Introduction

Individuals with asthma have chronic inflammation in the bronchi (air passages). As a consequence the bronchial walls swell causing the bronchi to narrow, which can lead to breathlessness. Muscles around the air passages also become irritable so that they contract, causing sudden worsening of symptoms in response to various stimuli, including exposures encountered at work.  The inflammation can also make mucus glands in the bronchi produce excessive sputum which further blocks up already narrowed air passages. If the inflammation is not controlled with treatment, as well as causing acute attacks, it can lead to permanent narrowing and scarring of the air passages¹.

There is no universally accepted definition of "occupational asthma". It can be defined as adult asthma caused by workplace exposures and not by factors outside the workplace. The wide definition of "work-related" asthma includes all cases where there is an association between symptoms and work, and includes "work aggravated asthma", meaning pre-existing or coincidental new onset adult asthma which is made worse by non-specific factors in the workplace.

Asthma caused by specific work factors is of two broad types: "allergic occupational asthma" and "irritant-induced occupational asthma". The former accounts for the majority of cases and typically involves a latency period between first exposure to the specific cause (the "respiratory sensitiser") in the workplace and the onset of symptoms. The latter typically occurs within a period of hours following exposure to high levels of an irritant gas, fume or vapour in the workplace.

The causal mechanisms for occupational asthma vary from one substance to another. Because the range of industries which use substances with the potential to cause asthma is quite broad, and not all employees in these industries will necessarily be exposed, it is difficult to estimate with any confidence the total number of workers at risk. Estimation of the overall scale of the disease, trends in incidence, and identification of high risk occupations and activities, relies on a variety of sources of data each with different strengths and weaknesses.