What Are Carcinogens and How Do They Cause Cancer?

A carcinogen is any substance or agent that causes cancer. It does so by altering the cellular metabolism or by damaging DNA in our cells, interfering with normal cellular processes.  The identification of substances in the environment that cause people to become ill with cancer helps in prevention efforts.

Carcinogens can be chemical substances, viruses, or even medications and radiation therapies used to treat cancer. While a carcinogen or a combination of carcinogens can cause cancer, it may not always the only reason since the tendency to develop cancer can be inherited.

Carcinogenic substances can cause cancer in the following different ways:

• By directly damaging the DNA in cells leading to mutations (disrupting the normal process of cells)
• By not affecting the DNA directly, but instead causing cells to divide at a faster rate than normal, which can increase the chances that DNA changes and mutations will occur.

The cell’s DNA can be damaged by a wide range of substances and exposures, including:

• Lifestyle—what you eat, if you smoke, lack of physical activity
• Natural exposure—to ultraviolet light, radon gas, infectious agents
• Medical treatment—radiation and chemotherapy, hormones, immunosuppressants
• Workplace exposure—some jobs have increased exposure to industrial chemicals or products
• Household exposure—cleaning products, paints, herbicides, and pesticides, etc.
• Pollution—outdoor air pollution or even secondhand tobacco smoke

Some carcinogens do not directly cause cancer but can lead to cancer. Some carcinogens cause cancer if you have long-term, high levels of exposure. How your body reacts to this level of exposure, the length, and the time, the intensity of the exposure, combined with your genetic makeup will determine the risk of developing cancer.

Carcinogens are classified by The International Agency for Research on Cancer (IARC). The IARC is part of the World Health Organization (WHO) and its main goal is to determine the cancer-causing potential of different substances and classify carcinogens accordingly.

Carcinogens are classified into one of the following groups:

• Group 1: Carcinogenic to humans
• Group 2A: Probably carcinogenic to humans.
• Group 2B: Possibly carcinogenic to humans.
• Group 3: Unclassifiable as to carcinogenicity in humans
• Group 4: Probably not carcinogenic to humans 

It can be difficult to test and classify substances as carcinogenic to humans as it is not ethical to test to see if something can cause cancer by exposing people to it. Therefore researchers have only been able to classify a little over 100 substances as “carcinogenic to humans.” 

Some of the most common substances known as being carcinogenic to humans include (there are many more):

• Alcoholic beverages
• Aluminum production
• Arsenic and inorganic arsenic compounds
• Asbestos (all forms) and mineral substances (such as talc or vermiculite) that contain asbestos
• Benzene
• Cadmium and cadmium compounds
• Coal, indoor emissions from household combustion
• Engine exhaust, diesel
• Epstein-Barr virus (infection with)
• Estrogen postmenopausal therapy
• Formaldehyde
• Helicobacter pylori (infection with)
• Hepatitis B virus (chronic infection with)
• Hepatitis C virus (chronic infection with)
• Human immunodeficiency virus type 1 (HIV-1) (infection with)
• Human papillomavirus (HPV) (infection with certain types) 
• Ionizing radiation (all types)
• Iron and steel founding (workplace exposure)
• Leather dust
• Mineral oils, untreated or mildly treated
• MOPP and other combined chemotherapy including alkylating agents
• Nickel compounds
• Outdoor air pollution
• Paint (workplace exposure as a painter)
• Processed meat (consumption of)
• Radon 
• Rubber manufacturing industry
• Shale oils
• Silica dust, crystalline, in the form of quartz or cristobalite