How could the environment be related to cancer?
Cancers are a diverse collection of diseases, each with a different clinical appearance, set of symptoms, and range of severity. What they have in common is the process occurring at the cellular level that brings these diseases about. Although this cellular process may not be the main concern among cancer patients and their families, it allows us to group cancers together, particularly when considering compounds in the environment that may be related to cancer.
All cancers are a result of a single type of cell, somewhere in the body, reproducing in a disordered way, usually at an abnormally fast pace. Knowing this helps us understand what kinds of environmental factors might be associated with cancer. Possibilities include:
- Chemicals known to damage DNA, which contains the information cells use to reproduce normally
- Chemicals that alter how a cell regulates its own reproduction, which may have to do with its DNA or with cellular proteins that interact with DNA.
- Chemicals that induce epigenetic modifications. Epigenetic modifications are reversible, heritable changes that impact how genes are expressed without actually changing DNA. In particular, they may play a role in cancers thought to be associated with infectious agents.
- Infectious agents (particularly viruses) that take over cellular functions and may re-arrange pieces of DNA as part of their infectious process
- Agents that harm the immune system. A person's immune system is believed to play a role in clearing the body of at least some types of cancerous cells, so damage to the immune system may encourage some forms of cancer.
Scientists have found at least one example for each of these scenarios, but it's generally agreed that there may be others yet to be observed. In its 2008-2009 report entitled Reducing Environmental Cancer Risk, the President's Cancer Panel notes that environmental sources of cancer have been largely underestimated and understudied, particularly in light of the growing incidence of a number of cancer types.
To what degree is cancer an environmental disease?
It is not easy to answer for several reasons, the most prominent of which is that the majority of cancers take years, or even decades, to develop. Scientists rarely have the opportunity to take a large number of people, measure the amount of a specific pollutant they are exposed to, and then follow them over time to see if they develop cancer.
There are two exceptions to this problem, however, and they form the foundation of much of what we do know about the environment and cancer:
- Behavioral risk factors: Behaviors such as cigarette smoking or diet frequently persist over many years, so it can be relatively simple for people to quantify exposures they have experienced, even decades after the fact.
- Occupational risk factors: People are frequently able to recall their work histories during the course of their lives, and in some cases records of the types and quantities of chemicals to which they may have been exposed have been kept.
In spite of how little we know, there is concern that environmental factors play a larger role in cancer than has previously been thought. Much of this concern stems from the fact that the incidence rates of a large variety of cancers have been increasing over the last several decades. Most traditional explanations for this increase fail to explain why this is happening. For example:
- Rates of alcohol and tobacco consumption, the behaviors most consistently linked to cancer, have been declining during the same period
- It is unlikely that genetic susceptibility to cancer has changed at a rate rapid enough to account for the increase
- Cancer rates have increased even among young and middle-aged people, so increased life-expectancy cannot account for the shift
- Obesity has been increasing to alarming rates in the U.S. and abroad, and obesity may be related to some-but not all-of the cancer types for which rates have been increasing. Recent meta-analyses have linked increased body weight with esophageal cancer, thyroid, colon, and kidney cancers in men and with endometrial, gallbladder, and esophageal cancers in women.
Meanwhile, the number and quantities of compounds that could cause cancer-at least in theory-in the global environment have been increasing.
Breast cancer is the most commonly diagnosed cancer among women. Some breast cancers are known to respond to hormone levels in a woman's bodies, and a variety of persistent chemicals such as DDT, PCBs, and dioxin have the potential to alter hormone activities in people.
Lung cancer is the leading cause of cancer death in the United States. Occupational exposure to certain metals, polycyclic aromatic compounds, and vinyl chloride have been associated with lung cancer. Certain types of air pollution have also been associated with this disease.
Occupational exposures to truck exhaust and compounds used in the textile and leather dye industries have been associated with bladder cancer.
Occupational exposures to radiation and a variety of metallic, petrochemical, and organic compounds have been associated with brain cancer. There is some concern that children may be at risk when their parents are exposed to these substances, particularly during pregnancy.
Increases in risk for thyroid cancer have been observed with exposure to ionizing radiation.
