GHS Hazard Classes, Categories, and Severity Levels

The Globally Harmonized System sorts chemical dangers into 30 hazard classes grouped under three headings — physical, health, and environmental — and then assigns each classified chemical a numbered category showing how severe the danger is within that class. Category 1 is always the most severe, and the numbers rise as the danger drops. The GHS hazard classes and categories were developed by the United Nations, are now in the eleventh revision of the system, and give manufacturers, importers, and employers a single set of criteria for evaluating chemicals instead of navigating conflicting national standards.1United Nations Economic Commission for Europe. About the Globally Harmonized System of Classification and Labelling of Chemicals

In the United States, OSHA enforces the GHS through the Hazard Communication Standard at 29 CFR 1910.1200. The standard currently aligns primarily with GHS Revision 7 and applies to every workplace where employees could be exposed to hazardous chemicals under normal conditions or in a foreseeable emergency.2eCFR. 29 CFR 1910.1200 – Hazard Communication

How Classes and Categories Fit Together

A hazard class describes the type of danger — flammability, acute toxicity, aquatic toxicity. A hazard category describes the severity of that danger within the class. A Category 1 acute toxicant is lethal at much lower doses than a Category 4 substance in the same class.3Occupational Safety and Health Administration. Hazard Communication – Hazard Classification Guidance for Manufacturers, Importers, and Employers

Categories only compare severity within the same class. A Category 2 flammable liquid is not necessarily safer than a Category 1 skin irritant. The numbering tells you a chemical’s potency relative to other chemicals with the same type of hazard, and nothing about how one hazard type compares to another.3Occupational Safety and Health Administration. Hazard Communication – Hazard Classification Guidance for Manufacturers, Importers, and Employers

Not every class uses the same number of categories. Acute toxicity has four. Aspiration hazard has one. Several classes split Category 1 further into 1A, 1B, and sometimes 1C to capture finer distinctions. In carcinogenicity, Category 1A covers substances with confirmed human evidence of causing cancer, and Category 1B covers substances presumed to cause cancer based on strong animal data. Both fall under the most severe classification, but the distinction matters for risk management.

Physical Hazard Classes

Physical hazards are the inherent properties of a chemical that can cause fires, explosions, or dangerous pressure releases. GHS identifies 17 physical hazard classes.3Occupational Safety and Health Administration. Hazard Communication – Hazard Classification Guidance for Manufacturers, Importers, and Employers The main ones:

  • Explosives — substances capable of producing gas at a temperature, pressure, and speed that damages their surroundings.
  • Flammable gases, liquids, and solids. Flammable solids are tested by measuring their burning rate over a distance of 100 millimeters.
  • Aerosols — pressurized containers dispensing flammable or other hazardous contents.
  • Oxidizing gases, liquids, and solids — not necessarily combustible themselves, but capable of causing or intensifying the burning of other substances.
  • Gases under pressure — gases contained at 200 kPa or more at 20 degrees Celsius, including compressed, liquefied, and refrigerated liquefied gases.
  • Self-reactive substances — thermally unstable materials that can decompose violently even without oxygen.
  • Pyrophoric liquids and solids — materials that ignite within five minutes of contact with air, even in small quantities.3Occupational Safety and Health Administration. Hazard Communication – Hazard Classification Guidance for Manufacturers, Importers, and Employers
  • Self-heating substances — materials that heat up on contact with air with no external energy source, igniting only in large quantities over long periods.
  • Substances that emit flammable gas on contact with water.
  • Organic peroxides — thermally unstable substances that can decompose explosively or burn rapidly.
  • Corrosive to metals — chemicals that damage or destroy metals through chemical reaction.
  • Desensitized explosives — explosive materials treated to suppress their explosive properties for safer transport and handling.

Classification of physical hazards relies on standardized laboratory testing outlined in the UN Manual of Tests and Criteria, which prescribes specific procedures for measuring properties like flash points, burning rates, and decomposition temperatures. That keeps physical classification objective rather than a judgment call by individual manufacturers.

Health Hazard Classes

Health hazards cover the biological damage a chemical can do through ingestion, skin contact, or inhalation. OSHA recognizes 11 health hazard classes under the Hazard Communication Standard.3Occupational Safety and Health Administration. Hazard Communication – Hazard Classification Guidance for Manufacturers, Importers, and Employers

