GHS / HazCom term · Glossary
LC50
Verified vs OSHA sources · 2026-10-05
Lethal Concentration 50 — the concentration of a substance in air (or in water, for aquatic tests) that kills 50% of a test population over a set exposure time, typically a 4-hour inhalation exposure. It's the inhalation counterpart to the LD50 and is used to classify acute inhalation toxicity. A lower LC50 means a more toxic substance, because it takes less in the air to be lethal.
Also known as: lethal concentration 50, median lethal concentration
LC50 at a glance
- Measures inhalation (airborne) lethality — the concentration killing 50% of a test population.
- Standard exposure time is usually 4 hours.
- Units vary by form: ppm (gases), mg/L (vapors), mg/L or mg/m³ (dusts/mists).
- Lower LC50 = more toxic. HCS Category 1 (most toxic) inhalation cut-offs: gases ≤ 100 ppm, vapors ≤ 0.5 mg/L, dusts/mists ≤ 0.05 mg/L.
In plain English
The amount of a gas, vapor, or dust in the air that would kill half of a test group over a set time (usually 4 hours). The smaller the number, the more poisonous the substance — because a tiny amount in the air is enough to be deadly.
What the source says, in summary
Acute toxicity refers to those adverse effects occurring following ... an inhalation exposure of 4 hours [used to classify inhalation hazards via the LC50].
In context
A classification or labeling concept from the UN Globally Harmonized System, adopted into US law through OSHA's Hazard Communication Standard (29 CFR 1910.1200) and its appendices.
Where this is written in OSHA's rules
- 29 CFR 1910.1200 App A.1Read on eCFR / OSHA.gov
Example
A gas with a 4-hour LC50 of 50 ppm is far more toxic than one with an LC50 of 5,000 ppm — the first is lethal to half a test population at one-hundredth the airborne concentration. Under HazCom, that 50-ppm gas would fall in the most severe acute inhalation toxicity category.
Why it matters
LC50 is the backbone of acute inhalation toxicity classification, which drives the skull-and-crossbones pictogram, signal word, and hazard statements on a label. For responders and industrial hygienists, comparing a chemical's LC50 to values like the IDLH helps gauge how quickly an airborne release becomes life-threatening.
LC50 vs. LD50
Both mark the “50% lethal” point, but by different route. LC50 is a concentration in air (inhalation), reported in ppm or mg/L over an exposure time. LD50 is a dose that enters the body (oral or dermal), reported in mg/kg of body weight. Use LC50 for what's breathed, LD50 for what's swallowed or absorbed.
Acute inhalation LC50 — HCS Category 1 cut-offs (most toxic)
| Physical form | LC50 for Category 1 |
|---|---|
| Gas | ≤ 100 ppmV |
| Vapor | ≤ 0.5 mg/L |
| Dust / mist | ≤ 0.05 mg/L |
LC50: frequently asked questions
- What is LC50?
- Lethal Concentration 50 — the airborne concentration of a substance that kills 50% of a test population over a set exposure time (usually 4 hours). It's used to classify acute inhalation toxicity.
- What's the difference between LC50 and LD50?
- LC50 is a concentration in air (inhalation), in ppm or mg/L over time. LD50 is a dose delivered to the body (oral or dermal), in mg/kg of body weight. LC50 is for what's breathed; LD50 for what's ingested or absorbed.
- Does a lower LC50 mean more or less toxic?
- More toxic. A lower LC50 means a smaller airborne concentration is enough to be lethal to half the test population, so the substance is more dangerous to inhale.
Related terms
Free tools for this
Governing OSHA standards
More GHS terms
A GHS health-hazard class for the harmful effects that follow a single dose, several doses within 24 hours, or a 4-hour inhalation exposure. Chemicals are sorted into categories by route (oral, dermal, inhalation) using LD50/LC50 cut-offs — Category 1 is the most toxic. OSHA's HazCom standard uses Categories 1–4.
Aspiration HazardA GHS health-hazard class for chemicals that can be fatal if swallowed and then enter the airways — directly or by vomiting — causing chemical pneumonia or lung injury. Thin, low-viscosity hydrocarbons (gasoline, kerosene, lamp oil) are the classic example. It is Category 1, with the health-hazard pictogram (GHS08) and signal word “Danger.”
CarcinogenicityA GHS health-hazard class for chemicals that cause cancer or increase its incidence. GHS (and OSHA's HazCom standard) sorts carcinogens into Category 1A (known human carcinogen), 1B (presumed, from animal evidence), and Category 2 (suspected). Carcinogens carry the health-hazard pictogram (GHS08) and a hazard statement such as H350 (“May cause cancer”).
Chemicals Under PressureA physical-hazard class added by OSHA's 2024 HCS update (aligning with GHS Revision 7) for liquids or solids pressurized with a gas, held in pressure receptacles other than aerosol dispensers and not classified as gases under pressure. It is categorized by the flammability of its contents and always carries the gas-cylinder pictogram (GHS04) for the pressure hazard.
ClassificationThe process of evaluating the available scientific evidence for a chemical against the GHS criteria to decide whether it is hazardous and, if so, the hazard class and category of each effect. Classification is the first step in HazCom — its result determines every label element (pictogram, signal word, hazard and precautionary statements) and SDS content.
Corrosive to MetalsA GHS physical-hazard class for chemicals that, by chemical action, will materially damage or even destroy metals. It is a single category (Category 1), determined by corrosion-rate testing on steel and aluminum, and uses the corrosion pictogram (GHS05) with the hazard statement H290. It is a separate hazard from skin corrosion, even though both use GHS05.
Desensitized ExplosivesA physical-hazard class added by OSHA's 2024 HCS update (GHS Revision 7) for solid or liquid explosives that have been phlegmatized — wetted or diluted — to suppress their explosive properties for safe transport and storage. It has four categories based on the corrected burning rate, and uses the flame pictogram (GHS02), not the exploding-bomb.
Globally Harmonized System (GHS)An internationally agreed framework created by the United Nations (first adopted in 2003) that standardizes how chemical hazards are classified and communicated — using common hazard classes, pictograms, signal words (Danger / Warning), and coded hazard (H) and precautionary (P) statements on labels and safety data sheets. The US adopted GHS into law through OSHA's 2012 Hazard Communication Standard update.
Sources & verification
- 29 CFR 1910.1200 App A.1 (eCFR / OSHA.gov)
- OSHA — 1910.1200 Appendix A (Health Hazard Criteria)
- NIOSH — Immediately Dangerous to Life or Health (IDLH) Values
Reviewed by HazComFast against eCFR, OSHA.gov, NIOSH, and the Federal Register. Last reviewed 2026-10-05. This glossary is general information, not legal advice; OSHA State-Plan states (e.g. California, Michigan) may adopt stricter requirements.
Put LC50 into practice
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