Firefighters have higher rates of several types of cancers, and of all cancers combined, than the US population as a whole, according to a new study from the National Institute for Occupational Safety and Health (NIOSH).

The researchers found that cancers of the respiratory, digestive, and urinary systems accounted mostly for the higher rates of cancer seen in the study population.

The population of firefighters had a rate of mesothelioma - a cancer associated with exposure to asbestos - two times greater than the rate in the US population.

Firefighters can be exposed to contaminants from fires that are known or suspected to cause cancer. These contaminants include combustion by-products such as benzene and formaldehyde, and materials in debris such as asbestos from older structures.

The findings of the new study do not address other factors that can influence risk for cancer, such as smoking, diet, and alcohol consumption. In addition, few women and minorities were in the study population, limiting the ability to draw statistical conclusions about their risk for cancer.

The study analyzed cancers and cancer deaths among 29,993 firefighters from the Chicago, Philadelphia, and San Francisco fire departments who were employed since 1950. The study was led by NIOSH in collaboration with the National Cancer Institute and the Department of Public Health Sciences in the University of California at Davis.

The findings were reported in an article posted on-line by the peer-reviewed journal Occupational and Environmental Medicine.