When toxic substances penetrate the human body, they disseminate to various tissues, undergoing metabolic changes. This process yields reactive metabolites that may covalently bind with specific target molecules, resulting in toxicity.

Toxicity falls into two primary categories: local and systemic.

Local toxicity appears at the exposure site, such as protein denaturation caused by caustic substances.

In contrast, systemic toxicity requires the toxic agent's absorption and distribution, potentially disrupting a specific biochemical pathway.

Certain toxins can trigger both local and systemic effects. For example, tetraethyl lead can cause skin irritation upon contact, a local effect, and disrupt the central nervous system when absorbed into the bloodstream, which is a systemic effect.

The severity and reversibility of toxic effects depend significantly on the affected organ. Damage to the central nervous system is typically irreversible due to the limited regenerative capacity of brain neurons. On the other hand, injuries to organs like the liver might be reversible because of the organs inherent regenerative abilities.

The onset of toxic effects varies from immediate to delayed. For instance, an excess of acetylcholine at synapses due to malathion-mediated inhibition of acetylcholinesterase can quickly induce symptoms. On the other hand, asbestos may have long latency periods before causing diseases like mesothelioma, a cancer type that affects specific tissue linings.