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Any consideration of the relative safety of a chemical must also take into account the degree to which the response to the toxicant is reversible. In other words, as the concentration of the substance decreases in the tissues and it is eliminated from the body, will the effects of the toxicant be reversed? It is known that after a single exposure (a one-time ingestion of a toxicant), the body will in time eliminate the substance. But will the biological (adverse) effects diminish over time? Reversal of adverse effects depends on the type of effect. The reversibility of a toxicity response can be categorized as readily reversible, not readily reversible, or nonreversible. Most chemical-induced effects short of death are reversible in time if the chemical subsides over time.
However, once an effect is produced, it may outlast the presence of the original chemical. An example is the compound organophosphate and its target site choline esterase. Organophosphate toxicity results in an inactivated esterase, meaning that the effects are essentially irreversible (not readily reversible), at least till the time it takes to synthesize more esterase. The body must synthesize new esterase, which might take as long as a few weeks. In reversible toxicological effects, once the chemical has been removed, the deranged system will return to its normal functional state either immediately or later, after some regeneration has occurred. Specific toxicological effects that are irreversible (nonreversible), particularly when they are life threatening, include teratogenesis, mutagenesis, and carcinogenesis. Because of the consequences, exposure to such xenobiotics must be limited. However, many substances do not clearly fall under or out of these categories; thus, toxicologists rely on a battery of in vivo and in vitro tests to determine whether a compound produces toxicity.
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