Carbon dioxide measures up as a real hazard.

Carbon dioxide is the fourth most common gas present in the earth’s atmosphere, with an average ambient concentration (in fresh air) of about 350 ppm. Carbon dioxide is one of the most common byproducts of living organisms. With every exhaled breath we produce and release CO2 into the atmosphere (with an average concentration in exhaled breath of about 3.8%). According to one USDA study, an average person produces about 450 litres (900 grams) of CO2 per day. Liquid and solid carbon dioxide (dry ice) are widely used as refrigerants, especially in the food industry. Carbon dioxide is also used in many industrial and chemical industry processes. Carbon dioxide is particularly associated with the beer and wine making industries, where it is produced by yeast during the fermentation process that converts sugar into alcohol. Carbon dioxide in the headspace of fermentation vessels can easily reach 50% by volume or even higher concentrations. Carbon dioxide is also widely used in the oil industry, where it is commonly injected into oil wells to decrease the viscosity and aid in the extraction of oil from mature fields. It is also one of the most common atmospheric hazards encountered in confined spaces. Carbon dioxide is a primary byproduct of bacterial decomposition. As with people, “aerobic” or oxygen using bacteria produce carbon dioxide as a primary metabolic byproduct. In many confined spaces there is a direct relationship between low concentrations of oxygen and elevated concentrations of CO2. In the case of a confined space where CO2 is generated as a byproduct of aerobic bacterial action, a concentration of 19.5% O2 (the hazardous condition threshold for oxygen deficiency in most jurisdictions) would be associated with an equivalent concentration of at least 1.4% ( = 14,000 ppm) CO2. This is substantially higher than the generally accepted workplace exposure limit for CO2 (5,000 ppm calculated as an 8hour TWA). The true concentration of CO2 could be substantially higher if the oxygen deficiency is due to displacement rather than consumption of the oxygen in the confined space. Fresh air contains only 20.9% oxygen by volume. The balance consists mostly of nitrogen, with minor or trace concentrations of a wide variety of other gases including argon, water vapour and carbon dioxide. Because oxygen represents only about one-fifth of the total volume of fresh air, every 5% of a displacing gas that is introduced into a confined space reduces the oxygen concentration by only 1%. As an example, consider an oxygen deficiency due to the introduction of dry ice into an enclosed space. In this case a reading of 19.5% O2 would not be indicative of 1.4% CO2, it would be indicative of 5 X 1.4% = 7.0% (= 70,000 ppm) CO2. The bottom line is that if you wait until the oxygen deficiency alarm is activated, and the deficiency is due to the presence of CO2, you will have substantially exceeded the toxic exposure limit long before leaving the affected area. In spite of these considerations, in the past the majority of atmospheric monitoring programs have treated CO2 as only a “simple asphyxiant”. An asphyxiant is a substance that can cause unconsciousness or death by suffocation (asphyxiation). Asphyxiants which have no other health effects are referred to as “simple” asphyxiants. Because CO2 was not considered to be a toxic hazard, rather than directly measuring the CO2 in the confined space or workplace environment, it was seen as adequate to simply measure the oxygen concentration. This attitude is changing as it becomes more feasible (and affordable) to directly measure CO2 by means of compact, portable multisensor gas detectors equipped with miniaturised infrared sensors for the direct measurement of this gas.