both heat waves and ozone formation, such an interaction may be important for public health," said study researcher Cizao Ren, of the Harvard School of Public Health. (While ozone in the upper atmosphere protects Earth from the sun's harmful ultraviolet radiation, in the lower atmosphere it's a primary component of smog and acts as a lung irritant.)

The study involved 694 elderly men (average age 73 years) who lived in Boston Participants had their heart-rate variability measured at least once between November 2000 and December 2008. The researchers also analyzed temperature and air pollution data from the surrounding area up to 20 days prior to the participants' examinations.

The researchers found an association between temperature and heart-rate variability in the warm season, but not the colder months. One reason for this may be that people tend to stay indoors in the winter months, where the temperature is often controlled with heating.

Previous studies have found higher temperatures can increase the risk of death from cardiovascular disease, and this effect is exacerbated by air pollution. But the new study suggests what might be happening on a biological level to cause problems.

Air temperature and ozone may influence the way the automatic nervous system functions. The automatic nervous system is a part of the central nervous system that helps the body adapt to its environment, according to the American Heart Association. It regulates body functions, including the heart's electrical activity and airflow into the lungs. Heart-rate variability is an indicator of automatic nervous system function, Ren said.

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Air pollution may cause problems with reflexes in the airways to the lungs. In addition, higher temperatures may make the body more sensitive to toxins, such as ozone.

The researchers note the study involved elderly men in one part of the United States, and the findings may not be representative of the population as a whole.

The study was published in the March 8 issue of the American Journal of Epidemiology.

Pass it on: Air temperature and ozone may act synergistically to worsen cardiovascular health.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @RachaelRettner.

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