COSHH Assessments

  • The route of entry into the body
  • Chemical classification, categories of harm and risk phrases
  • Ill health effects: acute/chronic/target organs/ local agents
  • Chemical and physical properties
  • Solubility
  • Synergistic effects (see below)
  • Amount used
  • Does the substance have a workplace exposure limit (WEL)?
  • The health effects caused by hazardous substances can be broadly divided into three categories of harm:

– Toxic hazards (e.g. very toxic, toxic, harmful, carcinogenic, mutagenic, toxic for reproduction)
– Corrosive/irritant hazards
– Dermatitic hazards (e.g. irritant, sensitising)


The concentration of a particular chemical agent is an important parameter in determining the likelihood of harm occurring and also how much damage may be done. Chemical reactions usually proceed faster if the concentration is higher and therefore a toxic substance is likely to “poison” faster, or a corrosive one to “burn” quicker, if the concentration is higher. Normally the higher the concentration of a chemical agent we are handling, the greater the risk of harm.

Physical Nature of Airborne Contaminants

Airborne contaminants can be grouped under the general term of aerosols. An aerosol is a scientific term that applies to any disperse system of liquid or solid particles suspended in air, so it applies to a wide range of particulate systems encountered occupationally. In addition to size, the particle shape is important in determining the effects of inhalation of airborne contaminants as this affects the way that particles behave in the air and also how they behave after they have been deposited in the respiratory tract. Particle shapes include: Fibres – (long, thin needle-shaped particles) e.g. asbestos and mineral fibre.

Solubility in Body Fluids

Ammonia gas which can dissolve in moist areas of the respiratory tract (there are many other examples of soluble gases which can cause harm by this route); toxic metals that are only soluble in water at a particular pH which determines where in the gastrointestinal tract dissolution and absorption can take place; and organic chemicals that are not water soluble but will dissolve in fats or lipids in the body and consequently accumulate in target organs which are susceptible (e.g. chlorinated hydrocarbons).


Chemical agents are able to exert a harmful effect on the body if exposure to the substance in isolation occurs. However, if exposure to a combination of agents occurs it is possible, in some cases, for one or other of the agents to enhance the harmful effect of the other. This is called synergy and means that the effect of the combination of agents on the body is greater than the sum of the individual effects. The most common example of this is the effect of exposure to asbestos fibres on smokers. Tobacco smoke is known to contain carcinogenic substances and it is possible to estimate the likelihood of a smoker contracting lung cancer. Asbestos fibres are also a recognised carcinogen and epidemiological data gives us the probability of an asbestos worker developing lung cancer following exposure to asbestos dust. However, if epidemiological data for asbestos workers who smoke is studied, it is found that the likelihood of these individuals contracting lung cancer is much higher than that expected from exposure to each agent individually.


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