A risk factor is any factor that is associated with increasing someone’s chances of developing a certain condition, such as cancer. Some risk factors are modifiable, such as lifestyle or environmental risk factors, and others cannot be modified, such as inherited factors, age or sex. 
 
Having 1 or more risk factors does not mean that you will develop cancer. Many people have at least 1 risk factor but will never develop cancer, while others with cancer may have had no known risk factors. Even if a person with cancer has a risk factor, it is usually hard to know how much that risk factor contributed to the development of their disease.  
 
Most people with pleural mesothelioma have been exposed to asbestos, which might be present in the home or the workplace. It usually takes a very long time (20–50 years) for mesothelioma to develop after exposure to asbestos. 
 
Asbestos is a naturally occurring mineral made up of tiny fibres. Because it is resistant to heat and fire, asbestos was previously commonly used in building materials, such as walls and roofing, and in insulation. When the asbestos breaks up – for example, when the building materials decompose over time, or during renovations – asbestos fibres can be inhaled and enter the lungs. The fibres can move to the pleural lining of the lung and chest wall, where they eventually cause normal cells to become cancer cells.  
 
The risk of developing mesothelioma is greater in people who have been exposed to asbestos for a long time and at higher levels than in people who have had a lower level of exposure. However, most people who have been exposed to asbestos do not develop mesothelioma. The process that leads to the development of mesothelioma is not well understood. 
 
Although exposure to asbestos is the only known risk factor for mesothelioma, some people who have had no clear exposure to asbestos develop mesothelioma. This suggests that there might be other causes, such as a gene mutation or prior radiation therapy to the chest for another type of cancer. However, these links have not been proven. 
 
Smoking does not seem to be a risk factor for mesothelioma. However, people with mesothelioma who smoke may be advised to quit because smoking might interfere with cancer treatment.