I'm exposed to substances at work that worsen my asthma. What can I do to prevent this?

Take precautions. I agree with dr. Diamond's answer. I would also recommend taking any steps to minimize environmental exposures e.g. Using appropriate gear to reduce breathing in fine particles or irritants. I would strongly advocate seeing a pulmonologist or occupational medicine specialist and getting lung function testing. Chronic allergen or irritant exposure can lead to permanent loss of lung function.
Do the Best. Prevention and long-term control includes avoiding triggers of asthma, such as allergens, smoking and infections. Sometimes daily medicines such as inhaled bronchodilators, steroids, or oral leukotriene inhibitors singularly or in combination are necessary to prevent asthma. For work, either the medical approach or avoid as best you can. For medical therapy, consult your doctor.