Environmental Risk Factors for Cancer

Environmental Risk Factors for Cancer

Also called: Occupational Factors

Reviewed By:
Mark Oren, M.D., FACP

Summary

Environmental factors are among the leading contributors to cancer risk. Every day, the average person encounters or uses many potentially hazardous substances that may contribute to various forms of cancer. These include:

  • Tobacco smoke
  • Radiation
  • Infectious agents such as certain viruses
  • Toxic chemicals or wastes

Substances that have been shown to lead to cancer are known as carcinogens. People may be exposed to carcinogens in their everyday environment inside and outside the home. Exposures that occur in the workplace are known as occupational risk factors.

Federal and international agencies are responsible for identifying and regulating the use of substances that may act as carcinogens. These organizations work to help protect the public from these potentially harmful substances and to thus reduce the overall incidence of cancers caused by environmental factors.

About environmental factors

Environmental factors are potential hazards in a person’s everyday surroundings that may cause changes in cell structure, which may eventually result in cancer. Environmental factors are responsible for up to two-thirds of a person’s cancer risk, according to the National Cancer Institute.

Unrepaired abnormalities in a cell’s DNA (genetic blueprint) can cause cancer. In some cases, these abnormalities are linked to characteristics inherited from parents. In other cases environmental factors are the source of the abnormality. This can occur after exposure to chemicals, radiation or infectious agents. It can also result from lifestyle choices, such as diet and use of alcohol and tobacco.

A substance that causes changes that lead to cancer is known as a carcinogen. In some cases, carcinogens directly damage DNA. In other cases, the effect is more indirect. For example, some carcinogens cause cells to divide more quickly. This increased number of cells also boosts the odds that a cell will appear with a DNA abnormality that might eventually lead to cancer. 

Environmental carcinogens do not cause cancer with every exposure. Some carcinogens are more likely to cause cancer than others, and the cancer risk after exposure to a carcinogen depends on several factors. These factors include the length and intensity of exposure, an individual’s genetic predisposition to cancer and other health related elements.

Before a substance is labeled as a carcinogen, it must undergo rigorous testing and analysis. Laboratory and animal testing helps determine whether or not the substance is likely to cause cancer in humans. There are far too many natural and manmade substances to test for all carcinogens. As a result, scientists select and test chemicals based on the following:

  • Knowledge of the substance’s chemical structure
  • Results of other lab tests
  • Information about the extent of human exposure to the chemical

Carcinogens are also identified through a process known as epidemiologic studies. These investigations look at human populations and search for links between various factors and cancer. 

Scientists combine data from these various studies when deciding whether or not to classify a substance as a carcinogen. Several organizations have their own methods for classifying substances, but the most widely used system is maintained by the International Agency for Research on Cancer (IARC). This branch of the World Health Organization focuses on human cancer and the relationship to it of individuals and the environment. The IARC has identified around 900 substances and assigned them to one of the following categories:

  • 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

The process of identifying carcinogens is not perfect. To ensure human safety, most potential carcinogens are tested in high dosages on animals. The information gleaned from these experiments is then extrapolated and applied to humans in establishing levels of the substance that are safe. Because animals and humans differ, and because of other mitigating factors, these standards are less precise than experts would like.

Epidemiologic studies have their shortcomings. It is very difficult to link a single substance to cancer because humans do not live in a controlled environment and are exposed to many different substances at the same time. In addition, cancer often does not manifest itself until years or decades have passed between the time a person was exposed to a carcinogen and the onset of cancer.

As a result, experts generally err on the side of safety and make conservative assumptions about what constitutes a safe level of human exposure to suspected or known carcinogens. Many federal regulating agencies define acceptable cancer safety risk as being met if the substance does not increase cancer risk by more than one case per million persons over a lifetime. These safety standards are incorporated into laws and procedures established by the Food and Drug Administration (FDA), the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA). Such regulations govern chemical and radiation exposures in the United States.

