Danish researchers think they have stumbled on a cause of heart attacks: noisy streets.

While tracking 57,000 people over 10 years as part of a long-term cancer study, the researchers found that a high percentage of first-time heart-attack victims lived on busy streets. In a report published June 20, the researchers said they originally blamed car exhaust, but follow-up tests ruled that out. They also factored in other data gathered during the study, including the subjects' age, gender and living habits.

When they got done, the only consistent factor they could find was the noise. "Traffic noise was associated with a 12 percent higher risk for incident MI [myocardial infarction]," the report says. They theorized that the noise interrupts sleep, which in turn can lead to heart problems.

"I can see a reasonable association between fragmented sleep and coronary events," said Dr. Michel Cramer Bornemann of the Minnesota Regional Sleep Disorders Center at Hennepin County Medical Center. Every time a sleeper is aroused, the body releases adrenaline, "and over an extended time, that could take its toll," he said.

The researchers concede that there are gaps in the study. They didn't have access to medical records indicating whether the victims had a family history of heart problems. "We also had no information on bedroom location, noise from neighbors and ventilation or hearing impairment, all of which might influence exposure to noise," they said.

Nonetheless, Cramer Bornemann thinks they might be on to something. "It certainly warrants further investigation," he said.