  • Acute toxicity — harmful effects from a single dose or short-term exposure, whether swallowed, absorbed through the skin, or inhaled over four hours. Classification uses numerical values like LD50 (the dose lethal to 50 percent of test animals) to sort substances into four severity categories.4Occupational Safety and Health Administration. Appendix A to 1910.1200 – Health Hazard Criteria
  • Skin corrosion and irritation — irreversible destruction of skin tissue (corrosion) or reversible inflammation (irritation).
  • Serious eye damage and eye irritation — tissue damage to the eye that does not fully reverse within 21 days (serious damage) or that does reverse (irritation).
  • Respiratory or skin sensitization — substances that trigger allergic reactions after repeated exposure, such as occupational asthma from inhaled sensitizers or contact dermatitis from skin sensitizers.
  • Germ cell mutagenicity — chemicals that cause genetic mutations in reproductive cells that can be passed to offspring.
  • Carcinogenicity — substances that cause cancer or increase its incidence.
  • Reproductive toxicity — chemicals that harm sexual function, fertility, or fetal development, with a separate category for effects on breastfeeding.
  • Specific target organ toxicity, single exposure — damage to organs like the liver or kidneys from one-time exposure.
  • Specific target organ toxicity, repeated exposure — organ damage built up from chronic contact.
  • Aspiration hazard — the risk of a liquid or solid entering the lungs directly during swallowing or vomiting, causing chemical pneumonia. Category 1 aspiration hazards include hydrocarbons with a kinematic viscosity of 20.5 mm²/s or less at 40°C.5Occupational Safety and Health Administration. Appendix A to 1910.1200 – Health Hazard Criteria
  • Simple asphyxiants — gases that displace oxygen in the air, creating a suffocation risk without being toxic themselves.

Health hazard classification often requires expert judgment because the data is rarely clean. Some substances have limited human studies, and classifiers must weigh animal data, structural similarity to known hazards, and the quality of available research.

Environmental Hazard Classes

Environmental hazard classes look at damage to ecosystems rather than to people. GHS recognizes two:

  • Hazardous to the aquatic environment, divided into acute and chronic aquatic toxicity. Acute toxicity measures how quickly a substance kills fish, crustaceans, or algae at low concentrations. Chronic toxicity looks at long-term effects and considers whether a substance persists in water and accumulates in living organisms. The key bioaccumulation threshold is a Bioconcentration Factor (BCF) of 500 or higher; when BCF data is unavailable, classifiers use an octanol-water partition coefficient (log Kow) of 4 or above as a stand-in.6United Nations Economic Commission for Europe. GHS Rev. 7 Annex 9 – Guidance on Hazards to the Aquatic Environment
  • Hazardous to the ozone layer — substances that deplete the stratospheric ozone layer, aligned with the chemicals controlled under the Montreal Protocol.

OSHA does not enforce the environmental hazard sections in the United States. Sections 12 through 15 of Safety Data Sheets, which cover ecological information, are included for international consistency but fall under other agencies like the EPA.7Occupational Safety and Health Administration. Hazard Communication – Safety Data Sheets

How the Class and Category Show Up on a Label

Classifications drive label content. Two signal words exist: “Danger” for more severe hazards and “Warning” for less severe ones. If a chemical falls into multiple categories, some warranting “Danger” and others “Warning,” only “Danger” appears. You will never see both on the same container.8Occupational Safety and Health Administration. Hazard Communication Standard – Labels and Pictograms

Hazard statements describe the nature of the danger in standardized language tied to each classification category. The same category produces the same statement no matter who manufactures the chemical.

Nine standardized pictograms, each a black symbol on a white background inside a red diamond border, map to the classifications:9Occupational Safety and Health Administration. Hazard Communication Standard Pictogram

  • Flame — flammables, pyrophorics, self-heating substances, materials that emit flammable gas, self-reactive substances, organic peroxides, and desensitized explosives.
  • Flame over circle — oxidizers.
  • Exploding bomb — explosives, self-reactive substances, and organic peroxides with explosive potential.
  • Gas cylinder — gases under pressure and chemicals under pressure.
  • Corrosion — skin corrosion, serious eye damage, and materials corrosive to metals.
  • Skull and crossbones — acute toxicity at fatal or toxic levels, Categories 1 through 3.
  • Health hazard — carcinogens, mutagens, reproductive toxicants, respiratory sensitizers, target organ toxicants, and aspiration hazards.
  • Exclamation mark — skin and eye irritants, skin sensitizers, Category 4 acute toxicity, and respiratory tract irritants.
  • Environment — aquatic toxicity. This pictogram is not mandatory under OSHA.

A pictogram appears only once on a label, even if multiple hazards call for it. When more than one pictogram could apply, the more severe wins. A chemical qualifying for both skull and crossbones (fatal acute toxicity) and exclamation mark (irritation) shows only the skull and crossbones.

Which Version of GHS Applies in the U.S. Right Now

OSHA published a 2024 update to the Hazard Communication Standard that brings U.S. requirements closer to GHS Revision 8, adding updated label elements for chemicals under pressure and correcting several classification thresholds.10Federal Register. Hazard Communication Standard The original compliance deadline for manufacturers and importers to evaluate chemicals under the new criteria was January 19, 2026, but OSHA extended it to May 19, 2026, with all other deadlines pushed back by four months. During this transition, employers can comply with either the old or the updated version of the standard.11Occupational Safety and Health Administration. HCS 2024 Compliance Date Extension Notice If you are reading a Safety Data Sheet or a label today, expect to see classifications built on Revision 7 criteria until suppliers transition their catalogs.

Under the standard, chemical manufacturers and importers carry the primary responsibility for classifying chemicals. Employers who use those chemicals do not have to perform independent classification as long as they rely on the manufacturer’s or importer’s evaluation.2eCFR. 29 CFR 1910.1200 – Hazard Communication For a downstream user, the class-and-category numbers on the incoming label and SDS are the working record of the chemical’s hazards.