STATS ARTICLES 2010
Air pollution and heart attack
Rebecca Goldin Ph.D and & Jing Peng, July 1, 2010
Is air pollution a significant risk factor for heart attacks? We look at a new study.
Continuing our analysis of recent studies on heart disease and attack that made headlines, it turns out that air pollution is highly correlated with heart attacks. A study published in the June issue of Epidemiology examined the relationship between air pollution and out-of hospital cardiac arrest in Melbourne, Australia, a city with a population of about 3.8 million and which has one of the largest out-of-hospital registries in the world. A total of 8,434 heart attack cases were identified through the Victorian Cardiac Arrest Registry between January 2003 and December 2006 – an average of 5.8 arrests per day.
The Technical Details
Air pollution concentrations obtained from a central monitoring site were used to evaluate the concentration of specific substances in the air on the day that the study was conducted. Pollution was measured by the concentration of particles in the air obtained from a central monitoring site for the several days up to and including the day that the heart attack rate was measured. The measured pollutants included particles classified by size, such as PM2.5, defined as particles with diameters less than 2.5 micrometers; PM10, defined as particles with diameters less than 10 micrometers (these include all particles classified as PM2.5); ozone (O3); carbon monoxide (CO); nitrogen dioxide (NO2); and sulfur dioxide (SO2).
The authors controlled for temperature and humidity, as well as the age of the heart attack sufferer. They excluded cases of heart attacks that clearly did not originate with the heart, such as those following physical trauma.
The study was conducted over four years, from January, 2003 to December, 2006. During that period, the pollution data was collected and the range of pollution measured was broken into quartiles; the quarter of lowest pollution days had PM2.5 rates of under 3.2 micrograms per meter cubed, while the highest quartile had of PM2.5 rates of 7.45 micrograms per meter cubed. Heart attack rates were then correlated with quartile-increases of pollution – showing that days with increased pollution were followed (about two days later) with days of increased out-of-hospital heart attacks.
Of all the particulate fractions, PM2.5 had the strongest association with out-of-hospital cardiac arrest. Subgroup analysis showed higher risks for men than for women. The increase in risk of an out-of hospital cardiac arrest for an interquartile range increase in PM2.5 for men was 4 percent and for women 2.77 percent. Neither NO2 and SO2 showed an association with out-of-hospital cardiac arrest.
To give some perspective on the numbers, the concentrations for PM2.5 and PM10 in Melbourne averaged, respectively 6.35 µg/m3 and 19.6 µg/m3. A study on pollution levels in New York in 2006 showed the annual mean of concentration for PM2.5 and PM10 were, respectively, 14 µg/m3 and 23 µg/m3, which suggests that New Yorkers may be more vulnerable to air-pollution driven heart attacks than if they lived in a more bucolic setting.
News Coverage: the contrast between what was reported in the press and in the scientific article
The study’s conclusions were not quite as black and white as Reuters’ coverage suggested. For every 4.26 micrograms per cubic meter increase in PM2.5 concentrations, the risk of cardiac arrest for an interquartile range increase for men was 4.00% (95% CI 1.18% to 6.90%) and for women 2.77% (95% CI -1.28% to 6.99%).
Reuters reported the risk of cardiac arrest was 4 percent higher than average for the next 48 hours, boosting the overall number by ignoring the risk for women. Despite inflating the risks for women, Reuters correctly noted that while CO was also linked with increased heart attacks (though less strongly than PM2.5), the other measured pollutants were not.
To date, air pollution and out-of-hospital cardiac arrest or deaths were conducted in only 4 studies. Among them, one study from Boston (Increased particulate air pollution and the triggering of myocardial infarction, by Peters A, et al. Circulation. 2001;103:2810 –2815) included 772 participants on average four days after a heart attack. The study did not show a link for the 24 hours immediately before the heart attack, but it did for 24-48 hours before. Another study from Rome (A case-crossover analysis of out-of-hospital coronary deaths and air pollution in Rome, Italy. Am J Respir Crit Care Med. 2005; 172:1549 –1555) found air-pollution in that city was on average three times higher than in Melbourne. Although that study did not measure PM2.5, an increase of 29.7 µg/m3 in concentrations PM10 corresponds to a 10.1% increase in out-of-hospital cardiac arrest in Melbourne study.
Moreover, in subgroup analysis by age both Rome and Melbourne studies showed that people age 65-74s are most susceptible to particulate air pollution. This may imply older people are in higher risk of extensive CHD with exposure to air pollution.
There are a certain limitations of the study. One limitation was a shortage of information on personal risk factors, such as smoking, alcohol intake, and obesity. Another limitation was that researchers used a central monitoring station to estimate exposure for the entire Melbourne population. The distances from the central monitoring station and time spent indoors are both varied when people were recorded with cardiovascular arrests.
Other limitations include the nature of the heart attacks. Does pollution actually lead to heart attacks, or does it make a ticking time-bomb go off a day or two earlier? Air pollution is likely more harmful to people with existing health problem, demonstrated by the fact that the oldest group of people in the Melbourne study were the least likely to suffer a heart attack attributable to pollution.
It’s easy to overestimate the impact of air pollution on cardiovascular events. Sudden cardiac arrest with no cause affects about 1 in 2,000 people in North America, according to the American Heart Association, a number consistent with the observation in this study. If pollution caused a 4 percent increase in these attacks, it would affect about 1 in 50,000 people.