Now that icy weather is here, the danger of carbon monoxide poisoning due to unserviced heating systems has made its annual appearance.

Carbon monoxide poisoning is produced by the incomplete combustion of gas, said Ogden Fire Marshal Robert Wright."If a furnace leaks, if the flue of a wood stove is plugged or the damper closed in a fireplace, then carbon monoxide can escape in the home," he explained.

Carbon monoxide poisoning is the leading cause of poison deaths in the United States, said Kayleen L. Paul, nurse director of emergency services at McKay-Dee Hospital.

In 1985, 4,000 deaths were caused by carbon monoxide. Forty percent were accidental and 60 percent were cases of suicide, mostly involving cars left running in garages, she said.

In the same year, 110,000 Americans had to take time off from work because of symptoms such as dizziness, severe headaches, nausea, vomiting, abdominal pain and fatigue.

"The problem is that the symptoms are so vague. They are the same as those of the flu or even a dozen other illnesses," Paul said.

She said carbon monoxide poisoning usually involves a group of people such as family members or co-workers. Generally, the sickest member of the group seeks treatment, and that leads to the discovery of other victims.

Those at high risk of exposure are automobile mechanics, especially in winter when garage doors are kept shut, firefighters, smokers and people who work around smokers, she said.