LC50 stands for “Lethal Concentration 50,” the concentration of a substance in air or water that kills 50% of a test population within a set time period. It’s one of the most widely used measurements in toxicology for gauging how dangerous a chemical is when inhaled or dissolved in water. The key thing to understand: a lower LC50 number means a more toxic substance, because it takes less of the chemical to be lethal.
How LC50 Works
LC50 measures toxicity based on concentration in a surrounding medium, whether that’s air in a chamber or water in a tank. Researchers expose a group of test organisms to different concentrations of a chemical, then record how many die at each level over a fixed time period. The concentration that results in exactly 50% mortality is the LC50. That value is calculated statistically using a method called probit analysis, which fits a curve to the dose-response data rather than requiring that any single test group land exactly at the 50% mark.
The result is always reported with a time period attached. A “96-hour LC50” means the concentration that kills half the test population over four days. You’ll also see 24-hour, 48-hour, and 72-hour values. Without the time component, an LC50 number is incomplete, since a chemical that seems relatively safe over four hours could be devastating over four days.
Units of Measurement
For airborne chemicals (gases, vapors, dusts, and mists), LC50 is typically reported in milligrams per liter of air (mg/L), parts per million by volume (ppm/V), or milligrams per cubic meter (mg/m³). For water-based testing, such as evaluating pesticide risks to fish, the standard unit is milligrams per liter (mg/L), which is functionally the same as parts per million in water.
How LC50 Differs From LD50
These two values measure the same basic thing, lethality at the 50% mark, but through different routes of exposure. LD50 stands for “Lethal Dose 50” and measures how much of a substance it takes to kill half a test group when delivered directly into the body, usually by mouth or through the skin. It’s reported as a weight of chemical per unit of body weight (mg/kg).
LC50 applies when the chemical enters the body through a surrounding medium. The organism breathes contaminated air or absorbs a chemical dissolved in water. You can’t easily measure the exact dose each individual receives, so researchers measure the concentration in the environment instead. This makes LC50 the standard metric for inhalation toxicity in workplace safety and for aquatic toxicity in environmental science.
To illustrate, the insecticide dichlorvos (commonly used in household pest strips) has an oral LD50 of 56 mg/kg in rats and a dermal LD50 of 75 mg/kg. Its inhalation LC50 in rats is 15 mg/m³ over a 4-hour exposure. Each number captures a different exposure route for the same chemical.
How LC50 Values Are Determined
Standard fish toxicity tests, following U.S. EPA guidelines, run for a minimum of 96 hours. Researchers use species with well-established sensitivity profiles, with rainbow trout and bluegill sunfish being the preferred freshwater species. Other approved test fish include fathead minnows, common carp, zebrafish, channel catfish, and several salmon species. For saltwater testing, the list includes Atlantic silversides and sheepshead minnows.
During testing, mortality is recorded at 6, 24, 48, 72, and 96 hours. Researchers calculate separate LC50 values at each time point along with 95% confidence intervals, which gives a picture of how toxicity develops over time. The final 96-hour LC50 is the figure most commonly cited in regulatory decisions and safety data sheets.
Reading LC50 Numbers
The relationship between LC50 values and danger is inverse. A chemical with a 96-hour LC50 of 0.06 mg/L for fish is far more toxic than one with an LC50 of 100 mg/L, because it takes a much smaller concentration to kill. Copper, for example, has a 96-hour LC50 as low as 0.06 mg/L for sensitive freshwater fish species like fountain darters and Atlantic sturgeon, while hardier species like fathead minnows tolerate concentrations up to 0.47 mg/L. That range shows why the species tested matters enormously when interpreting these numbers.
For aquatic toxicity, chemicals with a 96-hour fish LC50 at or below 1 mg/L are generally classified as highly toxic. Values between 1 and 10 mg/L fall into a moderate category, and values between 10 and 100 mg/L are considered lower toxicity. Anything above 100 mg/L is typically treated as relatively low concern for aquatic life.
How Regulators Use LC50 Categories
The Globally Harmonized System (GHS) of chemical classification uses LC50 ranges to sort chemicals into hazard categories, which determine the warning labels and safety symbols on chemical products. For inhalation toxicity involving dusts and mists, the categories break down as follows:
- Category 1 (most dangerous): LC50 below 0.05 mg/L
- Category 2: LC50 between 0.05 and 0.5 mg/L
- Category 3: LC50 between 0.5 and 1.0 mg/L
- Category 4: LC50 between 1.0 and 5.0 mg/L
Gases and vapors have their own separate scales. For gases, Category 1 starts below 100 ppm, while Category 4 tops out at 5,000 ppm. These thresholds directly determine what hazard pictograms appear on a product label, what protective equipment is required, and how a chemical must be stored and transported.
For aquatic environments, the GHS classifies chemicals as Acute Toxicity Category 1 if the 96-hour fish LC50 is at or below 1 mg/L. Category 2 covers 1 to 10 mg/L, and Category 3 covers 10 to 100 mg/L. These ratings shape how chemicals are regulated near waterways and what environmental precautions manufacturers must follow.
Limitations of LC50
An LC50 value is a snapshot of acute toxicity only. It tells you about short-term lethality, not about cancer risk, reproductive harm, organ damage from long-term exposure, or effects at lower concentrations over months or years. A chemical can have a relatively high LC50 (appearing less immediately dangerous) while still causing serious chronic health problems at much lower concentrations.
Species sensitivity also varies widely. The insecticide permethrin has a 96-hour LC50 as low as 1.2 micrograms per liter for sturgeon species, but 25.0 micrograms per liter for bonytail chub, a roughly 20-fold difference. Extrapolating from one species to another, or from animal data to human risk, requires additional safety factors and professional judgment.
There is also growing movement to reduce the number of animals used in LC50 testing. U.S. regulatory agencies now accept several alternative approaches, including “up-and-down” procedures that use far fewer animals to estimate lethal thresholds. New approach methodologies using computer modeling and cell-based assays are under active development, with a major new database of validated alternatives expected to launch in 2025 through the National Toxicology Program.