Chemical factors and cancer risk

There are many environmental factors that contribute to cancer and have been identified as carcinogens. Among these are certain chemicals that are known to potentially increase the risk of cancer. These include:

  • Asbestos. A group of naturally occurring fibrous minerals found in soil and rocks in many parts of the world. Because asbestos fibers are resistant to heat, they have been used as an insulating material for centuries. However, it is now recognized that inhaling or ingesting asbestos fibers can trigger diseases such as lung cancer (with added risk from smoking) and mesothelioma, a rare cancer that affects the thin membranes lining the abdomen and chest. It also can result in scarring of the lungs. Evidence suggests that asbestos exposure may contribute to stomach cancer, colon cancer and cancer of the larynx. The IARC has classified asbestos as carcinogenic to humans (Group 1).
  • Arsenic. A naturally occurring toxic chemical used in pesticides, as a wood preserve and in the manufacture of products including dyes, paints, medicines, soaps and semiconductors. Workplace exposures are known as occupational risk factors. Arsenic can also be found in drinking water, soil, air and food. It has been linked to cancers of the bladder, lung, kidney, liver, nasal passages, prostate and skin (Group 1).
  • Benzene. A colorless, flammable liquid with a sweet-smelling odor that is inhaled or is absorbed through the skin. Benzene is primarily used as a solvent, as a starting material in the synthesis of other chemicals and as a gasoline additive. Workplaces are the most common site of exposure to high levels of benzene, especially for people who work in the rubber industry, oil refineries, chemical plants, shoe manufacturing sites and gasoline-related industries.

    People are regularly exposed to lower doses in the general environment due to gasoline, automobile exhaust fumes, cigarette smoke and emissions from coke ovens. It canbe found in industrial processes and wastewater from certain industries. Benzene is also found in glues, cleaning products, detergents, art supplies and paint strippers.

    Benzene appears to contribute to acute myeloid leukemia (AML). It can be measured in the blood and the breath. Quitting smoking can dramatically reduce the levels of benzene in a person’s blood. People should also avoid gasoline fumes by pumping gasoline carefully and choosing gas stations with vapor recovery systems that capture fumes (Group 1).
  • Vinyl chloride. A chemical used in the manufacture of plastics. According to the American Cancer Society, exposure to vinyl chloride increases the risk of liver cancer, lung cancer and possibly other cancers (Group 1).

Other chemicals have also been associated in cancer. In some cases, the links between the chemicals and cancer are suspected, but not yet confirmed. These chemicals include:

  • Lead. A bluish-gray metal found in small amounts in the earth’s crust that is also found in plants, animals, air, water, dust and soil. Lead is an indestructible element that persists once released into the environment. It can move from one medium to another. Exposure to lead can occur from swallowing or inhaling it, with known toxic effects in the human body. Lead is a suspected factor in cancers of thelungs, brain, stomach, kidneys and colon. Although it is not proven that lead contributes to cancer, the IARC classifies it as possibly carcinogenic to humans (Group 2B), and the EPA considers it a probable carcinogen in humans.
  • Methyl tertiary-butyl ether (MTBE). Part of a group of chemicals known as fuel oxygenates, which are added to increase gasoline’s oxygen content. Federal legislation in the early 1990s required oxygenates to be added to gasoline to make it burn better and to decrease carbon monoxide and other vehicle emissions. However, MTBE is more soluble in water than most gasoline components and is more resistant to biodegradation. As a result, it has begun showing up in detectable amounts in the air and drinking water of many communities. It is not known whether or not MTBE causes cancer in humans, but it has been shown to cause lymphomas and leukemias leukemias in laboratory animals. Some of these animals also have developed cancer of the kidneys, liver, testicles and uterus. The IARC considers MTBE to be unclassifiable as to carcinogenicity in humans (Group 3), and use of MTBE is being phased out.

There are some chemicals that, although suspected, have not been positively linked to cancers in epidemiological studies, including:

  • Aspartame. A low-calorie artificial sweetener about 200 times sweeter than sugar that once was suspected of causing brain cancer. It can be found in tabletop sweeteners, foods and beverages. In the early 1980s, the Food and Drug Administration (FDA) approved aspartame for use in many products, including breakfast cereals, chewing gum, dry beverage mixes, carbonated beverages, instant tea and coffee, gelatins and puddings.

    Recent studies appear to have discredited the theory that aspartame causes cancer. The FDA, World Health Organization and other organizations have declared aspartame safe for use for most individuals, with the exception of people with the genetic disorder phenylketonuria. These individuals cannot metabolize the amino acid phenylalanine, which appears in aspartame.

Radiation and cancer risk

Radiation is the emission of a particular type of energy from any source. It is typically associated with radioactive materials or nuclear reactions. Most forms of radiation have not been linked to cancer, but high-frequency radiation such as ionizing radiation or ultraviolet radiation can cause the type of genetic damage that leads to cancer.

