Skip directly to local search Skip directly to A to Z list Skip directly to navigation Skip directly to site content Skip directly to page options
CDC Home


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: 10102-44-0; Chemical Formula: NO2

Both the ACGIH and NIOSH have recommended occupational limits for nitrogen dioxide. The current ACGIH recommendation is for a 3 ppm TWA and a 5 ppm STEL. The NIOSH REL is 1 ppm as a 15-minute short-term limit. OSHA's former PEL was 5 ppm as a ceiling value. The Agency proposed, and the final rule establishes, a permissible exposure limit for nitrogen dioxide of 1 ppm as a 15-minute STEL. NIOSH (Ex. 8-47, Table N1) agreed with the selection of this PEL. Nitrogen dioxide is a reddish-brown gas.

The previous ACGIH TLV of 5 ppm as a ceiling concentration (the basis for the former OSHA limit) was based primarily on the animal studies of Gray, MacNamee, and Goldberg (1952/ Ex. 1-154), Gray, Goldberg, and Patton (1954/Ex. 1-165), and Wagner, Duncan, Wright, and Stokinger (1965/Ex. 1-102). Gray, MacNamee, and Goldberg (1952/Ex. 1-154), and Gray, Goldberg and Patton (1954/Ex. 1-165) demonstrated lung injury among rats exposed for eight or more weeks to an 8-ppm concentration of a mixture of NO2 and nitric acid, but these authors did not see such lesions in rats exposed for six months to 4-ppm concentrations of this mixture. Wagner, Duncan, Wright, and Stokinger (1965/Ex. 1-102) reported transient, mild, acute effects but no adverse chronic effects in rats exposed to 1 ppm, 5 ppm, or 25 ppm pure NO2 for 18 months. The ACGIH's recommendation that the 5-ppm TLV be defined as a ceiling rather than as an 8-hour TWA was based on reports that NO2 accelerated lung tumor development among lung-tumor-susceptible mice; in the late 1960s, the ACGIH believed that a TLV-ceiling value would minimize the risk of accelerating lung tumor development.

The current ACGIH TLVs for NO2 are a 3-ppm 8-hour TWA and a 5-ppm STEL, and they are based on human studies that indicate that normal respiratory function may be compromised at exposures below the current OSHA ceiling limit of 5 ppm NO2. In particular, Kosmider, Ludyga, Misiewicz et al. (1972/ Ex. 1-224) reported a slight reduction in vital capacity and maximum respiratory volume in 70 men exposed to 0.4- to 2.7-ppm concentrations of the oxides of nitrogen six to eight hours daily for four to six years. These authors also reported an unspecified number of cases of chronic bronchitis among men in this group. Another study by Vigdortschik, Ancheeva, Matussevistch et al. (1937/Ex. 1-49) reported possible cases of chronic bronchitis and emphysema among 127 workers generally exposed below 2.8 ppm NO2; these workers were also believed to be exposed to sulfuric acid mist at levels sufficient to cause dental erosion.

The NIOSH REL for NO2 of 1 ppm as a 15-minute STEL is based on the two human studies discussed above, as well as some human studies involving short-term exposure. Abe (1967/Ex. 1-98) found a 40-percent decrease in effective lung capacity among healthy adult males 30 minutes after a 10-minute exposure to 4-to 5-ppm NO2. Expiratory and inspiratory maximum viscous resistance also increased by 92 percent after exposure. NIOSH (1976j/Ex. 1-265) concluded that Abe's results "document a definite and undesirable effect" at exposures approaching the former OSHA limit. A significant decrease in carbon monoxide diffusing capacity was observed by Von Nieding, Krekeler, Fuchs et al. (1973/Ex. 1-770) in healthy adults exposed to 5 ppm for 15 minutes. NIOSH also cites the work of Von Nieding, Wagner, Krekeler et al. (1971/Ex. 1-1204) and by Von Nieding and Krekeler (1971/Ex. 1-1175), who reported significant increases in airway resistance among 88 chronic bronchitis patients after a 15-minute exposure to a concentration of NO2 as low as 1.5 ppm. NIOSH (1976j/ Ex. 1-265) concluded that the specific concentration of NO2 required to produce pulmonary changes in normal, healthy adults is unknown, but "is likely to be about the same or perhaps a slightly higher concentration than the one inducing pulmonary changes in humans with existing chronic bronchitis" (1.5 ppm). Therefore, NIOSH recommended a 1-ppm 15-minute short-term limit for nitrogen dioxide. To provide additional support for a short-term rather than a TWA limit, NIOSH cites several animal studies that indicate that the toxic effects associated with exposure to NO2 are primarily determined by peak, and not average, concentrations of exposure.

In its posthearing submission, NIOSH (Ex. 150, Comments on Nitrogen Dioxide) reported on a recent study by Mohsenin (1988, as cited in Ex. 150) in which no significant pulmonary function changes were noted among 18 healthy subjects exposed to NO2 for one-hour periods. NIOSH (Ex. 150) noted that, in 1984, the World Health Organization, after an independent review of cross-sectional occupational health surveys, recommended a short-term occupational exposure limit of 1.8 mg/m3 (0.9 ppm) for NO2 and 8-hour TWA limit of 0.9 mg/m3 (0.45 ppm). NIOSH also reviewed studies that suggest that NO2 is mutagenic and is embryotoxic and teratogenic in rats.

