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Tuesday, April 12, 2011


Diepenbeek, Belgium - A comparative risk assessment of various triggers for MI suggests that cocaine is most likely to trigger an event in an individual, but traffic and exposure to air pollution has the greatest effect on triggering an MI at the population level [1].

"Many papers have studied the effects of various triggers of myocardial infarction, but our question was, well, at the population level, which one is the most important one?" lead investigator Dr Tim Nawrot (Hasselt University, Diepenbeek, Belgium) told heartwire. "Cocaine use, in this paper, is definitely the most important risk factor, with a 24-fold increased risk of having an event while using it, but not that many people use cocaine, making it rather weak at the population level. On the other hand, air pollution, in general, has quite a weak individual risk, but because many people are exposed to it at the population level, about 5% to 7% of myocardial infarctions are triggered by this risk factor."

The results of the study are published online February 23, 2011 in the Lancet.

Exposure prevalence ranges from 0.04% to 100%

In the paper, the researchers reviewed 36 epidemiologic studies assessing population-attributable fractions (PAF) of various triggers for MI. The PAFs are calculated using the odds ratios and frequencies of each trigger and provide an estimate of the proportion of MIs that could be avoided if the risk factor were removed. As Nawrot noted, the exposure prevalence in the analysis ranged from 0.04% for cocaine use to 100% for air pollution.

In ranking the triggers based on the odds ratios, cocaine use was associated with 24-fold increase risk of MI, far and away the most significant trigger for an event. The consumption of a heavy meal, smoking marijuana, experiencing negative emotions, physical exertion, experiencing positive emotions, getting angry, sexual activity, traffic exposure, coffee consumption, and air pollution were also associated with a significantly increased risk of MI.

In calculating the PAF, however, traffic exposure was the most important trigger for an acute event at the population level, report investigators. This was followed by physical exertion, alcohol consumption, coffee consumption, and a change of 30 µg/m3 of particulate matter with an aerodynamic diameter of 10 µg or less (PM10). Experiencing negative emotions, anger, eating a heavy meal, experiencing positive emotions, and sexual activity had a PAF ranging 3.9% to 2.2%, respectively, while cocaine and marijuana use had a PAF of 0.9% and 0.8%, respectively.
Prevalence of exposure, pooled odds ratio, and PAF for various triggers of myocardial infarction

Triggers of MI Prevalence of exposure(%) Odds ratio(95% CI) PAF, %(95% CI)
Air pollution, 10 µg/m3 100 1.02(1.01-1.02) 1.57(0.89-2.15)
Air pollution, 30 µg/m3 100 1.05(1.03-1.07) 4.76(2.63-6.28)
Alcohol 3.2 3.1 (1.4-6.9) 5.03(2.91-7.06)
Anger 1.5 3.11(1.8-5.4) 3.07(1.19-6.16)
Cocaine use 0.04 23.7(8.1-66.3) 0.90(0.28-2.55)
Coffee 10.6 1.5(1.2-1.9) 5.03(2.08-2.71)
Physical exertion 2.4 4.25(3.17-5.68) 6.16(4.20-8.64)
Sexual activity 1.1 3.11(1.79-5.43) 2.21(0.84-4.53)
Traffic exposure 4.1 2.92(2.22-3.83) 7.36(4.81-10.49)

Commenting on the results, Nawrot said the traffic exposure could be a combination of pollution, stress, noise, and other factors, although it is assumed that air pollution plays a large role, as drivers are exposed to peak levels while idling in traffic. In studies directly measuring air pollution, decreasing PM10 levels 30 µg/m3 would reduce MIs 5%, while reducing PM10 levels 10 µg/m3 would reduce events by 1.6%.

"Not only in this study, but in the many studies that have been conducted, the acute risk of air pollution on acute myocardial infarction is well established and is recognized as such by the American Heart Association," Nawrot told heartwire. "What this study adds is the perspective from the population level. Improving the air we breathe is a very relevant target to reduce the incidence of this disease in the general population."

In an editorial accompanying the study [2], Dr Andrea Baccarelli (Harvard University, Boston, MA) and Dr Emelia Benjamin (Boston University, MA) call the analysis "an exemplary piece of epidemiological work," saying that the evidence stands as a "warning against overlooking the public-health relevance of risk factors with moderate or weak strength that have high frequency in the community."

They note that reducing PM10 levels 30 µg/m3 would bring several European cities toward the 20 µg/m3 annual mean limit recommended by the World Health Organization, but the decrease is larger than needed, or achievable, in most US cities as well as several European cities. In 2000, most US cities had PM10 levels below 30 µg/m3 and many had levels below 20 µg/m3, but the PAF for reducing air pollution 10 µg/m3 is "far from negligible," at 1.57%, and would go a long way toward reducing the triggering burden in communities exposed to low to moderate levels of air pollution. A 10-µg/m3 and 30-µg/m3 reduction in air pollution is well less than what is needed in most Asian cities, they point out.

1. Nawrot TS, Perez L, Künzli N, et al. Public health importance of triggers of myocardial infarction: A comparative risk assessment. Lancet 2011; DOI: 10.1016/S0140-6736(10)62296-9. Available at:
2. Baccarelli A, Benjamin EJ. Triggers of MI for the individual and the community. Lancet 2011; DOI: 10.1016/S0140-6736(10)62348-3. Available at:

February 23, 2011 | Michael O'Riordan

Posted by Steven Almany, M.D.

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