Under WHMIS, hazardous products are sorted into two hazard groups, physical hazards and health hazards, and each group is split into hazard classes. Each class is then divided into categories that show how severe the hazard is, and Category 1 is always the most severe. The class and category decide what goes on the label: the pictogram, the signal word and the hazard statements. Below is every WHMIS hazard class, how classification works, and what changed in the 2022 update.
The 2 WHMIS hazard groups
Every hazardous product in WHMIS falls into one or both of these groups:
- Physical hazards: what the product can do because of its physical or chemical properties, such as catch fire, explode, react or corrode metal.
- Health hazards: what the product can do to your body, such as poison you, burn your skin, damage your lungs or cause cancer.
A single product can be in several classes from both groups. Gasoline, for example, is a flammable liquid (physical) and can also cause cancer, harm your organs and be an aspiration hazard (health).
Group, class, category: how classification works
- Hazard group: physical or health.
- Hazard class: the type of hazard, for example "Flammable liquids" or "Acute toxicity".
- Category (and sometimes subcategory): how severe it is within the class. Category 1 is the most severe, then 2, 3 and so on. Some classes use letters instead, such as Types A to G for organic peroxides.
The supplier classifies the product and puts the results on the supplier label and in section 2 of the Safety Data Sheet. The class and category decide the pictogram, the signal word ("Danger" for more severe, "Warning" for less severe) and the hazard statements.
The 19 physical hazard classes
| Physical hazard class | What it means |
|---|---|
| Flammable gases | Gases that can catch fire, including gases that ignite on contact with air |
| Aerosols | Spray cans; flammable ones are in categories 1 and 2 |
| Oxidizing gases | Gases that can cause or feed a fire |
| Gases under pressure | Compressed, liquefied, refrigerated or dissolved gases |
| Flammable liquids | Liquids that give off vapour that can catch fire |
| Flammable solids | Solids that ignite easily or cause fire through friction |
| Self-reactive substances and mixtures | Unstable products that can react strongly on their own, even without air |
| Pyrophoric liquids | Liquids that catch fire within minutes of contact with air |
| Pyrophoric solids | Solids that catch fire within minutes of contact with air |
| Self-heating substances and mixtures | Products that can heat up and catch fire in large amounts over time |
| Substances and mixtures which, in contact with water, emit flammable gases | Products that react with water to give off flammable gas |
| Oxidizing liquids | Liquids that can cause or feed a fire |
| Oxidizing solids | Solids that can cause or feed a fire |
| Organic peroxides | Unstable chemicals that can burn rapidly or explode (Types A to G) |
| Corrosive to metals | Products that can damage or destroy metal |
| Combustible dusts | Fine dust that can explode or flash fire when spread in the air |
| Simple asphyxiants | Gases that push out oxygen, which can cause suffocation |
| Chemicals under pressure | Liquids or solids pressurized with a gas (added in 2022) |
| Physical hazards not otherwise classified | Physical hazards that do not fit any other class |
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The 12 health hazard classes
| Health hazard class | What it means |
|---|---|
| Acute toxicity | Can cause serious harm or death after a single short exposure (swallowed, on the skin or inhaled) |
| Skin corrosion/irritation | Burns or irritates the skin |
| Serious eye damage/eye irritation | Damages or irritates the eyes |
| Respiratory or skin sensitization | Causes allergy or asthma-like reactions after exposure |
| Germ cell mutagenicity | Can cause genetic changes that may be passed on |
| Carcinogenicity | Can cause cancer |
| Reproductive toxicity | Can harm fertility, an unborn child, or a breastfed child |
| Specific target organ toxicity: single exposure | Damages a specific organ after one exposure, or causes drowsiness or breathing irritation |
| Specific target organ toxicity: repeated exposure | Damages a specific organ after repeated or long exposure |
| Aspiration hazard | Can cause serious lung damage if swallowed and then breathed into the lungs |
| Biohazardous infectious materials | Organisms or toxins that can cause disease (a WHMIS-only class) |
| Health hazards not otherwise classified | Health hazards that do not fit any other class |
Each class has its own pictogram rules. See every WHMIS symbol explained to match classes to the symbols on a label.
What changed in the 2022 WHMIS update
In December 2022, Health Canada amended the Hazardous Products Regulations to line up with a newer edition of the global GHS. The main changes to the hazard classes were:
- A new class, chemicals under pressure.
- "Flammable aerosols" became aerosols, with a new category for non-flammable aerosols.
- Pyrophoric gases were moved into the flammable gases class.
Suppliers had until December 15, 2025 to update their labels and Safety Data Sheets. The 9 WHMIS pictograms did not change, and there is no separate "WHMIS 2025": training on WHMIS 2015 / GHS is still the current standard.
Products WHMIS does not cover
Some products are regulated by other laws, so they do not get WHMIS supplier labels or SDSs. They include explosives, cosmetics, pest control products, consumer products sold in stores, nuclear substances, food and drugs. Your employer still has to protect you from their hazards and train you on safe use.
Do workers need to know the hazard classes?
You do not need to memorize the full list. You do need to understand what the class, category, pictogram and signal word on a label tell you, and where to find more in the SDS. That is part of general WHMIS education, which every province requires for workers who use or work near hazardous products, followed by site-specific training at your workplace.
Our online WHMIS course covers hazard classes, labels and SDSs in about 90 minutes, with unlimited free exam retakes and an instant certificate. Test yourself first with the free WHMIS symbols quiz.
Frequently asked questions
Under WHMIS, hazardous chemicals are sorted by what?
They are sorted into two hazard groups, physical hazards and health hazards. Each group is divided into hazard classes, and each class into categories by severity.
What are the two main WHMIS hazard groups?
Physical hazards (such as fire, explosion, reactivity and corrosion of metals) and health hazards (such as toxicity, skin and eye damage, sensitization, cancer and organ damage).
How many WHMIS hazard classes are there?
Under the current Hazardous Products Regulations there are 19 physical hazard classes and 12 health hazard classes, counting the two "not otherwise classified" classes.
Which hazard category is the most severe?
Category 1. The higher the category number, the less severe the hazard within that class.
What is the difference between a hazard class and a hazard category?
The class is the type of hazard, such as flammable liquids. The category shows how severe it is within that class, with Category 1 the most severe.
Is there a WHMIS 2025?
No. Health Canada updated the Hazardous Products Regulations in 2022, with a transition that ended in December 2025, but the system is still called WHMIS 2015 and the pictograms did not change.
Where do I find a product’s hazard class?
In section 2 (hazard identification) of its Safety Data Sheet, and through the pictograms, signal word and hazard statements on the supplier label.
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