GHS hazard categories make up the fundamental hierarchal elements of this systems’ building block approach. The GHS has 3 main hazard groups; physical, health, and environmental, each acting as an umbrella for a set of distinct hazard classes. How the hazard enacts its dangerous effects, is what classifies it into any one particular group.
GHS hazard categories define the next level of organization within the building block model, in which categories act as a means to compare the severity of a hazard within a specific hazard class. The GHS hazard categories are carefully chosen based on classification criteria and analysis of scientific/experimental data; both of which are harmonized to align with globally approved scientific standards. An example which illustrates use of the categorical, building-block approach would be how a manufacturer classifies a particular chemical substance:
1. Hazard Group = Health – Grouped into health hazards, as intrinsic properties of this substance may impose adverse health effects
2. Hazard Class = Acute Toxicity – Put into the acute toxicity class if, adverse effects occur via oral or dermal administration of this substance either as a single dose or multiple doses within 24 hours; if adverse effects occur upon inhalation exposure of 4 hours.
3. Hazard Category = Oral, Category 2 – Administration via the oral route fulfills the cut-off criteria for an Acute Toxicity Estimate (ATE) of 50mg/kg, putting it into category 2; 1 of 5 categories for this class.
GHS hazard categories are arranged such that, ascension into higher numerical values typically equates to a decreasing degree of hazard severity e.g. category 2 is less severe than category 1. At minimum, each hazard class must always contain at least 1 hazard category, which can sometimes be further divided by letter into sub-categories e.g. 1A & 1B act as sub-categories, with B indicating a hazard of lesser severity. Some countries may adopt all categories of a particular hazard class, others may only adopt the first 2, but the overall recommendation, is to always incorporate the category of highest hazard severity. Each hazard category is also associated with some combination of specific hazard—symbols, signal words, and statements, all of which are prescribed according to the nature of the hazard.