Ionizing radiation consists of high-energy waves that penetrate cells and cause ionization, the development of a positive charge in a molecule (group of atoms) that is normally neutral (without a charge). Ionized molecules are unstable and undergo chemical changes that lead to the formation of free radicals, which can damage the molecule or nearby molecules. DNA is a molecule potentially affected by this process, and a mutation in a DNA molecule can contribute to cancer.

Exposure to ionizing radiation comes from natural background radiation (such as the cosmic rays of the solar system or the radioactive gas, radon) non-medical synthetic radiation (such as from nuclear weapons testing and occupational and commercial sources) or medical radiation (such as from diagnostic x-rays or radiation therapy). Radon, an invisible gas released when uranium breaks down, can accumulate indoors at dangerous levels. Houses are now usually tested for radon when they are sold, especially in areas with high levels of uranium in the soil.

Ionizing radiation is one of the few scientifically proven human carcinogens. Its effects are related to dose exposure – the higher the dosage level, the more likely it is to potentially trigger cancer. The thyroid gland and bone marrow are most susceptible to radiation, and the kidneys, bladder and ovaries are the least affected. Leukemias are the most common cancers associated with ionizing radiation. Other cancers associated with this form of radiation include breast cancer and lung cancer.  Although medical treatments with radiation therapy present risks for cancer, the benefits of the therapy usually far outweigh the risks.

The sun is the greatest source of ultraviolet radiation, which causes basal cell and squamous cell cancers, the most common types of skin cancer,and melanoma, the most dangerous form. Other forms of radiation include electromagnetic radiation (emitted from cell phones), infrared radiation, radiofrequency radiation and microwaves. Although there has been speculation that some of these forms of radiation may cause cancer, no link has ever been established. In addition, studies have examined the association between close approximation to power lines and cancer. The evidence remains inconclusive on all of these factors and further research is being conducted.

Infectious agents and cancer risk

Recently, science has begun to recognize the cancer risk posed by certain viruses, bacteria and parasites in humans. Only about 10 percent of cancers in the United States are linked to these agents, but they may contribute to as much as 20 percent of cancers in developing countries, according to the American Cancer Society (ACS).  Most people who have infections never develop cancer and even though infections are often contagious, the potential cancer is not. Infectious agents associated with cancer include:

  • Viruses. Consist of a small group of genes surrounded by a protein coating. They enter a cell and use the cell machinery to produce more viruses. In some cases, the virus inserts its DNA or RNA into the host cell. This can affect the host cell’s genes and set the stage for it to become cancerous. Viruses known to sometimes cause cancer include:
    • Human T-cell leukemia/lymphoma virus-1 (HTLV-1). Associated with a type of lymphocytic leukemia and non-Hodgkin’s lymphoma called adult T-cell leukemia/lymphoma (ATL). It is found mostly in southern Japan, the Caribbean, Central Africa, parts of South America and in some immigrant populations in the southeastern United States.
    • Human papillomaviruses (HPVs). A group of more than 100 related viruses that manifest as warts on the skin, mouth, genital organs or larynx. Certain types of HPV are the primary cause of cervical cancer. Others contribute to cancers of the penis, anus, vagina and vulva. In 2006, the Food and Drug Adminstration approved the first vaccine for several types of HPV linked to cervical cancer. Recent studies have suggested that testing for past exposure to these types of HPV eventually may be able to replace Pap tests for cervical cancer screening in women.
    • Human herpes virus 8 (HHV-8). Also known as Kaposi sarcoma–associated herpes virus (KSHV), it is found in virtually all tumors in patients who have Kaposi sarcoma. This is a rare, slow-progressing cancer that causes tumors just beneath the skin. Recent findings also indicate that it may be a factor in the development of plasma cell tumors such as multiple myeloma.
    • Epstein-Barr virus (EBV). Another herpes virus probably best known for causing infectious mononucleosis, also known as “mono.” It infects and resides in white blood cells and increases the risk of nasopharyngeal cancer, stomach cancer, Hodgkin’s lymphoma and some types of non-Hodgkin’s lymphoma. Research has suggested it also may play a role in aggressive forms of breast cancer. If an individual is infected with EBV, it usually occurs before the age of 20. The great majority of people infected with EBV never develop cancer. Cancers associated with this virus are more common in Africa and parts of Southeast Asia.
    • Human immunodeficiency virus (HIV). HIV, the virus that causes AIDS, it is not thought to directly contribute to the development of cancers, but does increase the risk of some types of cancer such as Kaposi sarcoma, Hodgkin’s lymphoma, certain types of non-Hodgkin’s lymphoma, testicular cancer and penile cancer.
    • Hepatitis B virus (HBV) and hepatitis C virus (HCV). These viruses cause a type of liver infection known as viral hepatitis that can become chronic and increase the risk of hepatocellular carcinoma, the chief form of liver cancer. Between 25 percent and 50 percent of hepatocellular carcinomas are related to these infections. HCV may also be a factor in some types of non-Hodgkin’s lymphomas.
  • Bacteria. Microscopic one-celled organisms that are self-sufficient and that multiply by subdivision. In some cases, bacteria are not harmful to the body, and some are actually helpful. However, infectious bacteria can cause illness. Bacteria associated with cancer include:
    • Helicobacter pylori (H. pylori). This bacterium can cause chronic atrophic gastritis, inflammation and damage to the inner layer of the stomach. This is a change in the stomach lining that may become precancerous. Patients with the most common form of stomach cancer (adenocarcinoma) have a higher rate of infection with the bacteria than people without the cancer. H. pylori infection also has been associated with some stomach lymphomas.
    • Chlamydia trachomatis. Bacteria that infects the female reproductive system that may raise the risk for cervical cancer.
  • Parasites. Certain parasitic worms that live in the human body can raise the risk of cancer. These organisms are not found in the United States, but are a potential danger to people who live or travel in other regions of the world. This parasites include:
    • Opisthorchis viverrini and Clonorchis sinensis. These are types of flatworms known as liver flukes that have been associated with increased risk of cancer of the bile ducts. These infections occur almost exclusively in East Asia.
    • Schistosoma haematobium. Infection is known as schistosomiasis, and it has been associated with cancer of the urinary bladder. The parasite is found in developing countries in Africa and Asia.

