Enviromental sources of CO are the most important with respect to clinical CO poisoning, yet baseline values of CO levels in the blood are determined by the endogenous production of the gas by the catabo- lism of hemoglobin and other heme-containing com­pounds, coupled to its intake as a low level pollutant in the ambient air. For most urban locales, this leads to a normal COHb level of less than 2 percent of total hemoglobin in nonsmokers. This value rises some­what in pregnant women due to endogenous fetal CO production. For smokers, “normal” values may range from 10 percent to 15 percent immediately after a cigarette to chronic values ranging from 3 percent to 8 percent; thus, a smoking history is of significance in a patient with suspected CO exposure.

The major environmental source of CO is the incomplete combustion of organic material, either in internal combustion engines or in the burning of fuels such as kerosene, wood, or coal for home or industrial purposes. Automobiles are the most prolific of all CO sources, producing an estimated 0.37 kg of the gas for each mile travelled. Vehicle exhaust is particularly dangerous in enclosed spaces or areas where there is poor ventilation. In a closed garage, lethal concentra­tions can be reached in ten minutes or less. Toxic levels of CO have been noted in heavily utilized tunnels, in stationary as well as moving buses, and in ice rinks due to ill-maintained ice-resurfacing machin­ery.

Malfunctioning equipment has further been respon­sible for several instances of industrial exposure, as recently highlighted in a garment factory in North Carolina. In this occurrence, workers in the cutting room of the plant began complaining of headaches, nausea, faintness, and dizziness. Their symptoms seemed to coincide with the use of a propane-powered forklift used in the area on occasion. Investigators, suspecting CO poisoning, took measurements of CO concentration in the cutting room both before and during operation of the forklift. Finding a basal con­centration of 35 ppm, they found that within 30 minutes of forklift start-up, the CO concentration had risen to 250 ppm on continuous monitoring. Spot checks with a different device showed levels of up to 300 ppm. The OSHA guidelines suggest no more than a 50-ppm exposure over an eight-hour time-weighted average. Upon replacement of the forklift with an electric model, no further symptoms were encoun­tered by the workers. canada drugs online

Fires of all types and the smoke that they produce are a common source of CO and, as with engine exhaust, rapidly produce hazardous levels of the gas in enclosed environs. The smoke inhalation and CO poisoning resulting from dwelling fires is estimated to be a direct cause of 50 percent of all fire-related fatalities. As alluded to earlier, tobacco smoke contains significant amounts of CO. In particular, secondary smoke to which nonsmokers are exposed contains about two and one-half times the amount of CO as directly inhaled smoke.

Virtually any inadequately vented indoor appliance that relies on combustion of fuels can give rise to toxic levels of CO. Even though natural gas burns quite cleanly, proper oxygenation is required to avoid incom­plete combustion; however, what makes natural gas particularly dangerous is that potentially lethal levels of CO may be reached without the warning indicator of irritating fumes.

Less common causes of CO intoxication are expo­sure to fumes from Sterno, a type of canned heating fuel, and to solvents containing methylene chloride (which is converted to CO via hepatic metabolism).