so as to be considered in the development of allowable concentrations used in the hazard index calculation.

Guidelines for human exposure to HCN have been developed by several agencies. Following a review of the literature on occupational exposures, a National Research Council Subcommittee concluded that a 1-hour exposure to HCN at 8 ppm should cause no more than mild headache in healthy adults (Lam and Wong 2000). The American Conference of Governmental Industrial Hygienists Ceiling value, a concentration that should not be exceeded during any part of a working exposure, is 4.7 ppm (ACGIH 1996). The National Institute for Occupational Safety and Health (NIOSH) short-term exposure limit is also 4.7 ppm, and the Immediately Dangerous to Life and Health (IDLH), a 30-minute exposure, is 50 ppm (NIOSH 2005). The Occupational Safety and Health Administration Permissible Exposure Limit (OSHA) is 10 ppm (NIOSH 2005). A guideline value considered safe for the general public includes the 1-hour Acute Exposure Guideline Level-1 (AEGL-1) of 2.0 ppm (NRC 2002).

CARBON MONOXIDE

Data on correlations between exposure concentrations and levels of blood COHb are lacking. A number of sources including Coburn and Forman (1987), WHO (1999), and EPA 2005 reviewed COHb levels in the blood of smokers and symptoms in healthy adults associated with COHb levels in the blood. A physiologic background concentration of 0.5-0.8% is due to endogenous formation. A concentration of 5% COHb may be found in one pack/day smokers, and concentrations of 10-15% in two and three pack/day smokers. Up to 10% COHb has no appreciable effect except shortness of breath on vigorous exertion. Concentrations of 10-20% may result in symptoms such as headache. As concentrations of COHb increase from 30 to 70%, successive symptoms reported are headache, fatigue, dizziness, confusion, unconsciousness, and possibly death. A concentration of 80% COHb is rapidly fatal. Clinical studies with humans indicate that a COHb of about 34-56% are not lethal in healthy adults EPA 2005. CO levels in homes are usually lower than 9 ppm, but may range up to 30 ppm in homes with wood stoves. Levels inside motor vehicles are generally around 9-25 ppm, but may range up to 35 ppm EPA 2005.

In a study with cynomolgus monkeys exposed to 900 ppm CO, no signs of intoxication occurred during the first 20-25 minutes (corresponding to COHb of about 16-21%) (Purser and Berrill 1983). At 25 minutes, the animals’ performance in a behavioral test was significantly decreased. At 30 minutes the monkeys were lying down. In rodent studies, incapacitation in rats exposed to CO occurred at 5 and 35 minutes at concentrations of 5706 and 1902 ppm, respectively. Blood COHb values were 81 and 71%, respectively (Chaturvedi et al. 1995). Hartzell et al. (1985a,b) reported a higher value, incapacitation at 8000 ppm at 5.1 minutes. Blood levels were not reported. Lethal (LC50) data in the rat ranges from a 5-minute value of 10,000-14,000 ppm to a 60-minute value of approximately 4000 ppm EPA 2005.

Guidelines for human exposure to carbon monoxide include the following: ACGIH 8-hour TLV-TWA of 25 ppm; NIOSH 8-hour TWA of 35 ppm and IDLH of 1200 ppm; OSHA PEL of 50 ppm; 1-hour Emergency Response Planning Guidelines (ERPG-1) of 200 ppm; and the NRC Emergency Exposure 1-hour Guidance Level of 400 ppm.



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