Guidance for Laboratory Students and Occupational Workers

The most important factor is the dose-time relationship. The dose-time relationship forms the basis for distinguishing between two types of toxicity acute and chronic. Acute toxicity of a chemical refers to its ability to inflict systemic damage as a result (in most cases) of a one-time exposure to relative large amounts of the chemical. In most cases, the exposure is sudden and results in an emergency situation. 

Chronic toxicity refers to a chemical s ability to inflict systemic damage as a result of repeated exposures over a prolonged time period, to relatively low levels of the chemical. Some chemicals are extremely toxic and are known primarily as acute toxins (e.g., hydrogen cyanide) some are known primarily as chronic toxins (e.g., lead). Other chemicals, such as some chlorinated solvents, can cause either acute or chronic effects. 

Toxic effects of chemicals can range from mild and reversible (e.g., headache from inhaling petroleum naphtha vapors that disappears with fresh air) to serious and irreversible (e.g., liver or kidney damage from excessive exposures to chlorinated solvents). Toxic effects from chemical exposure depend on the severity of the exposures. Greater exposure and repeated exposure generally lead to more severe effects. 

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