When assessing how dangerous a chemical substance is, one of the most widely used measures is the LD50 — the lethal dose that kills 50% of a test population under specified conditions. Understanding LD50 and other acute toxicity parameters is fundamental to toxicology and chemical safety management.
What is LD50?
LD50 stands for “Lethal Dose, 50%.” It is expressed in units of mass of chemical per unit body weight (typically mg/kg) and represents the single dose predicted to cause death in 50% of a defined animal population under standardized experimental conditions. A lower LD50 indicates higher acute toxicity — meaning a smaller amount of the substance is needed to cause lethal effects. For example, botulinum toxin has an LD50 of approximately 1-2 ng/kg, making it one of the most acutely toxic substances known, while table salt (NaCl) has an LD50 of about 3,000 mg/kg in rats.
GHS Acute Toxicity Categories
The Globally Harmonized System (GHS) classifies acute toxicity into five categories based on LD50 values for oral, dermal, and inhalation routes. Category 1 represents the most toxic substances (oral LD50 ≤5 mg/kg), while Category 5 covers substances with relatively low acute toxicity. These categories determine the signal word, hazard statement, and pictogram required on chemical labels and Safety Data Sheets.
LC50 for Inhalation Toxicity
For inhalation toxicity, the equivalent measure is the LC50 (Lethal Concentration, 50%), expressed in mg/L or ppm for gases, or mg/L for dusts and mists over a specified exposure duration (typically 4 hours). The LC50 is especially relevant for occupational health, where workers may be exposed to airborne chemicals.
Limitations of LD50 in Risk Assessment
While LD50 values provide useful acute hazard information, they have significant limitations. LD50 values vary by species, sex, age, and route of administration. They do not account for chronic effects, sublethal toxicity, or mixture interactions. Modern risk assessment increasingly uses additional endpoints including subacute, subchronic, and chronic toxicity data, as well as mechanistic toxicology and in vitro studies to complement LD50 data.
