Criteria for Major Hazard Chemicals

Typically any assessment of thedegree of hazard from a chemical is based on multiple parameters which include:values of relevant physico-chemical properties, operating conditions, availableinventory etc. Of these, the property values generally provide a very criticalinput to hazard assessment. Thus, several international and national bodiesacross the world have defined criteria based on substance physico-chemicalproperties, primarily for classifying substances into various hazardcategories.  One of the widely acceptedclassification criteria is due to the International Labour Organization (, which recommends the following taxonomy forhazardous substances that pose significantly high level (major) hazard: 

Toxic substances:

Toxic substances are classifiedinto hazard categories according to their acute toxicity. Classification can bedone by determining the acute toxicity in animals, expressed in LD50 or in LC50 values (see table 1). In general three categories are suggested: Very Toxic(Category 1 and 2) and Toxic substances (Category 3). These are demarcated by LD50 and LC50 values as shown in Table 2.


Table 2. Classification of toxicchemicals based on LD50and LC50


LD50 absorbed orally in rat

(mg/kg bodyweight)

LD50 dermal absorption in rat or rabbit

(mg/kg bodyweight)

LC50 absorbed by inhalation in rat

(mg/litre per 4 hours)








0.1 - 0.5


25 - 200

50 - 400

0.5 - 2


Flammable substances:

  • Gases which form flammable mixtures with air
  • Highly or extremely flammable liquids with flash points lower than 21 °C
  • Flammable liquids with flash points lower than 55 °C

Explosive substances:

Thiscategory includes substances which may explode when brought in contact with asource of ignition or which are more sensitive to shock and friction than dinitrobenzene.