A hazard class is a grouping system that categorizes dangerous materials based on the type of threat they pose. The term shows up in two major contexts: transporting hazardous materials (where the Department of Transportation uses 9 numbered classes) and workplace chemical safety (where the Globally Harmonized System, or GHS, sorts chemicals into physical and health hazard classes). Both systems exist to make sure anyone handling, shipping, or working near a dangerous substance knows exactly what kind of danger they’re dealing with.
The Two Systems That Use Hazard Classes
If you’re seeing “hazard class” on a shipping document or placard on the side of a truck, you’re looking at the DOT system. If it’s on a chemical label or Safety Data Sheet (SDS) at a workplace, it’s the GHS system enforced by OSHA. The DOT system classifies materials for transportation. The GHS system classifies chemicals for anyone who manufactures, stores, or works with them. Both systems assign a class based on the nature of the hazard, then use categories or divisions to indicate how severe that hazard is within the class.
The 9 DOT Hazard Classes for Transportation
The DOT divides all hazardous materials into nine numbered classes. Every substance listed in the federal Hazardous Materials Table gets assigned one of these classes (or is marked “Forbidden” for transport entirely). The nine classes are:
- Class 1: Explosives. Subdivided into six divisions (1.1 through 1.6) based on the type of blast or projection risk.
- Class 2: Gases. Includes flammable gases (Division 2.1), non-flammable/non-toxic compressed gases (Division 2.2), and gases that are poisonous by inhalation (Division 2.3).
- Class 3: Flammable liquids. Any liquid with a flash point at or below 140°F (60°C). The lower the flash point, the more dangerous the liquid is considered. Diethyl ether, for example, has a flash point below 73°F and a boiling point below 100°F, putting it in the most hazardous subgroup. Diesel fuel, with a flash point between 101°F and 140°F, falls into a less severe category.
- Class 4: Flammable solids. Covers three types: solids that burn readily (Division 4.1), materials that can ignite spontaneously in air (Division 4.2), and materials that release flammable or toxic gas on contact with water (Division 4.3). A pyrophoric material, for instance, can catch fire within five minutes of air exposure without any ignition source.
- Class 5: Oxidizers and organic peroxides. These materials can cause or intensify fire by supplying oxygen to a reaction.
- Class 6: Toxic substances. Poisons and materials that are dangerous through inhalation, ingestion, or skin contact.
- Class 7: Radioactive materials.
- Class 8: Corrosives. Materials that destroy living tissue or corrode metals on contact.
- Class 9: Miscellaneous dangerous goods. A catch-all for hazardous materials that don’t fit neatly into Classes 1 through 8.
Each substance also gets a four-digit UN identification number. Numbers prefixed with “UN” are recognized for both domestic and international shipping, which means a firefighter in Germany can look up the same UN number and know exactly what’s in a container.
GHS Hazard Classes for Workplace Chemicals
The GHS system, adopted by OSHA through its Hazard Communication Standard, takes a different approach. Instead of nine numbered buckets, it sorts chemicals into two broad groups: physical hazards and health hazards.
Physical Hazard Classes
Physical hazards describe what a chemical can do to its surroundings. These classes include explosives, flammable gases, flammable liquids, flammable solids, flammable aerosols, oxidizing gases, oxidizing liquids and solids, gases under pressure, self-reactive chemicals, pyrophoric chemicals (those that ignite spontaneously in air), self-heating chemicals, chemicals that emit flammable gas on contact with water, organic peroxides, combustible dusts, and materials corrosive to metals. There’s considerable overlap with the DOT classes here, but the GHS system is more granular because it’s designed for people working directly with chemicals rather than just transporting them.
Health Hazard Classes
Health hazards describe what a chemical can do to your body. The classes include acute toxicity (immediate poisoning from a single exposure), skin corrosion and irritation, serious eye damage, respiratory or skin sensitization (allergic reactions), the ability to cause genetic mutations, carcinogenicity (cancer risk), reproductive toxicity, organ damage from a single exposure, organ damage from repeated exposure, aspiration hazard (danger from inhaling liquid into the lungs), and simple asphyxiants (gases that displace oxygen). Biological hazards are not covered by this system.
Hazard Class vs. Hazard Category
This is a distinction that trips people up. The class tells you the type of danger. The category (or division, in DOT language) tells you the severity within that type. Think of it like this: “flammable liquid” is the class, describing the nature of the risk. The category then ranks how flammable, on a numbered scale.
In the GHS system, Category 1 (or Category A) always represents the most severe hazard. The higher the category number, the lower the danger. Some classes have five severity categories while others have only one. This is counterintuitive if you’re used to rating systems where higher numbers mean more of something, so it’s worth remembering: lower category number means greater risk.
In the DOT system, divisions work similarly. An explosive classified as Division 1.1 presents a mass explosion hazard, while Division 1.6 covers extremely insensitive items with negligible blast probability.
How Hazard Classes Appear on Labels and Documents
On workplace chemical labels, you won’t see the hazard category number itself. OSHA requires the category number on Section 2 of the Safety Data Sheet, but on the physical label, the classification translates into three things you can see: a pictogram, a signal word, and a hazard statement.
The GHS uses nine standardized pictograms, each a red-bordered diamond with a black symbol inside. An exploding bomb means explosives. A flame indicates flammable materials. A flame over a circle represents oxidizers. A gas cylinder means compressed gas. A corrosion symbol (liquid eating through a surface and a hand) covers corrosives. A skull and crossbones signals acute toxicity. An exclamation mark indicates irritants and less severe hazards. A silhouette of a person with a starburst on the chest flags serious longer-term health hazards like carcinogenicity or organ damage. And an image of a dead tree and fish marks environmental hazards.
On shipping placards and documents, the DOT class number appears directly. A truck carrying Class 3 materials will display a red diamond placard with a flame symbol and the number 3. The shipping papers list the proper shipping name, hazard class, and UN identification number for every hazardous material on board.
How a Substance Gets Classified
Classification isn’t subjective. Federal regulations lay out specific, testable criteria for each class. A liquid qualifies as Class 3 (flammable) only if its flash point, the lowest temperature at which it produces enough vapor to ignite, is at or below 140°F. A gas qualifies as Division 2.1 (flammable) if it ignites in a mixture of 13% or less by volume with air, or has a flammable range of at least 12% in air. A solid is considered a flammable solid (Division 4.1) if it burns faster than 2.2 millimeters per second in standard testing.
For health hazards under GHS, classification relies on toxicity data, including lethal concentration values from animal studies, evidence of cancer risk from epidemiological research, and standardized skin and eye irritation tests. Manufacturers and importers are responsible for performing or gathering this testing and assigning the correct class and category before a chemical enters the market or a workplace.