In this case, the substance causes the worker to become sensitised or to develop an allergic reaction some time after initial contact. The type of allergic mechanism is known as Type IV or delayed hypersensitivity. People do not become allergic to a substance immediately at first contact. The sensitisation period( the time between contact and the development of an allergy) can vary from a number of days to months or even years. The risk of becoming allergic depends on several factors:

  • The nature of the substance. Substance with a higher likelihood to cause allergy is known as a skin sensitiser.
  • The nature of contact. The higher or more repeated the exposure the more likely it is for the individual to develop sensitisation.
  • The vulnerability of the host. Typically people with other allergies are NOT particularly more vulnerable to developing contact allergic dermatitis. Individuals with a previous history of non allergic dermatitis ARE more vulnerable. This may be because the sensitiser may more easily enter the bloodstream in those individuals.

Once the individual becomes sensitised, each time he/she comes into contact with the sensitising substance, even in very small amounts, dermatitis will develop. This is different to irritant dermatitis which is dose related. The long term health consequences and ability to remain at work can be significant. In general the majority of an exposed occupational group do NOT become sensitised. It is an idiosyncratic or individual reaction.

Sensitisation is specific to one substance or to a group of substances that are chemically similar. Once sensitised a person is likely to remain so for life.  In allergic dermatitis the rash can occur in areas of the skin not in direct contact with the substance - the so called “Id” reaction.

Common sensitisers are chromate's (found in cement), nickel (cheap jewellery), epoxy resins, formaldehyde, wood dust, flour, printing plates, chemicals and adhesives.