are chronically toxic substances whose adverse effects are not immediately apparent. At the present time, because of the limited data available in this area, significant uncertainties attend the assessment of risks associated with work with neurotoxic substances.

3.C.2.8 Toxins Affecting Other Organs

Target organs outside the reproductive and neurological systems can also be affected by toxic substances found in the laboratory. Most of the chlorinated hydrocarbons, benzene, other aromatic hydrocarbons, some metals, carbon monoxide, and cyanides, among others, can produce one or more effects in target organs. Such an effect may be the most probable result of exposure to the particular chemical. Although this chapter does not include specific sections on liver, kidney, lung, or blood toxins, many of the LCSSs mention those effects in the toxicology section.

3.C.3 Assessing Risks Due to the Toxic Effects of Laboratory Chemicals

The first step in assessing the risks associated with a planned laboratory experiment involves identifying which of the chemicals to be used in the proposed experiment are potentially hazardous substances. The OSHA Laboratory Standard (29 CFR 1910.1450) defines a hazardous substance as

a chemical for which there is statistically significant evidence based on at least one study conducted in accordance with established scientific principles that acute or chronic health effects may occur in exposed employees. The term ''health hazard" includes chemicals which are carcinogens, toxic or highly toxic agents, reproductive toxins, irritants, corrosives, sensitizers, hepatotoxins, nephrotoxins, neurotoxins, agents which act on the hematopoietic systems, and agents which damage the lungs, skin, eyes, or mucous membranes.

The OSHA Laboratory Standard further requires that certain chemicals be identified as "particularly hazardous substances" and handled using special additional procedures. Particularly hazardous substances include chemicals that are "select" carcinogens (those strongly implicated as a potential cause of cancer in humans), reproductive toxins, and compounds with a high degree of acute toxicity. Highly flammable and explosive substances make up another category of hazardous compounds, and the assessment of risk for these classes of chemicals is discussed in section 3.D. This section considers the assessment of risks associated with work with specific classes of toxic chemicals, including those that pose hazards due to acute toxicity and chronic toxicity.

3.C.3.1 Acute Toxicants

Acute toxicity is the ability of a chemical to cause a harmful effect after a single exposure. Acutely toxic agents can cause local toxic effects, systemic toxic effects, or both, and this class of toxicants includes corrosive chemicals, irritants, and allergens (sensitizers). Among the most useful parameters for assessing the risk of acute toxicity of a chemical are its LD50 and LC50 values, selected with due regard for the possible routes of exposure. In interpreting these lethal dose and lethal concentration values, the following points should be considered. The LD50 is the mean dose causing death in animals, and it should be recognized that the minimum dose causing death in some proportion of the test population will be much lower, with significant illness or harm short of lethality probably occurring at even lower doses. Finally, it is assumed that the lethal dose for animals (usually rodents) is an appropriate predictor of the lethal dose in humans.

In assessing the risks associated with acute toxicants, it is useful to classify a substance according to the acute toxicity hazard level as shown in Table 3.1. LD50 values can be found in the Laboratory Chemical Safety Summary (LCSS) or MSDS for a given substance, and in references such as Sax's Dangerous Properties of Industrial Materials (Lewis, 1992), Sigma-Aldrich Library of Chemical Safety Data (Lenga, 1988), and A Comprehensive Guide to the Hazardous Properties of Chemical Substances (Patnaik, 1992). Table 3.2 relates test animal LD50 values expressed as milligrams or grams per kilogram of body weight to the probable human lethal dose, expressed in easily understood units, for a 70-kilogram (kg) person.

Special attention must be given to any substance classified according to the above criteria as having a high level of acute toxicity hazard. Chemicals with a high level of acute toxicity make up one of the categories of "particularly hazardous substances" defined by the OSHA Laboratory Standard. Any compound rated as highly toxic in Table 3.1 meets the OSHA criteria for handling as a particularly hazardous substance.

Table 3.3 lists some of the most common chemicals with a high level of acute toxicity that are encountered in the laboratory. These compounds must generally be handled using the additional procedures outlined in Chapter 5, section 5.D. In some circumstances, it may not be necessary to employ all of these special precautions, such as when the total amount of an acutely toxic substance to be handled is a small fraction of the harmful dose. It is an essential part of prudent experiment planning to determine whether a chemical with a high

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