Toxic chemicals entering the body get distributed to tissues where they may be metabolized. The reactive metabolites may interact covalently with target molecules to produce toxicity. Local toxicity occurs at the site of exposure. An example is the denaturation of proteins by caustic substances. Systemic toxicity usually requires absorption and distribution of the toxicant which may function by interrupting a specific biochemical pathway. Some toxic effects may be both local and systemic. For example, exposure to tetraethyl lead can cause skin irritation, and its absorption into the circulation can affect the CNS. Injuries to the CNS are irreversible due to the limited capacity of brain neurons to regenerate. Injuries to organs like the liver may be reversible due to its regenerating ability. Most toxic effects are quick. For instance, excess acetylcholine at synapses, due to malathion-mediated inhibition of acetylcholinesterase, produces immediate symptoms. Some toxic chemicals, like asbestos, exhibit long latency periods before leading to mesothelioma—a cancer of the linings of certain tissues.