COSHH Assessments

Substances hazardous to health

Control of Substances Hazardous to Health Regulations 2002 (As amended)

COSHH applies to a wide range of substances and preparations (mixtures of two or more substances) which have the potential to cause harm to health if they are ingested, inhaled, or are absorbed by, or come into contact with, the skin, or other body membranes. Hazardous substances can occur in many forms, including solids, liquids, vapours, gases and fumes. They can also be simple asphyxiants or biological agents.


Dust of any kind can also become a substance hazardous to health under COSHH when it is present at concentrations in the air equal to or greater than 10 mg/m3 (as a time-weighted average over an eight-hour period) of inhalable dust or 4 mg/m3 (as a time-weighted average over an eight-hour period) of respirable dust. However, there may be dusts with no formal workplace exposure limit (WEL) which are not listed in CLP, but for which limits lower than 10 mg/m3 or 4 mg/m3 would be appropriate because of evidence of potential hazards to health.

Gases & Vapours:

COSHH covers those gases and vapours which, when present at high concentrations in air at the workplace, act as simple asphyxiants. These can displace the oxygen content to such an extent that life cannot be supported. Many of these asphyxiant gases are odourless, colourless and not readily detectable. Monitoring the oxygen content of the air is a means of assessing whether their presence poses a risk to the health of employees.

Biological Agents:

Biological agents may also be considered to be substances hazardous to health and include:

micro-organisms such as bacteria, viruses, fungi, and the agents that cause transmissible spongiform encephalopathies (TSEs);

cell cultures, if the cell being cultured is itself hazardous;

parasites that live inside their host, including single-cell organisms, eg malarial parasites, amoebae and trypanosomes and the microscopic infectious forms of larger parasites, eg the microscopic ova and infectious larval (helminths).

External Larger Parasites:

External larger parasites, such as ticks and mosquitoes, are not biological agents, but their bite may be the route by which a parasite or pathogen can infect a human.

About COSHH Assessments

Further information about COSHH can be obtained from the Health & Safety Executive (HSE) by browsing through their website at: