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 said.
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% survive.
What causes cardiac arrest?
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
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% 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 US, according
to a 10-year study by the US Environmental Protection Agency (EPA). But lung
function in healthy people can start to suffer at about 70 ppb, which is still
within the US 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 metre of air in
the prior two days, cardiac arrests rose by 4.6%.
A 2010 study from New York City found comparable effects for particulate
pollution: cardiac arrests rose by between four and 10% with every
10-microgram-per-cubic-metre increase in fine particulates. The EPA safe air
quality standard for such small-particle pollution is 35 micrograms per cubic
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."