Glossary / Documents & labels
Hazard category
A hazard category is a subdivision within a GHS hazard class that shows the relative severity of the hazard. Categories are numbered, and Category 1 is generally the most severe, though some classes use letters or sub-numbers.
Explanation
If the hazard class tells you what kind of danger a product presents, the category tells you how bad it is. A class such as acute oral toxicity has five categories in the UN GHS (OSHA adopted Categories 1 to 4), and a lower number means a smaller dose is enough to cause harm. The upper LD50 cutoffs for categories 1 through 4 are 5, 50, 300, and 2000 mg/kg.
Categories matter because they decide the label. A product in a more severe category usually gets the signal word Danger, while less severe categories may get Warning, and the lowest categories may carry no signal word. The matching hazard statement changes with the category as well.
Not all classes have the same number of categories. Flammable liquids have four, based on flash point and initial boiling point, whereas other classes have as few as one or two. Some use sub-categories, such as 1A and 1B for skin sensitization or corrosion, to distinguish finer differences.
Different systems may use categories in the opposite direction. In NFPA 704 and HMIS, a higher number means a more severe hazard, which is the reverse of GHS where Category 1 is the most severe. Mixing up these scales is a common error.
To interpret a category correctly, always check the class it belongs to.
What to do and watch for
- Remember that in GHS, a lower category number means higher hazard.
- Do not compare GHS categories with NFPA or HMIS ratings directly.
- Read the category together with the hazard class on the SDS.
Where it appears on an SDS
- Section 2: Hazard identification. Each classification entry gives the hazard class together with its category, for example a flammable liquid with a specific category number.
SDS content and wording vary by supplier. This page is general reference and does not replace the current SDS for your product.