June 20, 2012 -- Ah, traffic. Whether you're sitting in it, walking near it or breathing in fumes from it, traffic seems to be bad for our health in a lot of ways. Today, researchers in Denmark are reporting on another link between gridlock and health -- traffic noise may raise the risk of a heart attack.
The researchers found that people who lived with higher levels of traffic noise around their homes had a higher risk. For every 10-decibel increase of noise, the risk of a first heart attack went up by 12 percent.
Although previous studies have found some association between traffic noise and heart health, Dr. Mette Sorensen, the study's lead author, said she was surprised that this study showed such a specific relationship between noise levels and increased heart attack risk.
"Previously, there seemed to be no effect up to around 60 decibels," she said. "But I see increases at around 40 decibels up to the highest level, around 82 decibels. It doesn't seem to be a level where there are no effects."
The scientists studied more than 50,000 50- to 64-year-olds in two of Denmark's largest cities, Copenhagen and Aarhus, keeping track of many aspects of their health, including everywhere they lived for a 10-year period. Based on the locations of their homes and an analysis of traffic patterns, Sorensen and her team calculated how much noise each person had been exposed to. Of the people in the study, 1,600 had their first heart attack during the decade of the research.
The study was published today in the journal PLoS One.
Cardiologists say the link between traffic noise levels and heart health is interesting. But the findings don't spell out the exact reasons for that link.
But they have several guesses for what may be going on. One is a well-known culprit in increased heart attack risk: stress, which likely affects many people who live in bustling, noisy cities.
"The noise itself probably does increase stress and the levels of stress hormones like adrenaline. Your blood pressure is probably going up as well," said Dr. Robert Bonow, a professor of medicine at the Northwestern Feinberg School of Medicine.
Past studies of traffic and heart health have also suggested stress as a mediator between gridlock and increased health risks.
Dr. Chip Lavie, medical director of cardiac rehabilitation and prevention at the Ochsner Medical Center in New Orleans, said scientists have found associations between heart health and psychological stress of all kinds.
Earthquakes, traumatic events like Hurricane Katrina, "even sporting events have the common theme of increased psychological stress, which one could theorize is also in play during heavy traffic, with or without traffic noise," Lavie said.
Sorensen and her colleagues speculated that all the noise might prevent people from getting adequate sleep, another known risk factor for heart attacks.
Where there are high volumes of noisy traffic, air pollution is likely to follow, and cardiologists said that might also have contributed to the higher heart attack risk for the people in the study because particles in polluted air are damaging to the heart and blood vessels. The link between heart health and air pollution is also well-studied: Most recently, researchers noticed that risk factors for heart disease decreased in Beijing during the 2008 Olympics, when traffic and air pollution were kept to lower-than-normal levels.
Sorensen and her team also studied how much air pollution the study participants experienced, but Sorensen said those results will be published in a separate study later on.
The people who had heart attacks in the study were at greater risk from other factors as well: They smoked more, were less physically active and had poorer diets. Although the researchers tried to rule out those factors in their statistical analysis, it's often difficult for researchers to rule out absolutely every variable that might affect an association between two factors, even when using the best statistical methods. Dr. David Frid, a cardiologist at the Cleveland Clinic, said it's hard to discount how other cardiovascular risk factors may have affected the study participants who had heart attacks.
"Cardiovascular disease is a complex process and there are multiple factors that could lead to a heart attack," Frid said. "As we identify these various factors, we have to then be cognizant of what we can do to ultimately reduce our risk of developing heart disease."
It may not always be possible for people to escape the noisy environment they live in, so those who hear lots of horns honking and brakes squealing night and day may need to step up their efforts to get regular exercise and eat a healthy diet to compensate for that risk.
"Ultimately, if you're living in a place with a lot of noise, it could potentially put you at higher risk, but we don't know whether reducing that noise reduces your risk," Frid said.