Causes of Cancer

Age
Advancing age is the most important risk factor for cancer overall and for many individual cancer types. The incidence rates for cancer climb steadily as age increases. 

Tobacco
Tobacco use is a leading cause of cancer and of death from cancer. People who use tobacco products or who are regularly around environmental tobacco smoke (also called second hand smoke) have an increased risk of cancer because tobacco products and second hand smoke have many chemicals that damage DNA. Tobacco use causes many types of cancer, including cancer of the lung, larynx (voice box), mouth, esophagus, throat, bladder, kidney, liver, stomach, pancreas, colon and rectum, and cervix, as well as acute myeloid leukemia. People who use smokeless tobacco (snuff or chewing tobacco) have increased risks of cancers of the mouth, esophagus, and pancreas. There is no safe level of tobacco use. People who use any type of tobacco product are strongly urged to quit.  People who quit smoking, regardless of their age, have substantial gains in life expectancy compared with those who continue to smoke. Also, quitting smoking at the time of a cancer diagnosis reduces the risk of death.

Obesity
People with obesity may have an increased risk of several types of cancer, including cancers of the breast (in women who have been through menopause), colon, rectum, endometrial (lining of the uterus), oesophagus, kidney, pancreas, and gallbladder. Conversely, eating a healthy diet, being physically active and keeping a healthy weight may help reduce risk of some cancers. These healthy behaviours are also important to lessen the risk of other illnesses, such as heart disease, type II diabetes, and high blood pressure.

Alcohol
Drinking alcohol can increase your risk of cancer of the mouth, throat, oesophagus, larynx (voice box), liver, and breast. The more you drink, the higher your risk. The risk of cancer is much higher for those who drink alcohol and also use tobacco.
 

Infectious agents
Certain infectious agents, including viruses, bacteria, and parasites, can cause cancer or increase the risk that cancer will form. Some viruses can disrupt signalling that normally keeps cell growth and proliferation in check. Also, some infections weaken the immune system, making the body less able to fight off other cancer-causing infections. And some viruses, bacteria, and parasites also cause chronic inflammation, which may lead to cancer. Examples are given below.
 Epstein- Barr Virus (EBV)
Hepatitis B Virus and Hepatitis C Virus (HBV and HCV)
Human Immunodeficiency Virus (HIV)
Human Papillomaviruses (HPVs)
Human T-Cell Leukemia/Lymphoma Virus Type 1 (HTLV-1)
Helicobacter pylori (H. pylori)
 

 Lifestyle
Changes in the food we eat and our unhealthy habits increase our risk of cancer. Eating little or no vegetables and fruits, decreased physical activity or sedentary habit, lack of exercise, obesity, habits like tobacco use, alcohol etc may increase our risk of cancer. Overweight and obesity are causally associated with several common cancer types, including cancers of the esophagus, colorectal, breast in postmenopausal women, endometrium and kidney.
 

Outdoor air pollution
Exposure to particulate matter, automobile exhaust. 

Carcinogens

Carinogens known to cause cancer in Human beings.
Given below is the list of Group 1 carcinogens which are classified by International Agency for Research on Cancer as carcinogenic to humans.

•    Acetaldehyde (from consuming alcoholic beverages)
•    Aflatoxins
•    Alcoholic beverages
•    Aluminium production
•    Areca nut
•    Arsenic and inorganic arsenic compounds
•    Asbestos (all forms) and mineral substances (such as talc or vermiculite) that contain asbestos
•    Benzene
•    Benzidine and dyes metabolized to benzidine
•    Benzo[a]pyrene
•    Beryllium and beryllium compounds
•    Betel quid, with or without tobacco
•    1,3-Butadiene
•    Cadmium and cadmium compounds
•    Chromium (VI) compounds
•    Clonorchis sinensis (infection with), also known as the Chinese liver fluke
•    Coal, indoor emissions from household combustion
•    Coke production
•    Engine exhaust, diesel
•    Epstein-Barr virus (EBV) (infection with)
•    Ethanol in alcoholic beverages
•    Fission products, including strontium-90
•    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 papilloma virus (HPV) types 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59 (infection with) (Note: The HPV types that have been classified as carcinogenic to humans can differ by an order of magnitude in risk for cervical cancer)
•    Human T-cell lymphotropic virus type I (HTLV-1) (infection with)
•    Ionizing radiation (all types)
•    Kaposi sarcoma herpes virus (KSHV), also known as human herpes virus 8 (HHV-8) (infection with)
•    Leather dust
•    Mineral oils, untreated or mildly treated
•    Neutron radiation
•    Nickel compounds
•    N'-Nitrosonornicotine (NNN) and 4-(N-Nitrosomethylamino)-1-(3-pyridyl)-1-butanone (NNK)
•    Opisthorchis viverrini (infection with), also known as the Southeast Asian liver fluke
•    Opium consumption
•    Outdoor air pollution (and the particulate matter in it)
•    Painter (workplace exposure as a)
•    Phosphorus-32, as phosphate
•    Plutonium
•    Processed meat (consumption of)
•    Radon-222 and its decay products
•    Salted fish (Chinese-style)
•    Schistosoma haematobium (infection with)
•    Shale oils
•    Silica dust, crystalline, in the form of quartz or cristobalite
•    Solar radiation
•    Soot (as found in workplace exposure of chimney sweeps)
•    Talc containing asbestiform fibres
•    2,3,7,8-Tetrachlorodibenzo-para-dioxin (TCDD); "dioxin"
•    Thorium-232 and its decay products
•    Tobacco, smokeless
•    Tobacco smoke, second hand
•    Tobacco smoking
•    Ortho-Toluidine
•    Ultraviolet (UV) radiation, including UVA, UVB, and UVC rays
•    Ultraviolet-emitting tanning devices
•    Welding fumes
•    X- and Gamma-radiation