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CARBON MONOXIDE

OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project extracted from 54FR2332 et. seq. This rule was remanded by the U.S. Circuit Court of Appeals and the limits are not currently in force.

CAS: 630-08-0; Chemical Formula: CO

OSHA's former limit for carbon monoxide was 50 ppm as an 8-hour TWA. The ACGIH has a TLV-TWA of 50 ppm with a TLV-STEL of 400 ppm. NIOSH (1973d/Ex. 1-237) recommends an 8-hour TWA limit of 35 ppm with a 200-ppm ceiling. The proposed PEL and ceiling were 35 ppm and 200 ppm, respectively; NIOSH (Ex. 8-47, Table N1) concurs that these limits are appropriate, and they are established in the final rule. Carbon monoxide is a flammable, colorless, practically odorless gas.

Carbon monoxide readily combines with hemoglobin to form carboxyhemoglobin (COHb). Excessive accumulations of COHb cause hypoxic stress in healthy individuals as a result of the reduced oxygen-carrying capacity of the blood. In patients with cardiovascular disease, such stress can further impair cardiovascular function. The ACGIH (1986/Ex. 1-3) cites a number of studies showing that exposure to 50 ppm TWA carbon monoxide generally results in COHb levels of 8 to 10 percent, and that such levels are not generally associated with overt signs or symptoms of health impairment in healthy individuals under nonstressful conditions. However, the ACGIH comments that a TLV of 25 ppm, which results in COHb levels of 4 percent or less, may be necessary to protect workers with cardiovascular disease, because this condition places workers at higher risk of serious cardiovascular injury (ACGIH 1986/ Ex. 1-3, p. 106). The NIOSH recommendation of 35 ppm TWA is also aimed at protecting workers with chronic heart disease; NIOSH believes that such workers should not be allowed to have carboxyhemoglobin levels that approach 5 percent. The rationale for the ACGIH's recommendation of a 400-ppm TLV-STEL for CO is not entirely clear, but may be based on a study by Schulte (1964/Ex. 1-366), which stated that exposure to 100 ppm carbon monoxide for four hours is excessive.

Several commenters (Exs. 133, 188, 3-675, 3-673, L3-1330, 3-902, 3-660, 3-349, 3-1123, and 129) submitted comments on the Agency's proposed limits for carbon monoxide. Some of these commenters (Exs. 3-675 and 3-673) were concerned that the revised limits would have serious economic impacts on their industries (electric utilities, steel, and nonferrous foundries). However, OSHA has determined that it is feasible for facilities in these sectors to comply with the proposed CO limits (see Section VII of the preamble).

Many rulemaking participants questioned the health basis for lowering the former CO limit of 50 ppm as an 8-hour TWA to 35 ppm and supplementing this limit with a 200-ppm STEL (Exs. 133A, 188, 3-660, 3-349, 3-1123, and 129). These commenters pointed out that the discussion of CO's health effects in the preamble to the proposal (53 FR 21171) stated that the carboxyhemoglobin levels associated with CO exposures of 50 ppm "are not associated with toxic effects in healthy individuals." According to the American Iron and Steel Institute (Ex. 3-1123), whose remarks were typical of the views of these commenters:

  • The proposed PEL should not be adopted because there is not adequate evidence that exposure to carbon monoxide at levels of 50 ppm TWA poses a significant risk to workers with heart or pulmonary disease...(Ex. 3-1123, p. 23).

H.K. Thompson, Corporate Industrial Hygiene Manager of Caterpillar, Inc. (Ex. 3-349), stated:

  • PELs or TLVs are not set to protect individuals with chronic heart disease. In our industry we transfer people with disabilities to jobs where the risk for them is minimal (Ex. 3-349, p. 3).

In response to these commenters, OSHA quotes the ACGIH (1986/Ex. 1-3):

  • Each molecule of CO combining with hemoglobin reduces the oxygen carrying capacity of the blood and exerts a finite stress on man. Thus, it may be reasoned that there is no dose of CO that is not without an effect on the body. Whether that effect is physiologic or harmful depends upon the dose of CO and the state of health of the exposed individual. The body compensates for this hypoxic stress by increasing cardiac output and blood flow to specific organs, such as the brain or the heart. When this ability to compensate is overpowered or is limited by disease, tissue injury results [emphasis added]. Exposure to CO sufficient to produce COHb saturations in the 3-5% range impairs cardiovascular function in patients with cardiovascular disease and in normal subjects.... The primary effect of exposure to low concentrations of CO on workmen results from the hypoxic stress secondary to the reduction in the oxygen-carrying capacity of blood....workmen with significant disease, both detected and undetected, may not be able to compensate adequately and are at risk of serious injury. Forsuch workers, a TLV of 25 ppm...might be necessary. Even such aconcentration might be detrimental to the health of some workers who might have far advanced cardiovascular disease....It would appear to the Committee that the timeweighted TLV of 50 ppm for carbon monoxide might also be too high under conditions of heavy labor, high temperatures, or at high elevations (ACGIH 1986/Ex. 1-3, p. 106).

