The primary risk factor for developing mesothelioma is exposure to asbestos. Between 70-80% of people diagnosed with mesothelioma have been exposed to asbestos, usually in the workplace. Although it is possible to develop mesothelioma without any exposure to asbestos, it is very rare. While one person for every million people in the US will be diagnosed with mesothelioma, about 7-13 men who have been exposed to asbestos will be diagnosed with the disease. Symptoms frequently take 20 years to develop, but can take as long as 50 years to occur.