Toxicity in humans refers to the ability of a substance to cause harmful health effects, which can strike a single cell, a group of cells, an organ system, or the entire body. A toxic effect may be visible damage, or a decrease in performance or function measurable only by a test. All chemicals can cause harm at a certain level, and when a small amount can be harmful, the chemical is considered toxic. The toxicity of a substance depends on three factors: its chemical structure, the extent to which the substance is absorbed by the body, and the body’s ability to detoxify the substance and eliminate it from the body.
Toxic substances can be found in products that we use daily, such as household cleaners, prescription and over-the-counter drugs, gasoline, alcohol, pesticides, fuel oil, and cosmetics. Any chemical can be toxic or harmful under certain conditions, and the potency and, therefore, the toxicity of a chemical can be affected by its breakdown within the human body.
Heavy metals are a type of toxic substance that can cause various disorders and can also result in excessive damage due to oxidative stress induced by free radical formation. Mercury is considered the most toxic heavy metal in the environment, and its toxicity is a common cause of acute heavy metal poisoning.
Toxicity can be measured by its effects on the target (organism, organ, tissue, or cell), and individuals typically have different levels of response to the same dose of a toxic substance. A population-level measure of toxicity is often used which relates the probabilities of an outcome for a given individual in a population, such as the LD50. Acute toxicity is generally thought of as a single, short-term exposure where effects appear immediately and are often reversible, while chronic toxicity is generally thought of as frequent exposures where effects may be delayed (even for years) and are generally irreversible.
