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Categories of Danger


There are 3 general classifications of hazards, each of which contains a number of such categories:


·         Physico-chemical hazards – those that are caused by the intrinsic physical or chemical

properties of the substance.

·         Toxicological hazards – those that arise from a chemical causing harmful effects to living organisms, which in practice normally means death, injury or adverse effects in humans when ingested, inhaled or absorbed through the skin. Toxic effects may be acute or chronic, local or systemic, and reversible or irreversible.

·         Environmental Impacts – those that relate to the potential of a chemical to damage 1 or more environmental compartments (i.e. the air, soil or water, including groundwater).


The categories of danger within each classification are shown in the following table


Classification of Hazardous Substances









Highly flammable



Very toxic








Toxic for reproduction



Toxic or harmful to aquatic


Long-term effects such as


Toxic to the non-aquatic


Dangerous for the ozone




The definitions of the categories of danger posed by chemicals within the general toxicology

classification are set out below.


·         Very Toxic: Very toxic substances and preparations are those that in very low quantities cause death or acute or chronic damage to health when inhaled, swallowed or absorbed via the skin.

·         Toxic: Toxic substances and preparations are those that in low quantities cause death or acute or chronic damage to health when inhaled, swallowed or absorbed via the skin.

·         Harmful: Harmful substances and preparations are any that may cause death or acute or chronic damage to health when inhaled, swallowed or absorbed through the skin.

·         Corrosive: Corrosive substances and preparations are those that may on contact destroy living tissues. The following examples of corrosive substances may be encountered in the course of construction work:

·         Acids – Sulphuric acid and hydrochloric acid in chemical cleaners, e.g. for masonry, brickwork.

·         Alkalis – cement, lime or agents used as chemical cleaners.

·         Gases and vapors – hydrogen sulphide.

·         Vapours – from resins, paints and thinners.

·         Irritant: These are non-corrosive substances and preparations which through immediate, prolonged or repeated contact with the skin or mucous membrane may cause inflammation.

·         Sensitizing: These are substances and preparations that may cause an allergic reaction.

·         Carcinogenic: Carcinogenic substances and preparations are those which, if inhaled or ingested or absorbed by the skin, may induce cancer or increase its incidence.


Note that, in classifying a particular chemical, the hazards it presents may lie within any or all of the general classifications and more than 1 class of danger may be identified. Thus, nitric acid is classified as both oxidising and corrosive, and asbestos is classified as a carcinogen (Category 1) and toxic.


Distinction Between Acute and Chronic Health Effects


Acute health effects arise where the quantity of a toxic or harmful substance absorbed into the body produces harmful effects very quickly – i.e. within seconds, minutes or hours.


In an occupational setting, acute toxicity does not often occur because the conditions required to produce it are either too complicated, or the results would be so serious that stringent safety measures are observed, thus preventing its occurrence. Gassing accidents producing toxic conditions are an exception.


The term ’chronic toxicity’ describes a condition where the harmful effects of a substance absorbed into the body take a very long time to appear – months or perhaps years. The conditions produced by the toxin usually result from absorption of small quantities over a period of time. In terms of occupational safety, chronic toxicity, or at least its prevention, presents the most difficult control problems.


This is particularly true if materials have little-known or poorly-documented toxicity levels, or if hygiene control strategies are breaking down. The following points illustrate how insidious are the effects of chronic toxicity and give an indication of the difficulties of achieving effective control:


  •       The level of contamination required to produce chronic effects is often tolerated by people because they do not experience acute symptoms.
  • ·         Symptoms occur slowly, so they are not recognised until an advanced condition of harm has developed.
  • ·         When symptoms are recognised, the harm may be too advanced for full recovery – sometimes no recovery is possible.
  • ·         Symptoms are often confused with ‘normal’ ill-health or with ‘getting older’.
  • ·         Symptoms are not always easily identifiable in groups of people with the same exposure, owing to the effect of differing ‘personal’ metabolisms.

It is important to note that for both acute and chronic toxicity, time is involved in relation to their definition, but that the level of toxic action is not defined. Acute toxic action does not necessarily mean death. Intoxication from drinking alcohol is an acute toxic condition, but only in rare cases is it the direct cause of death. Cirrhosis of the liver related to intoxication by alcohol is a chronic toxicity condition from which death can occur. Some toxic substances, such as cyanide and paraquat, are generally considered to be acute toxins only