Smoke from wildfires may send people -- particularly seniors -- to hospital emergency rooms (ERs) with heart, stroke-related complaints, according to new research in the Journal of the American Heart Association, the Open Access Journal of the American Heart Association/American Stroke Association.

Previous studies have shown that wildfire smoke exacerbates respiratory conditions but yielded inconsistent results for effects on the heart, brain or blood vessels.

The study was the product of a collaboration between researchers at the University of California San Francisco, California Department of Public Health and the U.S. Environmental Protection Agency. Researchers reviewed more than one million ER visits in northern and central California during intense wildfires in the summer of 2015. They examined the relative risk of daily heart-, brain- and blood vessel-related ER visits on light, medium and dense smoke days relative to days without wildfire smoke exposure.

They found that smoke exposure was associated with increased rates of ER visits, not just for breathing trouble, but also ischemic heart disease, irregular heart rhythm, heart failure, pulmonary embolism and stroke. The risk was greatest for adults age 65 and older.

The greatest increased relative risk was noted within a day of dense wildfire smoke. During these times, researchers found rates of ER visits among adults 65 and older increased:

42 percent for heart attack; and

22 percent for ischemic heart disease.

Overall, ER visits for all cardiovascular and cerebrovascular causes were elevated across all smoke days, with the greatest increase on dense smoke days and among adults age 65 and older. Respiratory conditions also were increased, as anticipated.

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"This is one of the most extensive studies of wildfire health impacts in California to date," said Ana Rappold, Ph.D., study senior author and statistician with the U.S. Environmental Protection Agency in Durham, North Carolina.

Wildfire smoke contains many pollutants including ozone, carbon monoxide and fine particulate matter -- which is linked to cardiovascular risk. People with underlying cardiovascular disease risk factors may be at risk for an acute heart, brain or blood vessel event when exposed to wildfire smoke.

A 2010 AHA statement on air pollution noted that particulate matter has been associated with increased risks of heart attack, stroke, irregular heart rhythm and heart failure exacerbation within hours to days of exposure in susceptible individuals. In addition, long term exposure to particulate matter can reduce life expectancy by a few years.

"The findings have public health and clinical implications," said Wayne E. Cascio, M.D., study author and acting director for the National Health and Environmental Effects Research Laboratory in the EPA's Research and Development Office. "I think it will have a significant impact on how clinicians and public health officials view future wildfire events and the smoke that's generated from them."

Research has shown that wildfire season in North America is increasing in intensity and duration. As wildfires likely become a bigger problem, it is important to continue research for wider-ranging health impacts from smoke exposure and ways to help people prevent these health outcomes, said Zachary S. Wettstein, B.A., study first author and graduating medical student from the University of California San Francisco.

"We need to study effective interventions that might decrease exposure to smoke and the associated health impacts," Wettstein said. "These findings urge us to study these impacts over longer periods of time and within susceptible populations."

The study did not examine the effect of personal traits such as past medical history, race, socioeconomic status and other factors that might affect the association between smoke exposure and ER visits but did examine differences by sex and age.