(Reuters Health) - Cardiac arrests are more likely when levels of air pollution - especially soot-like particles and ozone - have been high in recent days or even hours, according to a large study from Texas.

Evidence already links airborne particles with heart disease and lung problems but the new findings are the first to show that high ozone may immediately raise the risk that a person's heart will stop beating.

"Heart patients should consider when there are high ozone levels that they should take extra care of themselves," lead author Katherine Ensor of Rice University in Houston told Reuters Health.

About 300,000 Americans experience cardiac arrest - when the heart abruptly stops and therefore can't get blood to the rest of the body - outside of hospitals each year and less than 10 percent survive. Cardiac arrest can be caused by electrical problems in the heart muscle, sudden trauma or longstanding disease.

Previous studies have found that living in polluted cities or near highways for many years can raise the risk of heart disease in general, but they mainly point the finger at small airborne particles. Ozone is more often associated with short-term worsening of asthma and other lung diseases.

To see whether various air pollutants have any direct effect on cardiac arrest rates, Ensor and her colleagues compared a database of cardiac arrests that took place outside of hospitals in Houston with air quality records for the city between 2004 and 2011.

Among the more than 11,000 cardiac arrests without an obvious cause (such as a traumatic injury), researchers found a slight rise when ozone levels where higher than usual.

Cardiac arrest risk went up by 4.4 percent for every 20 parts per billion of ozone above average within the previous three hours, according to the results published in the journal Circulation.

A difference of 20 ppb in ozone would be significant, according to Ensor. "In general I think people would notice, the air would feel thick," she said.

Summer ozone levels typically hover between 50 and 60 ppb in the U.S., according to a 10-year study by the U.S. Environmental Protection Agency (EPA). But lung function in healthy people can start to suffer at about 70 ppb, which is still within the U.S. National Ambient Air Quality Standard.

Ensor's team found a similar rise in cardiac arrest risk with elevated small-particle pollution. For every increase by 6 micrograms of fine particulates per cubic meter of air in the prior two days, cardiac arrests rose by 4.6 percent.

A 2010 study from New York City found comparable effects for particulate pollution: cardiac arrests rose by between four and 10 percent with every 10-microgram-per-cubic-meter increase in fine particulates.

The EPA safe air quality standard for such small-particle pollution is 35 micrograms per cubic meter.

Though researchers don't fully understand how air pollution is connected to heart problems, some evidence suggests that irritants like particles and ozone entering the respiratory system create inflammation and a spike in destructive molecules called free radicals, which in turn can stress the heart.

Because there are many risk factors for cardiac arrest, an additional link to ozone could have big implications for people with chronic medical conditions living in urban areas, according to Dr. Comilla Sasson of the University of Colorado, who was not involved in the new study.

Among the most common conditions that raise the risk for cardiac arrest are "stents, heart attacks, bypass surgery, and the standard risk factors of high blood pressure, high cholesterol, asthma, emphysema, smoking and a family history or genetic factors," according to Sasson, who works to identify communities at greatest risk for cardiac arrest.

"I would think there are a lot of folks in (urban) areas that have one or two of these conditions," Sasson said.

It's too early to make recommendations based on the new results, Ensor said, but the ultimate goal of the research is to recommend more advanced warning systems to policymakers, to alert physicians who treat high risk patients, and to make ozone forecasts more available to the public.

For example, she added, scheduling outdoor sporting events, like marathons, could be affected by pollution levels.

Sasson said, "On high ozone days or high particulate days should we tell people at high risk to stay inside? If your grandma has heart problems, maybe keep her inside today."

SOURCE: bit.ly/134vV5n Circulation, online February 13, 2013.