Carbon monoxide poisoning is the most common kind of accidental poisoning in the United States, playing a role in 40,000 emergency rooms visits a year.

But while it is increasingly unusual for victims to die at the hospital, a new study suggests that when many are sent home, their problems are far from over.

Researchers say that carbon monoxide victims who suffer damage to their heart muscles, a common occurrence, are at much higher risk for heart attacks in later years. The study, led by Christopher R. Henry of the Minneapolis Heart Institute Foundation, appears in the current Journal of the American Medical Association.

Carbon monoxide poisoning can occur when people are exposed to combustion, including car exhaust, fires and faulty heating systems. From 1968 to 1998, the researchers said, it was responsible for more than 1,000 accidental deaths.

In this study, the researchers looked at what happened to 230 people treated from 1994 to 2002 for moderate to severe CO poisoning at a Midwestern hospital. The study followed their health until 2005.

Five percent of the patients died in the hospital. But "despite appearing to be a low-risk population from a cardiovascular standpoint," the researchers wrote, after seven and a half years, about a quarter of the rest were dead — a rate three times as high as expected.

For those patients whose heart muscles had been injured, the figure was about 38 percent, with almost half dying of what appeared to be cardiovascular problems.

Given this, the study says, doctors who suspect their patients have been exposed to carbon monoxide should screen them for heart muscle damage, and the patients should be monitored.