Rates of this disease have increased dramatically over the last few decades, although no reasons for this increase have yet been found.
Rates of various forms of leukemia are increasing among children, although the reasons for this are not known. Exposure to ionizing radiation, benzene, and some agricultural chemicals has been associated with increased risk.
Mesothelioma is a rare cancer affecting the linings of internal organs, most often the lungs. It is most often caused by exposure to asbestos.
Ultraviolet light (UV) radiation from prolonged sun exposure or tanning beds has been associated with this cancer. Exposures to environmental pollutants such as arsenic or dioxins may also contribute to increased risk.
Exposures to environmental pollutants such as solvents and persistent organic compounds may contribute to increased risk of these cancers.
In addition to smoking and long-term use of certain pain medications, there is also concern that exposure to cadmium, arsenic, and disinfection byproducts in drinking water may increase risk for these cancers.
- A metal that is naturally occurring but becomes available to humans through industrial processes and mining
- Exposure most commonly occurs through contaminated water, foods, or medications
- A chemical that is both synthetic (i.e. human-made) and from natural sources (i.e. emissions from volcanoes and forest fires)
- Commonly used in industries to make other chemicals and products, and is a component of crude oil, gasoline, and cigarette smoke
- Exposure most commonly occurs through breathing contaminated air; gases from products (i.e. glues, paints, wax); or working in industries that make or use benzene
- A synthetic insecticide that was banned for use in the US in 1972, but remains persistent in the environment
- Exposure most commonly occurs through eating contaminated foods, breathing contaminated air, or drinking contaminated water near landfills and waste sites
- A synthetic chemical that is a byproduct of incineration and chemical manufacturing
- Exposure most commonly occurs through eating contaminated foods (responsible for 90% of intake); living near contaminated sites; or working in paper mills, incinerators, or occupations that produce dioxins as a byproduct
- One of the first chemicals noted to be associated with cancer-benzidine-came to attention because of excess cases of bladder cancer seen among workers in these industries.
- Energy in the form of particles or rays that is emitted from radioactive material, high-voltage equipment, and nuclear reactions
- Exposure most commonly occurs through low levels in the environment; working as a pilot, flight attendant, astronaut, nuclear power plant worker, or x-ray technician; or receiving an x-ray exam
- A group of chemical compounds that are metals, including lead, tungsten, and mercury
- Exposure can occur through eating fish, from lead paint in older buildings, or through contaminated water passing through aging pipes
- A group of nitrogen compounds; the greatest use of nitrates is as fertilizer
- Exposure most commonly occurs through contaminated drinking water
- A large group of chemical compounds that contain carbon, including benzene and DDT, and can be both synthetic and naturally forming
- Exposure can occur in many ways including use of household chemicals, vehicle exhaust, and from pesticides
- A group of chemical compounds that are made from petroleum or natural gas
- Exposure most commonly occurs through use of gasoline pumps, spilled oil on pavement, and chemicals used at home or work
- A synthetic chemical that was used prior to 1977 in electrical equipment; manufacturing was stopped in the U.S. in 1977 due to health concerns
- Exposure can occur through: use of old fluorescent lights and appliances made 30 or more years ago; eating contaminated food; breathing air or drinking water near contaminated waste sites; or working to repair, maintain, clean-up, or dispose of products that contain PCBs (e.g. transformers, fluorescent lights, electrical devices
- A group of chemicals formed from the incomplete burning of coal, oil and gas, garbage, or other organic materials like tobacco or charbroiled meat, emitted from volcanoes and forest fires
- Exposure can occur through: working in industries such as coal-tar and asphalt production and incinerators; breathing air contaminated with cigarette smoke, wood smoke, or vehicle exhaust; eating grilled or contaminated foods; or drinking contaminated water
- Trihaloamines (THM) form when chlorine used to disinfect water supplies react with organic and inorganic material in water to form disinfection by products (DBPs)
- Exposure most commonly occurs through drinking water
- A synthetic substance used in the production of certain plastics
- Exposure most commonly occurs through breathing air that is contaminated from plastics industries, hazardous waste sites, and landfills; working in an industry that uses vinyl chloride; or drinking contaminated water