A physical hazard category is a ranking within a chemical hazard class that tells you how severe or dangerous a substance’s physical properties are. Under the Globally Harmonized System (GHS), chemicals that can explode, catch fire, react violently, or damage materials are grouped into 17 physical hazard classes, and each class is then divided into numbered categories (Category 1 being the most dangerous) based on measurable thresholds like flash point, corrosion rate, or ignition time.
This system exists so that anyone handling, storing, or shipping a chemical can look at a Safety Data Sheet or label and immediately understand what kind of physical danger they’re dealing with and how serious it is.
Physical Hazards vs. Health Hazards
Chemical hazards fall into two broad groups. Physical hazards describe what a chemical can do to its surroundings: explode, ignite, corrode metal containers, or release dangerous amounts of energy. Health hazards describe what a chemical can do to your body: cause cancer, damage organs, irritate skin, or trigger allergic reactions. A single chemical can carry both types of hazard. Gasoline, for example, is both a flammable liquid (physical hazard) and harmful if inhaled (health hazard). The physical hazard categories covered here focus entirely on the energy-release and reactivity side.
The 17 Physical Hazard Classes
OSHA’s Hazard Communication Standard, updated in May 2024 to align with Revision 7 of the GHS, recognizes 17 physical hazard classes. That 2024 update also added an 18th class, desensitized explosives, for explosive substances that have been chemically stabilized for safer transport. The original 17 are:
- Explosives
- Flammable Gases
- Flammable Aerosols
- Oxidizing Gases
- Gases Under Pressure
- Flammable Liquids
- Flammable Solids
- Self-Reactive Chemicals
- Pyrophoric Liquids
- Pyrophoric Solids
- Pyrophoric Gases
- Self-Heating Chemicals
- Chemicals That Emit Flammable Gases on Contact With Water
- Oxidizing Liquids
- Oxidizing Solids
- Organic Peroxides
- Corrosive to Metals
Each class targets a specific type of physical danger. Some classes have multiple numbered categories (like flammable liquids, which have four), while others have only one (like corrosive to metals). The number of categories depends on how many meaningful severity levels exist for that particular hazard.
How Categories Work Within a Class
Category 1 always represents the greatest danger, and higher numbers represent progressively lower risk. The boundaries between categories are set by specific, testable criteria, not by judgment calls. For flammable liquids, those criteria are flash point (the lowest temperature at which vapors can ignite) and boiling point. For pyrophoric materials, the criterion is how quickly the substance catches fire when exposed to air. For corrosive-to-metals substances, it’s how fast the chemical eats through steel or aluminum.
This numbered system is what makes labels and Safety Data Sheets useful at a glance. If you see “Flammable Liquid, Category 1” on a container, you know immediately that it’s more dangerous than a “Category 4” flammable liquid, even if you don’t remember the exact flash point cutoffs.
Flammable Liquids: A Detailed Example
Flammable liquids are one of the most commonly encountered physical hazard classes, and their four categories illustrate how the system works in practice. Any liquid with a flash point at or below 199.4°F (93°C) qualifies as flammable. From there, the categories break down by specific temperature ranges:
- Category 1: Flash point below 73.4°F (23°C) and boiling point at or below 95°F (35°C). These are the most volatile and dangerous. Diethyl ether is a classic example.
- Category 2: Flash point below 73.4°F (23°C) but boiling point above 95°F (35°C). Gasoline and acetone fall here.
- Category 3: Flash point between 73.4°F (23°C) and 140°F (60°C). Many paints and solvents land in this range.
- Category 4: Flash point between 140°F (60°C) and 199.4°F (93°C). These are the least hazardous flammable liquids, but they still require caution, especially when heated close to their flash point.
One practical detail worth knowing: if you heat a Category 4 liquid to within 30°F of its flash point, it must be handled with the same precautions as a Category 3 liquid. The same rule applies when heating Category 3 liquids with flash points above 100°F. Temperature during use, not just the number on the label, determines real-world risk.
Explosives and Their Six Divisions
Explosives use a slightly different system. Instead of numbered categories, they are classified into six divisions (1.1 through 1.6) based on the type and scale of the explosion they can produce:
- Division 1.1: Mass explosion hazard, meaning the entire quantity can detonate almost instantaneously.
- Division 1.2: Projection hazard (flying fragments) but no mass explosion.
- Division 1.3: Fire hazard with minor blast or projection effects, but no mass explosion.
- Division 1.4: Minor explosion hazard. Effects are mostly confined to the package itself, with no significant fragment projection.
- Division 1.5: Very insensitive substances that technically have a mass explosion hazard but are extremely unlikely to detonate under normal conditions.
- Division 1.6: Extremely insensitive articles with negligible probability of accidental detonation and no mass explosion hazard.
The division number directly affects how explosives must be stored, transported, and separated from other materials.
Gases Under Pressure
Gases under pressure are classified into four groups based on how the gas is physically contained, not by severity level. The groups are compressed gas, liquefied gas, refrigerated liquefied gas, and dissolved gas. Each presents a distinct hazard profile. Compressed gas cylinders can rupture and become projectiles. Refrigerated liquefied gases can cause cryogenic burns or tissue injury on contact. All gases under pressure carry the risk of explosion if heated, which is why their labels include the warning “may explode if heated.”
Pyrophoric and Self-Heating Substances
Pyrophoric materials ignite spontaneously in air within five minutes or less. That makes them among the most immediately dangerous chemicals in a workplace. Both liquids and solids can be pyrophoric, and both are classified as Category 1 with no lower-severity categories, because any substance that catches fire on contact with air is inherently high-risk.
Self-heating chemicals are related but slower-acting. They react with air and generate heat gradually, without needing an external energy source. The danger comes when they’re stored in large quantities for extended periods, allowing heat to build up to the point of ignition. Self-heating substances are split into Category 1 (self-heating in small quantities) and Category 2 (self-heating only in large quantities), which directly influences how much material you can safely store in one place.
Corrosive to Metals
This class covers chemicals that can destroy metal containers, storage tanks, or equipment. The classification has only one category. A substance qualifies if it corrodes steel or aluminum surfaces at a rate exceeding 6.25 mm per year at a test temperature of 131°F (55°C). The chemical must be tested on both metals, and exceeding the threshold on either one triggers the classification. Strong acids and some alkaline solutions commonly fall into this class. The practical concern is container failure: if a corrosive chemical eats through its storage vessel, it can create secondary hazards like spills, toxic exposures, or reactions with other nearby materials.
Where You’ll See These Categories
Physical hazard categories appear in three places you’re likely to encounter at work. Safety Data Sheets list them in Section 2 (Hazard Identification), along with the signal word (Danger or Warning), hazard statements, and the red-bordered GHS pictograms. Container labels display the same information in a condensed format. And shipping documents reference hazard classes and categories to determine how chemicals can be transported, what vehicles can carry them, and how far apart incompatible substances must be kept.
The category number determines which pictogram and signal word a product gets. Category 1 hazards almost always carry the signal word “Danger,” while higher-numbered (less severe) categories typically use “Warning.” Some Category 4 hazards don’t require a pictogram at all, reflecting their lower risk level. Understanding the numbering system lets you quickly compare the relative danger of two products in the same hazard class without needing to look up their technical test data.