Occupational asthma is caused by inhaling fumes, gases, dust or other potentially harmful substances while “on the job.” Irritants in high doses that induce occupational asthma include hydrochloric acid, sulfur dioxide or ammonia, which is found in the petroleum or chemical industries. If you are exposed to any of these substances at high concentrations, you may begin wheezing and experiencing other asthma symptoms immediately after exposure. Workers who already have asthma or some other respiratory disorder may also experience an increase in their symptoms during exposure to these irritants. Often, asthma symptoms are worse during days or nights you work, improve when you have time off and start again when you go back to work. People with a family history of allergies are more likely to develop occupational asthma, particularly to some substances such as flour, animals and latex. Allergies play a role in many cases of occupational asthma. This type of asthma generally develops only after months or years of exposure to a work-related substance.