Toxicity

Toxicity is the degree to which a substance can cause harm. It is studied within toxicology, the science that examines adverse effects of chemicals, biological agents, and physical hazards on living organisms. Toxic effects depend on the dose and exposure duration, the route of exposure (ingestion, inhalation, dermal, injection), and the organism’s age, health, genetics, and nutritional status. Small amounts of typically harmful substances can be harmless or beneficial; high doses or sensitive individuals increase risk. The principle "the dose makes the poison" summarizes this relationship.

Toxic effects are categorized as acute (immediate or short-term) or chronic (long-term). Acute toxicity often involves rapid onset organ damage; chronic toxicity can affect the liver, kidneys, nervous system, heart, or reproductive system. Common end-organ toxicities include hepatotoxicity, nephrotoxicity, neurotoxicity, and cardiotoxicity. Toxic effects may be reversible or irreversible and can result from direct injury, metabolic disruption, or interference with essential biological pathways.

Risk assessment uses hazard identification, exposure assessment, dose-response assessment, and risk characterization to estimate public health risk. Quantitative measures such as LD50 (lethal dose for 50% of test animals) and NOAEL/LOAEL (no/lowest observed adverse effect levels) guide regulatory decisions. Toxicokinetics describe absorption, distribution, metabolism, and excretion; toxicodynamics describe mechanisms of effect.

Mitigation includes exposure reduction, safety testing, labeling, and regulatory controls. In medicine, toxicity must be balanced against therapeutic benefits, with the therapeutic index guiding safe dosing. Ethical considerations increasingly favor alternative methods to animal testing where possible.