The AFL-CIO (Ex. 194) supported OSHA's proposed limit for NO 2. However, several commenters (Exs. 3-349, 3-670, 3-739, 3-666, 3-1144, 133, and 133A) objected to OSHA's proposal to establish the NIOSH REL for NO2 in the final rule, believing that the ACGIH TLVs of 3 ppm TWA and 5 ppm STEL were sufficiently protective. For example, David L. Van Lewen, Manager of Industrial Hygiene for BASF, referred to the Von Nieding et al. (1971/Ex. 1-1204) study as evidence that a 1-ppm short-term limit was not necessary:

    The Von Nieding study (1971/Ex. 1-1204) of chronic bronchitis patients...showed increased airway resistance when exposed to concentrations of nitrogen dioxide between 1.5 and 5.0 ppm. Lower concentrations had no significant effect. When this sensitive population does not show significant effects at concentrations below 1.5 ppm, it is not reasonable to set a workplace limit at a STEL of 1.0 ppm (Ex. 3-666).

Mr. Lawrence J. Ogden, representing the Intestate Natural Gas Association of America (INGAA) (Ex. 3-739), and Mr. Vincent D. Lajiness of the American Natural Resources Company (ANR) (Ex. 3-670) criticized the studies described in the NPRM, and in particular the Von Nieding et al. (1971/Ex. 1-1204) study; both rulemaking participants indicated that the data base developed by EPA to establish EPA's ambient air quality limit for NO2 is superior. Mr. Ogden stated that [a] far more extensive body of studies about NO2 health effects is available than is cited by OSHA in the proposed rulemaking. Much of this literature has been pulled together by the Environmental Protection Agency (EPA). The EPA review and assessment of scientific studies on the health effects of NO2 exists in the EPA NO2 Criteria Document and the Staff Memorandum, which have been provided to the record.... EPA's action should be addressed in the OSHA proposal because it represents a more recent evaluation than NIOSH, a far more concentrated Agency evaluation by research and regulatory personnel, and an extensive scientific peer review process. As a result of its evaluation, EPA decided in 1982 that evidence was insufficient that a short-term air standard for NO2 was needed. This conclusion has been re-examined annually by EPA and checked against the latest health studies related to NO2 effects (Ex. 3-739, pp. 7, 10). Mr. Ogden also referred OSHA to the 1979 National Academy of Science's Committee on Toxicology report on the health evidence for NO2. The EPA staff memorandum referred to by Mr. Ogden is the 1982 Office of Air Quality Planning and Standards (OAQPS) Staff Paper on the assessment of scientific information on NO2 (EPA/450/5-82/002, Ex. 3-2e). This document summarizes the findings expressed in EPA's Air Quality Criteria for Oxides of Nitrogen (EPA/600/8-82/026, Ex. 3-2f). Based on these reports, EPA issued a final rule retaining its 1971 ambient air quality standard for NO2, which is 0.053 ppm (100 mg/m3) as averaged over a one-year period.

The EPA Staff Paper concludes that the 1971 Von Nieding et al. (Ex. 1-1204) study "provides convincing evidence that chronic bronchitics exposed to NO2 concentrations of 1.6 ppm or greater for approximately 3 minutes experience increases in airway resistance" (Ex. 3-2e, p. 18). A number of other studies were cited by EPA in which healthy adults were exposed to NO2 concentrations in the range of 0.5 to 2.5 ppm. Folinsbee, Horvath, Bedi, and Delehunt (1978, as cited in Ex. 3-2e) reported no significant physiological changes in healthy adults exercising for up to one hour during a two-hour exposure to 0.6 ppm NO2. Suzuki and Ishikawa (1965, as cited in Ex. 3-2e) reported a 50-percent increase in inspiratory flow resistance in healthy adults 10 minutes after a 10-minute exposure to an NO2 concentration between 0.7 and 2 ppm.

Small changes in pulmonary function and a slight increase in the prevalence of respiratory symptoms occurred among healthy adults exposed to 1 ppm NO2 for two hours (Hackney, Thiede, Linn et al. 1978, as cited in Ex. 3-2e). Beil and Ulmer (1976, as cited in Ex. 3-2e) reported a statistically significant increase in airway resistance among healthy adults following exposure to 2.5 ppm NO2 for two hours, but not following exposure to 1 ppm. Based on their review of these data, the EPA staff paper concluded:

    [T]he lowest level of NO2 exposure that credible studies have associated with measurable impairment of pulmonary function appears to be in the range of 1.0 -1.6 ppm....Several CASAC members have expressed concern that a standard designed to prevent relatively small changes in pulmonary function (such as those observed in the Suzuki and Ishikawa (1965) and Von Nieding et al. (1971) studies) from occurring more than once per year would be unnecessarily stringent. The CASAC members indicated that they were more concerned about the health implications of repeated exposures to the peak concentrations observed in the two studies than the effects associated with a single exposure (Ex. 3-2e, p. 18).

