Carbon monoxide is the most common cause of accidental poisoning in the United States, sending more than 40,000 people to emergency rooms each year. A study published in the Journal of the American Medical Association finds that the colorless, odorless gas - which initially can cause dizziness, nausea, headaches, lethargy and confusion - can also lead to heart damage.

Lead author Dr. Timothy Henry with the Minneapolis Heart Institute Foundation was surprised with the results. "Almost 40 percent of patients had heart damage related to the carbon monoxide poisoning, which was much higher than we expected," he says.

The study was based on 230 patients with moderate to severe carbon monoxide poisoning. Researchers had also not anticipated long-term effects. "Most of us believed that since this was a one-time exposure to carbon monoxide, that if you were going to have problems you would have them right away," Dr. Henry says.

Dr. Henry says cardiac disease even showed up in younger people. It was a little surprising to find that these people, who would normally be at low risk for heart damage, actually ended up with significant heart damage.

Dr. Henry recommends that patients with suspected exposure to carbon monoxide be screened for possible heart damage. Carbon monoxide poisoning is associated with heating and cooling systems. Experts say these systems should be checked for leaks at least once a year.