Occupational asthma is new onset asthma or the recurrence of previously quiescent asthma directly caused by exposure to an agent at workplace. It is an occupational lung disease and a type of work-related asthma. Agents that can induce occupational asthma can be grouped into sensitizers and irritants.

Sensitizer-induced occupational asthma is an immunologic form of asthma which occurs due to inhalation of specific substances (i.e., high-molecular-weight proteins from plants and animal origins, or low-molecular-weight agents that include chemicals, metals and wood dusts) and occurs after a latency period of several weeks to years.

Irritant-induced (occupational) asthma is a non-immunologic form of asthma that results from a single or multiple high dose exposure to irritant products. It is usually develops early after exposure; however, it can also develop insidiously over a few months after a massive exposure to a complex mixture of alkaline dust and combustion products, as shown in the World Trade Center disaster. Unlike those with sensitizer-induced occupational asthma, subjects with irritant-induced occupational asthma do not develop work-related asthma symptoms after re-exposure to low concentrations of the irritant that initiated the symptoms. Reactive airways dysfunction syndrome (RADS) is a severe form of irritant induced asthma where respiratory symptoms usually develop in the minutes or hours after a single accidental inhalation of a high concentration of irritant gas, aerosol, vapor, or smoke.

Another type of work-related asthma is work-exacerbated asthma (WEA) which is asthma worsened by workplace conditions but not caused by it. WEA is present in about a fifth of patients with asthma and a wide variety of conditions at work, including irritant chemicals, dusts, second-hand smoke, common allergens that may be present at work, as well as other "exposures" such as emotional stress, worksite temperature, and physical exertion can exacerbate asthma symptoms in these patients. Both occupational asthma and work-exacerbated asthma can be present in an individual.

A number of diseases have symptoms that mimic occupational asthma, such as asthma due to nonoccupational causes, chronic obstructive pulmonary disease (COPD), irritable larynx syndrome, hyperventilation syndrome, hypersensitivity pneumonitis, and bronchiolitis obliterans.

Like other types of asthma, it is characterized by airway inflammation, reversible airways obstruction, and bronchospasm, but it is caused by something in the workplace environment. Symptoms include shortness of breath, tightness of the chest, coughing, sputum production and wheezing. Some patients may also develop upper airway symptoms such as itchy eyes, tearing, sneezing, nasal congestion and rhinorrhea.

Symptoms may develop over many years as in sensitizer-induced asthma or may occur after a single exposure to a high-concentration agent as in case of RADS.

At present, over 400 workplace substances have been identified as having asthmagenic or allergenic properties. Agents such as flour, diisocyanates, latex, persulfate salts, aldehydes, animals, wood dusts, metals, enzymes usually account for the majority cases; however, the distribution of causal agents may vary widely across geographic areas, depending on the pattern of industrial activities. For example, in France the industries most affected are bakeries and cake-shops, automobile industry and hairdressers, whereas in Canada the principal cause is wood dust, followed by isocyanates. Furthermore, the most common cause of occupational asthma in the workplace are isocyanates. Isocyanates are used in the production of motor vehicles and in the application of orthopaedic polyurethane and fibreglass casts.