Thus, the ACGIH also regards a lower limit for CO as necessary to protect workers with cardiovascular or pulmonary disease or those working under stressful conditions.

NIOSH (Ex. 150, Comments on Carbon Monoxide) submitted a substantial amount of posthearing evidence demonstrating the significant risk associated with CO exposure, particularly with respect to coronary heart disease. The following studies are particularly relevant to this issue. Atkins and Baker (1985, as cited in NIOSH/Ex. 150) report the case of two workers with preexisting coronary artery disease who died after exposure to CO at work. A study of firefighters in Los Angeles (Barnard and Weber 1979, as cited in NIOSH/Ex. 150) suggests that CO exposure during firefighting may be responsible for the high incidence of heart disease in firefighters; peak exposures during fire fighting were as high as 3000 ppm CO, with 40 percent of peak values in the 100- to 500-ppm CO range. A prevalence study by Hernberg et al. (1976, as cited in NIOSH/ Ex. 150) reports a clear dose-response relationship between CO exposure and angina pectoris in foundry workers. Stern and co-workers (1981, as cited in NIOSH/Ex. 150) suggest that the slight overall excess of deaths in motor vehicle examiners caused by cardiovascular disease is attributable to chronic exposure to low levels of CO (10 to 24 ppm as an 8-hour TWA). The AFL-CIO's posthearing comment (Ex. 194) agrees that the comments submitted by NIOSH are persuasive evidence of the need to reduce the 8-hour TWA for CO.

NIOSH also submitted recent data on carbon monoxide's reproductive effects and on its neurotoxic/behavioral effects. Based on a review of all of these studies, NIOSH concludes that "[t]he new data suggest a reevaluation of the REL and strongly support the inference that there is a significant risk of material impairment to health at the...[former] 50-ppm PEL which will be reduced by the proposed 35-ppm PEL" (Ex. 150, Comments on Carbon Monoxide).

OSHA notes that cardiovascular disease (detected or undetected) and pulmonary impairment are widespread in the general population in this country, and that workers constitute a significant part of this general population. In addition, workers regularly encounter complex and stressful situations at work, including heat stress, jobs demanding heavy exertion, and tasks requiring both judgment and motor coordination.

The AISI (Ex. 129) submitted an article (Redmond, Emes, Mazumdar et al. 1977, "Mortality of Steelworkers Employed in Hot Jobs") to OSHA which, in the opinion of the AISI, demonstrates that steelworkers who are exposed to high heat (and ostensibly also to CO) do not have coronary heart disease. Based on this article, the AISI asks that the steel industry be exempted from the revised PEL for CO. OSHA finds the article submitted by the AISI unconvincing on the point at issue; the article is not primarily concerned with CO exposures but with heat stress and, further, does not include a large enough sample to demonstrate the absence of an effect. Moreover, OSHA is establishing limits that will apply to all of general industry; the Agency does not customarily set standards based on the particular conditions prevailing in a specific operation or industry.

However, some evidence has been submitted by the AISI (Ex. 129) to the effect that the ceiling limit cannot regularly be achieved with engineering and work practice controls in specific operations in SIC 33. These operations are: blast furnace operations, vessel blowing at basic oxygen furnaces, and sinter plant operations. There is no evidence to the contrary in the record. For these operations, OSHA will therefore permit more flexibility in the use of respirators. The burden of proof will not be on employers to demonstrate that compliance with the ceiling by means of engineering and work practice controls is infeasible in any compliance action involving these operations in SIC 33.

There may be a few other operations that fall into this same category; however, the record is unclear on this point. Based on an appropriate showing pursuant to the OSH Act, OSHA will favorably consider requests for variances for specific operations in SIC 33 involving methods of compliance for the ceiling limit. Of course, all requests for variances or any other matters will be considered based on their merits.

OSHA thus finds that the reduced 8-hour TWA of 35 ppm for carbon monoxide is needed to reduce the significant risk of serious injury that has repeatedly been demonstrated to result from overexposure to CO in a host of occupational environments. The Agency concludes that a ceiling of 200 ppm is necessary to ensure that peak CO exposures do not reach levels demonstrated to be hazardous and that overall full-shift exposures remain under good control. In the absence of a ceiling, concentrations approaching the Immediately-Dangerous-to-Life-or-Health (IDLH) level of 1500 ppm could occur.

In the final rule, OSHA is establishing an 8-hour TWA of 35 ppm and a ceiling of 200 ppm as the PELs for carbon monoxide to ensure that employee COHb levels are maintained at or below 5 percent, in order to protect those workers at greater risk because of cardiovascular or pulmonary impairment. In addition, these revised limits will protect healthy workers who must work in environments involving exertion, heat stress, or other strenuous conditions. The Agency has determined that these limits will substantially reduce the significant occupational risk associated with both chronic and peak CO exposures in the workplace. OSHA concludes that the hypoxic stress associated with overexposures to carbon monoxide clearly constitutes a material impairment of health and functional capacity.

 

 
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