Myocardial infarction, or heart attack , is the leading causes of death from heart disease . Genetic predisposition and lifestyle factors such as diet, exercise, and stress levels chronically predispose an individual to heart disease. However, there are many acute risk factors that may trigger a heart attack event in a susceptible person, among them: a burst of strenuous exercise, anger, and the use of cocaine.

In the last decade, studies have suggested an association between the incidence of cardiovascular events, including heart attack and stroke deaths, and short- and long-term exposure to air pollution. (Air pollution has been a known contributor to asthma for some time.) Furthermore, a recent study out of the Netherlands found that people who lived near major roads had almost twice the risk of death from cardiovascular causes than those who did not. And an increased risk of death from heart disease has also been documented among people whose occupations regularly expose them to traffic, such as traffic control officers.

To further test the hypothesis that traffic contributes to acute cardiovascular complications, a study in the October 21, 2004 New England Journal of Medicine , looked at recent exposure to traffic in a group of heart attack survivors. They found that exposure to traffic within the past hour was, indeed, significantly associated with increased risk of an acute heart attack.

About the Study

The researchers enrolled 906 people between the ages of 25 and 74 who had recently survived a heart attack.

Trained nurses interviewed the participants an average of nine days following their heart attacks, and their medical records supplied supporting data. Information on the following possible risk factors was collected:

  • Social and demographic characteristics (for example, age and employment)
  • Medical history
  • Smoking status
  • On the day of and four days prior to their heart attack:
    • Time spent sleeping
    • Activity levels during the day
    • Time spent outdoors
    • Means of transportation
    • Location (according to postal codes)
    • Presence or absence of chest pain
    • Occurrence of extreme anger or joy
    • Any exposure to dust or solvents.

The Findings

In 691 patients (76% male, 70% over age 55) with complete information, exposure to traffic was associated with an almost three-fold increase in risk of heart attack within one hour. In addition, the time participants spent in cars, on public transportation, or on motorcycles or bicycles was consistently linked with increased overall risk. And while most participants had been exposed to traffic via cars, the increase in risk was also significant for those who rode on bicycles, buses, or trolleys.

After adjusting for other risk factors, 8% of the heart attacks in this study could be attributed to the short-term effects of exposure to traffic. Subgroup analyses indicated that women, those aged 60 or above, and those with diabetes were more susceptible to the hazards of traffic exposure compared to men, those under 60, and those without diabetes.

Other factors that were independently associated with an increased risk of heart attack within an hour were extreme exertion (six-fold increase in risk), being outside (more than two-fold increase), and having just woken up in the morning (70% increase).

Employment status was the only variable found to significantly modify the association between exposure to traffic and heart attack: participants who were unemployed were at higher risk, implying commuting to work was unrelated. Time of day was of borderline significance and indicated risk was higher in the mornings and afternoons—when traffic is most dense.

How Does This Affect You?

According to this study, exposure to traffic appears to be linked to an increase in the risk of a heart attack in people chronically predisposed to heart disease. Earlier research has already documented such an association for strenuous activity, cold weather, and early morning hours. It appears that encounters with traffic—in all its many forms—may be added to this list.

While the precise mechanisms by which traffic may exacerbate existing heart disease remains unknown, the researchers did identify poor local air quality from auto emissions as a likely explanation. Still, the researchers could not exclude other factors that could account for their findings – for example, the effects of stress associated with driving versus being driven—was not measured in this study.

Despite these unanswered questions, it now appears we have one more reason—in addition to frustration, air and noise pollution, energy consumption, expense, and risk of car accidents—to keep driving to a minimum, especially as we age and our risk of heart disease increases. And while public transportation may mitigate some of these problems, it is disappointing that this study did not find any benefit in taking the bus.