tory workers in measuring the air concentrations of chemicals.

3.C.3.2 Corrosive Substances, Irritants, and Allergens

Lethal dose and other quantitative toxicological parameters generally provide little guidance in assessing the risks associated with corrosives, irritants, and allergens (sensitizers), because these toxic substances exert their harmful effects locally. When planning an experiment that will involve the use of corrosive substances, basic prudent handling practices should be reviewed to ensure that the skin, face, and eyes are protected adequately by the proper choice of corrosion-resistant gloves and protective clothing and eyewear, including, in some cases, face shields. Similarly, LD50 data are not an indicator of the irritant effects of chemicals, and therefore special attention should be paid to the identification of irritant chemicals by consulting LCSSs, MSDSs, and other sources of information. Allergens are another class of acute toxicants whose effects are not included in LD50 data. Individuals may differ widely in their tendency to become sensitized to allergens, so it is prudent to regard compounds with a proven ability to cause sensitization as highly toxic agents. Once a person has become sensitized to an allergen, subsequent contact can lead to immediate or delayed allergic reactions. Furthermore, sensitization to a specific substance can persist for many years. Because an allergic response can be triggered in a sensitized individual by an extremely small quantity of the allergen, it may occur despite personal protection measures that are adequate to protect against the acute effects of chemicals. Laboratory workers should be alert for signs of allergic responses to chemicals.

3.C.3.3 Carcinogens

Because cancer is such a widespread cause of human mortality, and because exposure to chemicals may play a significant role in the onset of cancer, a great deal of attention has been focused on evaluation of the carcinogenic potential of chemicals. However, the vast majority of the substances involved in research, especially in laboratories concerned primarily with the synthesis of novel compounds, have not been tested for carcinogenicity. Compounds that are known to pose the greatest carcinogenic hazard are referred to as "select carcinogens," and they constitute another category of substances that must be handled as "particularly hazardous substances" according to the OSHA Laboratory Standard. A select carcinogen is defined in the OSHA Laboratory Standard as a substance that meets one of the following criteria:

  1. It is regulated by OSHA as a carcinogen.

  2. It is listed as "known to be a carcinogen" in the latest Annual Report on Carcinogens issued by the National Toxicology Program (NTP) (U.S. DHHS, 1991).

  3. It is listed under Group 1 ("carcinogenic to humans") by the International Agency for Research on Cancer (IARC).

  4. It is listed under IARC Group 2A ("probably carcinogenic to humans") or 2B ("possibly carcinogenic to humans"), or under the category "reasonably anticipated to be a carcinogen'' by the NTP, and causes statistically significant tumor incidence in experimental animals in accordance with any of the following criteria: (a) after inhalation exposure of 6 to 7 h per day, 5 days per week, for a significant portion of a lifetime to dosages of less than 10 mg/m3; (b) after repeated skin application of less than 300 mg/kg of body weight per week; or (c) after oral dosages of less than 50 mg/kg of body weight per day.

Table 3.4 lists some representative substances that meet the above criteria for classification as OSHA select carcinogens. These chemicals are classified as particularly hazardous substances and should be handled using the basic prudent practices given in Chapter 5, section 5.C, supplemented by the additional special practices outlined in section 5.D. Work with compounds that are possible human carcinogens may or may not require the additional precautions given in section 5.D. For these compounds, the LCSS should indicate whether or not the substance meets the additional criteria listed in category 4 and must therefore be treated as a select carcinogen. If an LCSS is not available, consultation with a safety professional such as a chemical hygiene officer may be necessary in order to determine whether a possible human carcinogen should be classified as a particularly hazardous substance.

Many chemical substances are encountered in the laboratory for which there is no animal test or human epidemiological data on carcinogenicity. In these cases, workers must evaluate the potential risk that the chemical in question is a carcinogenic substance. This determination can sometimes be made on the basis of knowledge of the specific classes of compounds and functional group types that have previously been correlated with carcinogenic activity. For example, chloromethyl methyl ether is a known human carcinogen and therefore is regarded as an OSHA select carcinogen requiring the handling procedures outlined in section 5.D. On the other hand, the carcinogenicity of ethyl chloromethyl ether and certain other alkyl chloromethyl ethers is not established, and these substances do not necessarily have to be treated as select carcinogens. However, because of the chemical similarity of

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