Accidental or intentional carbon monoxide poisoning is common throughout the year. In the Midwest, however, accidental exposures are more common during the winter months when the furnaces are overworked or malfunction. Consequently, enmasse exposures to the poisonous gas are frequently encountered during this season. Adding to the problem are the energy conservation efforts. Doors, windows and other ventilation avenues are tightly sealed, and solid fuel might be substituted to save other expensive conventional fuels. Other causes of carbon monoxide poisoning are blocked exhaust systems, automobiles with inadequate mufflers, fires and solvent (methylene chloride, etc.) usage in poorly ventilated areas, e.g., basements. The exact incidence of nonfatal subacute carbon monoxide poisoning is not available. National Clearinghouse Bulletin reported that in 1978, out of 376 reported exposures, there were eight fatalities. A figure which has been frequently quoted in the literature is that each year in the United States, approximately 3,500 deaths are caused by carbon monoxide poisoning.