Researchers at Rice University in Houston have found a direct correlation between out-of-hospital cardiac arrests and levels of air pollution and ozone. Their work has prompted more CPR training in at-risk communities.

Rice statisticians Katherine Ensor and Loren Raun announced their findings Sunday, 17 February, at the American Association for the Advancement of Science (AAAS) conference in Boston.

Their research, based on a massive data set unique to Houston, was published this month in the American Heart Association journal Circulation.

At the same AAAS symposium, Rice environmental engineer Daniel Cohan discussed how uncertainties in air-quality models might impact efforts to achieve anticipated new ozone standards by the US Environmental Protection Agency.

Given that the American Lung Association has ranked Houston eighth in the United States for high-ozone days, the Rice researchers set out to see if there is a link between ambient ozone levels and cardiac arrest. Ensor is a professor and chair of Rice's department of statistics, and Raun is a research professor in Rice's department of statistics.

For the new study, the authors analyzed eight years' worth of data drawn from Houston's extensive network of air-quality monitors and more than 11,000 concurrent out-of-hospital cardiac arrests (cardiac arrests) logged by Houston Emergency Medical Services (EMS).

They found a positive correlation between cardiac arrests and exposure to both fine particulate matter (airborne particles smaller than 2.5 micrograms) and ozone.

The researchers found that a daily average increase in particulate matter of 6 micrograms per day over two days raised the risk of cardiac arrests by 4.6 per cent, with particular impact on those with pre-existing (and not necessarily cardiac-related) health conditions.

Increases in ozone level were similar, but on a shorter timescale - each increase of 20 parts per billion over one to three hours also increased cardiac arrest risk, with a peak of 4.4 per cent. Peak-time risks from both pollutants rose as high as 4.6 per cent. Relative risks were higher for men, African-Americans and people over 65.

For the study, cardiac arrest events were defined as cases where medical service personnel performed chest compressions. Ensor and Raun noted the patients died in more than 90 per cent of the cases, which occurred more during the hot summer months (55 per cent of total cases).

The researchers also looked at the effects of nitrogen dioxide, sulfur dioxide and carbon monoxide levels, none of which were found to impact the occurrence of out-of-hospital cardiac arrests.

The work is expected to help Houston EMS fine-tune its deployment of personnel and equipment and provide early warnings to health officials and the public when weather and/or incidents warrant an alert for high ozone levels in specific areas, Ensor said.

Co-author David Persse, Houston Fire Department out-of-hospital cardiac arrests physician director and a public-health authority for the city, said it's long been thought by EMS workers that certain types of air pollution, including ozone, have significant negative effects on cardiac and respiratory health. ''But this mathematically and scientifically validates what we know,'' he said.