Carbon monoxide (CO) is a toxic gas known since prehistoric times when man first discovered fire. Accidental CO poisonings due to incomplete combustion of organic materials used for making fire or due to exposure to natural CO sources first reported by LEWIJ (29) and others (28). Although the main source of CO is incomplete burning of materials, several natural sources of CO in both biological (1,2,32) and nonbiological origin (11) have also been identified.As civilization developed and modern techniques spread over larger areas, the sources of CO increased both around our occupational and nonoccupational environment. In many countries the incidence of acute CO intoxications stili take the second place (21) while in some, the total number of deaths due to CO approaches the number of deaths because of other chemicals (15).The signs and symptoms of acute CO intoxication are well known and they only appear with carboxyhemoglobin (COHb) levels above 10 %. But these levels are seldom met in subjects exposed to traffic exhaust or industrical CO sources occupationally. As