Lifestyle factors and cancer risk

There is a direct link between food choices and 30 to 40 percent of cancers, according to the American Cancer Society (ACS). Choosing more vegetables, fruits and whole grains over animal fats may reduce the risk of colorectal, endometrial, esophageal, kidney, prostate, stomach and other cancers.

About one-third of all cancer deaths are attributed to use of tobacco products, according to the ACS. Smoking is responsible for about 87 percent of lung cancers and is associated with cancers of the mouth, pharynx, larynx, esophagus, pancreas, kidney and bladder.

The use of alcohol and recreational and illegal drugs weakens the body’s defense system. When the immune system is weakened, the body is less able to fight off disease, including cancer. For example, some studies have shown a connection between higher alcohol consumption and an increased risk of breast cancer. Alcohol is also a risk factor for cancers in other body areas, including the lungs, mouth, lips, colon, liver, esophagus and the larynx. Some research, however, has found that low to moderate consumption of red wine has cardiovascular and other health benefits, including reduced risk of prostate cancer, lung cancer and possibly other cancers. The association of alcohol consumption and cancer continues to be studied by researchers.

Other environmental factors and cancer risk

Several other environmental factors have been the source of speculation over their potential cancer risk. However, to date none of these have been positively linked to a higher cancer risk. They include:

  • Pesticides. Insecticides, herbicides and other chemicals are widely used in agriculture. High doses of such chemicals sometimes have been associated with cancer in animals. But concentrations found in foods are low, and have not been tied to increased cancer risk in humans. People in industry or farming who are exposed to higher levels of pesticides may be at increased risk of cancer. Research is ongoing to ensure that foods are safe and that alternative pest-control measures are developed that lower cancer risk.
  • Toxic wastes. Some toxic chemicals found at dump sites can be carcinogenic in high doses. Air, water and soil pollution are the chief avenues of exposure. However, most communities enforce rules to ensure that these sites emit very low or negligible levels.
  • Nuclear power plants. These plants emit ionizing radiation, which has been shown to cause cancer. These emissions are tightly controlled, which keeps exposure levels negligible for those in the surrounding community. Studies have shown that clusters of cancer are no more likely to occur near nuclear power plants than they do in other areas.

