ticular tissue or organ without affecting others. The affected organ (e.g., liver, lungs, kidney, and central nervous system) is referred to as the target organ of toxicity, although it is not necessarily the organ where the highest concentration of the chemical is found. Hundreds of systemic toxic effects of chemicals are known; they result from single (acute) exposures or from repeated or long-duration (chronic) exposures that become evident only after a long latency period.

Toxic effects are classified as either reversible or irreversible. Reversible toxicity is possible when tissues have the capacity to repair toxic damage, and the damage disappears after cessation of exposure. Irreversible damage, in contrast, persists after cessation of exposure. Recovery from a burn is a good example of reversible toxicity; cancer is considered irreversible, although appropriate treatment may reduce the effects in this case.

Laboratory chemicals are grouped into several classes of toxic substances, and many chemicals display more than one type of toxicity. The first step in assessing the risks associated with a planned laboratory experiment involves identifying which chemicals 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 that are carcinogens, toxic or highly toxic agents, reproductive toxins, irritants, corrosives, sensitizers, hepatotoxins, nephrotoxins, neurotoxins, agents that act on the hematopoietic systems, and agents that damage the lungs, skin, eyes, or mucous membranes.

The OSHA Laboratory Standard further requires that certain chemicals be identified as particularly hazardous substances (commonly known as PHSs) and handled using special additional procedures. PHSs 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. When working with these substances for the first time, it is prudent to consult with a safety professional prior to beginning work. This will provide a second set of trained eyes to review the safety protocols in place and will help ensure that any special emergency response requirements can be met in the event of exposure of personnel to the material or accidental release.

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 4.D. This section considers the assessment of risks associated with specific classes of toxic chemicals, including those that pose hazards due to acute toxicity and chronic toxicity.

The following are the most common classes of toxic substances encountered in laboratories.

4.C.2.1 Acute Toxicants

Acute toxicity is the ability of a chemical to cause a harmful effect after a single exposure. Acutely toxic agents cause local toxic effects, systemic toxic effects, or both, and this class of toxicants includes corrosive chemicals, irritants, and allergens (sensitizers).

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 4.1. LD50 values can be found in the LCSS or MSDS for a given substance, and in references such as Sax’s Dangerous Properties of Industrial Materials (Lewis, 2004), A Comprehensive Guide to the Hazardous Properties of Chemical Substances, 3rd Edition (Patnaik, 2007), and the Registry of Toxic Effects of Chemical Substances (RTECS) (NIOSH). Table 4.2 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-kg person.

Special attention is 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 PHSs defined by the OSHA Laboratory Standard. Any compound rated as highly toxic in Table 4.1 meets the OSHA criteria for handling as a PHS.

Table 4.3 lists some of the most common chemicals with a high level of acute toxicity that are encountered in the laboratory. These compounds are handled using the additional procedures outlined in Chapter 6,

TABLE 4.1 Acute Toxicity Hazard Level

Hazard Level Toxicity Rating Oral LD50 (rats, per kg) Skin Contact LD50 (rabbits, per kg) Inhalation LC50 (rats, ppm for 1 h) Inhalation LC50 (rats, mg/m3 for 1 h)
High Highly toxic <50 mg <200 mg <200 <2,000
Medium Moderately toxic 50 to 500 mg 200 mg to 1 g 200 to 2,000 2,000 to 20,000
Low Slightly toxic 500 mg to 5 g 1 to 5 g 2,000 to 20,000 20,000 to 200,000

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