EPA also reviewed research reports that have become available since publication of the EPA Criteria Document and Staff Paper, in particular the reports by Linn and Hackney (1983 and 1984) that reported finding no pulmonary effects among exercising healthy adults and asthmatics exposed to 4 ppm NO2. EPA concluded that these studies present "mixed and conflicting results," and that a more complete assessment of these studies was not possible because "many...have yet to be published in the peer-reviewed scientific literature" (50 FR 25535/Ex. 3-2d).

Regarding EPA's decision not to issue a short-term ambient-air-quality limit for NO2, a review of the preamble to EPA's final rule shows that EPA addressed this issue only with regard to existing ambient short-term levels of NO2. EPA reported that, under its current 0.053-ppm annual average limit, the vast majority of metropolitan areas would be expected to have fewer than two days with a daily maximum hourly value of 0.2 ppm or greater (50 FR 25536/Ex. 3-2d). Because of the uncertainties regarding the evidence for adverse effects at NO2 concentrations below 1 ppm, EPA concluded that the current annual average limit would "provide some measure of protection against possible short-term health and welfare effects" (50 FR 25537/Ex. 3-2d). It is also worth noting that, since 1971, EPA has designated a 2-ppm (one-hour average) level for NO2 as representing a "significant harm level" requiring an emergency response. Thus, OSHA finds that EPA's recent actions and reasoning regarding a short-term ambient limit for NO2 supports the establishment of 1 ppm as a STEL.

OSHA has also reviewed the most recent analysis of NO2 toxicity conducted by the National Academy of Science's (NAS) Committee on Toxicology for the Department of Defense (Emergency and Continuous Exposure Guidance Levels for Selected Airborne Contaminants, Vol. 4, pp. 83-96, National Academy Press 1985); the earlier 1979 review was cited by Mr. Ogden of the INGAA. In its more recent review, the NAS concluded that exposures to NO2 at levels between 0.5 and 1.5 ppm have demonstrated "little or no persistent change in pulmonary function" (NAS 1985, p. 89). The NAS Committee on Toxicology recommended short-term public emergency guidance levels (SPEGLs) for NO2 of 1 ppm, averaged over a 60-minute period, and 0.12 ppm as an 8-hour average.

OSHA concludes that the evidence reviewed by the EPA and the NAS and the several studies referenced by EPA and NAS reaffirm the conclusion expressed by NIOSH in its 1976 criteria document (NIOSH 1976j/Ex. 1-265) that "humans with normal respiratory function may be acutely affected by exposure [to NO2] at or below...[5 ppm]. Furthermore, the conditions of workers with chronic respiratory diseases, such as chronic bronchitis, may be aggravated by exposure to nitrogen dioxide at a concentration of approximately one-third of the current Federal standard" (NIOSH 1976j/Ex. 1-265, p. 117). In addition to the studies by Von Nieding et al. (1971/Ex. 1-1204) and Abe (1967/Ex. 1-98) described in the NPRM, both EPA (Ex. 3-2e) and the NAS (1985) cite a number of other published reports that show that exposure to NO2 at concentrations below 5 ppm causes increased airway resistance in both healthy adults and chronic bronchitics; these reports include the studies of Suzuki and Ishikawa (1965), Rokaw et al. (1968), Stresemann and Von Nieding (1970), and Beil and Ulmer (1976). Furthermore, these and other studies cited by EPA (Ex. 3-2e) and the NAS (1985) generally indicate that exposure to 1 ppm NO2 is not normally associated with significant airway resistance, even among workers with already-compromised respiratory function.

Thus, OSHA concludes that the former 5-ppm ceiling limit for NO2 is not sufficient to protect employees from experiencing increased airway resistance, and that establishing the ACGIH TLVs of 3 ppm TWA and 5 ppm STEL, as suggested by rulemaking participants (Exs. 3-349, 3-670, 3-739, 3-666, and 3-1144), would not provide sufficient protection. OSHA also concludes that the risk of increased airway resistance would be substantially reduced by promulgation of a 1-ppm short-term limit for NO2; a short-term limit is clearly indicated for NO2 since all of the studies cited above demonstrate that increased airway resistance is associated with exposure to NO2 for durations of between three minutes and two hours. OSHA considers the increased airways resistance caused by exposure to NO2 to be a material impairment of health. Therefore, to reduce the significant risk associated with short-term exposure to NO2, the Agency is establishing a 1-ppm limit, averaged over a 15-minute period, for nitrogen dioxide in the final rule.


Contact Us: The U.S. Government's Official Web PortalDepartment of Health and Human Services
Centers for Disease Control and Prevention   1600 Clifton Rd. Atlanta, GA 30333, USA
800-CDC-INFO (800-232-4636) TTY: (888) 232-6348 - Contact CDC–INFO