List of known carcinogens

The International Agency for Research on Cancer (IARC) has tested hundreds of substances as possible carcinogens. These substances have been categorized by the IARC according to their risk level. Group 1 is defined as agents that are known to be carcinogenic to humans and includes the following:

Agents and groups of agents

  • Aflatoxins (naturally occurring mixtures of this substance)
  • 4-Aminobiphenyl
  • Arsenic and arsenic compounds
  • Asbestos
  • Azathioprine
  • Benzene
  • Benzidine
  • Beryllium and beryllium compounds
  • N,N-Bis(2-chloroethyl)-2-naphthylamine (Chlornaphazine)
  • Bis(chloromethyl)ether and chloromethyl methyl ether (technical-grade)
  • 1,4-Butanediol dimethanesulfonate (Busulphan; Myleran)
  • Cadmium and cadmium compounds
  • Chlorambucil
  • 1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (Methyl-CCNU; Semustine)
  • Chromium [VI] compounds
  • Cyclophosphamide
  • Cyclosporin (ciclosporin)
  • Diethylstilbestrol (DES)
  • Epstein-Barr virus
  • Erionite
  • Estrogen therapy, postmenopausal
  • Estrogens, steroidal and nonsteroidal
  • Ethylene oxide 
  • Etoposide in combination with cisplatin and bleomycin
  • Formaldehyde
  • Gallium arsenide
  • Gamma radiation
  • Helicobacter pylori (infection with)
  • Hepatitis B or C virus (chronic infection with)
  • Herbal remedies containing plant species of the genus Aristolochia
  • Human immunodeficiency virus type 1 (infection with this virus)
  • Human papillomavirus types 16 and 18
  • Human T-cell lymphotropic virus type I
  • Melphalan
  • 8-Methoxypsoralen (Methoxsalen) plus ultraviolet A radiation
  • MOPP and other combined chemotherapy including alkylating agents
  • Mustard gas (sulfur mustard)2-Naphthylamine
  • Neutrons
  • Nickel compounds
  • Opisthorchis viverrini (infection with this parasite)
  • Oral contraceptives, combined (however, these agents have a protective effect against cancers of the ovary and endometrium)
  • Oral contraceptives, sequential
  • Phosphorus-32, as phosphate
  • Plutonium-239 and its decay products, as aerosols
  • Radioiodines, short-lived isotopes, including iodine-131, from atomic reactor accidents and nuclear weapons detonation (exposure during childhood)
  • Radionuclides, alpha- or beta-particle-emitting, internally deposited.
  • Radium-222, 224, 226 and 228 and their decay products
  • Schistosoma haematobium (infection with this parasite)
  • Silica, crystalline (inhaled in the form of quartz or cristobalite from occupational sources)
  • Solar radiation
  • Talc containing asbestiform fibers
  • Tamoxifen (however, there is also conclusive evidence that this agent reduces the risk of contralateral breast cancer)
  • 2,3,7,8-Tetrachlorodibenzo-para-dioxin
  • Thiotepa
  • Thorium-232 and its decay products, administered intravenously as a colloidal dispersion of thorium-232 dioxide
  • Treosulfan
  • Vinyl chloride
  • X- and Gamma radiation

Mixtures

  • Alcoholic beverages
  • Analgesic mixtures containing phenacetin
  • Areca nut
  • Betel quid with or
    without tobacco
  • Coal-tar pitches
  • Coal tars
  • Mineral oils, untreated and mildly treated
  • Salted fish
    (Chinese-style)
  • Shale oils
  • Soots
  • Tobacco products, smokeless
  • Wood dust

Circumstances of exposure

  • Aluminum production
  • Arsenic in drinking water
  • Auramine manufacture
  • Boot and shoe manufacture and repair
  • Coal gasification  
  • Coke production (from high heating of coal)
  • Furniture and cabinet making
  • Hematite mining (underground) with exposure to radon
  • Iron and steel founding
  • Involuntary smoking
  • Isopropanol manufacture (strong-acid process)
  • Magenta manufacture
  • Painter (occupational exposure)
  • Rubber industry
  • Strong inorganic acid mists containing sulfuric acid (occupational exposure)
  • Tobacco smoking

Questions for your doctor

Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor or healthcare professional the following questions about environmental factors and cancer:

  1. Are there elements in my home or work environment that are carcinogenic?
  2. Which cancers present the greatest risk based on my environment?
  3. Which cancers present the greatest risk based on my lifestyle habits?
  4. Am I using any products or medications that raise my risk of cancer?
  5. Are there any tests that can determine the extent of my exposure to hazardous materials?
  6. Which factors can I change in my life to reduce my risk of cancer?
  7. Can you refer me to programs to help me quit smoking?
  8. Who can I contact if I want information about a carcinogen?
  9. How can I determine if a product is hazardous to my health?
  10. If I have been exposed to a carcinogen in the past, how will it my future health?
  11. Who can I contact if I feel a company is not following regulations for hazardous materials?
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