Occupational asthma is caused by workers breathing in substances at work that produce a hypersensitive state in the airways - the small tubes that carry air in and out of the lungs - and trigger a subsequent response in them.

Not everyone who becomes sensitised goes on to get the clinical disease but once the lungs become hypersensitive further exposure to the substance, even at quite low levels, may provoke an attack.

Work-related asthma is broader and includes substances in the workplace that irritate the airways of individuals with pre-existing (eg childhood asthma). Respiratory irritants may provoke attacks in those with occupational asthma or pre-existing asthma of non-occupational origin. Examples include chlorine, general dust and even cold air. In this case, the individual does not become sensitised to that specific agent, but the attack is still work-related.

Symptoms of asthma of whatever cause, include attacks of wheezing, coughing chest tightness or shortness of breath. The symptoms can develop right after exposure. Sometimes, however, symptoms appear several hours after exposure, possibly at night, and so any link with workplace activities may not be obvious. Some sufferers become so disabled that they cannot work again. Other associated conditions are rhinitis (sneezing/runny nose) and/or conjunctivitis (itchy and inflamed red eyes).