Environmental Exposure
Population (epidemiological) and laboratory studies have led to the discovery of many potential environmental factors in the initiation, promotion and progression of cancer. Starting with Pott’s observations in the 18th century, certain occupations have been associated with an increased risk of cancer development. The recognition of increased scrotal cancer in chimney sweeps due to coal and tar exposure was followed by an observation in a British factory that all men distilling 2-napthylamine developed bladder cancer. ⁽¹⁾ Nickel refining, leather working and woodworking have also been associated with an increased risk of specific cancers due to chronic exposure to carcinogenic chemicals. Exposure to mustard gas, used as a chemical warfare agent in World War I has been associated with a higher risk of respiratory tract and lung cancers due to itsmutagenic properties.⁽²⁾ Of interest, some chemotherapy drugs are derivatives of mustard gas and are useful for the very same reason; they are highly mutagenic.

Geographical Influences
There have also been associations made between different geographical regions and particular cancers. Stomach cancer is 5-6 times higher among Japanese men, attributed to the consumption of fermented foods; breast cancer is 20 times higher among American women, attributed to the high fat American diet; and liver cancer is 10 times higher in Africa, which correlates with high rates of Hepatitis B infection. ⁽³⁾ Liver cancer may also be caused by aflatoxin, a food contaminant produced by fungi. This compound is prevalent in grain stores in tropical and subtropical regions because moist grain is a very good place for the fungi to live. ⁽⁴⁾

Lifestyle
The impact of many environmental factors can be reduced by making healthy lifestyle choices. One of the most potent carcinogens in humans is benzo[a]-pyrene, a compound found in cigarette smoke. ⁽¹⁾ In fact, the tar in cigarette smoke includes both initiators and promoters, making it especially dangerous. Alcohol is a promoter of carcinogenesis in humans, as is asbestos. Additionally, UV radiation, from exposure to the sun or tanning beds, is a powerful initiator in humans and is a cause of skin cancer. ⁽³⁾

See our detailed section on tobacco.

Drugs
Tamoxifen, a chemotherapeutic agent used to combat estrogen receptor positive (ER+) breast cancer, increases the risk ofendometrial cancer by increasing the rate of endometrial cell proliferation. For this reason, long-term tamoxifen treatment is losing popularity in favor of aromatase inhibitors.⁽⁵⁾ Estrogens themselves may also be important in the promotion of tumors, particularly in post-menopausal women receiving exogenous estrogens, due to their ability to increase mammary and endometrial cell division rates.⁽⁶⁾ This is an area of active research.

There are factors that can predispose an unborn fetus to developing cancer later in life. These include exposure to radiation or the synthetic estrogen diethylstilbestrol (DES).⁽⁷⁾

A Closer Look at Aflatoxin: Physical Impact and Associated Cancer

Exposure: Aflatoxin is produced by Aspergillus flavus and Aspergillus parasiticus fungi. The fungi synthesize aflatoxin when they are living in warm, moist conditions. These fungi are prevalent among crops such as rice, corn, cassava, nuts, peanuts, chilies, and spices. Countries with the highest amounts of these organisms lie within 40 degrees latitude north or south of the equator. Storage of food under warm, moist conditions increases the risk of aflatoxin contamination. Approximately 4.5 billion persons living in developing countries are chronically exposed to uncontrolled amounts of aflatoxin.⁽⁸⁾

Associated Cancer: Aflatoxicosis is the disease that results from ingestion of aflatoxin. This disease can present in one of two forms. The first is an acute illness that results from exposure to large amounts of the toxin over a short period of time. Adult humans have a high tolerance for aflatoxin. Ingestion of large amounts of aflatoxin usually causes liver damage and acute illness but is rarely fatal. However, exposure to high levels of the toxin can cause death in children. The second form of aflatoxicosis is due to low level chronic exposure to aflatoxin. Chronic aflatoxin exposure has an additive effect and can lead to the development of liver cancer. Aflatoxin increases the risk of liver cancer (usually in the form of hepatocellularcarcinoma or HCC) in all persons who ingest contaminated food. It can also increase the risk of lung cancer in workers who handle the grain. Infection with either the Hepatitis B or C viruses combined with exposure to aflatoxin can increase a person’s risk of developing liver cancer by as much as 30 fold over a person who is exposed to aflatoxin but is not infected with hepatitis virus. Infection with the Hepatitis B virus decreases a person’s ability to detoxify aflatoxin via the liver. This can partially account for the greatly increased risk of cancer development in individuals exposed to both aflatoxin and hepatitis virus.⁽⁸⁾

In its initial stages, HCC causes no noticable symptoms. It can grow for up to 3 years before causing physical symptoms.⁽⁹⁾For this reason, most HCC patients present with advanced stages of the disease, making treatment difficult. Non surgical treatments are only minimally effective. HCC patients have shown to have, at best, a 25% response rate to chemotherapy, as most HCC tumors are resistant to chemotherapy. Liver transplantation is the only current cure for HCC. Unfortunately, based on the number, size, location, and severity of the underlying disease, not all patients are candidates for a transplant

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