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.1
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?
This question has received increasing attention among scientists, policy makers, and the public. 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 shift2
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 analysis 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.
Reasons for concern2
For more information on specific chemicals, see table below.
|Breast||Breast cancer is the most commonly diagnosed cancer among women. Some breast cancers are known to respond to hormone levels in women’s bodies, and a variety of persistent chemicals such as DDT, PCBs, and dioxin have the potential to alter hormone activities in people.|
|Lung and bronchus||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.|
|Bladder||Occupational exposures to truck exhaust and compounds used in the textile and leather dye industries have been associated with bladder cancer.|
|Brain and other nervous system||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.|
|Thyroid||Increases in risk for thyroid cancer have been observed with exposure to ionizing radiation.|
|Non-Hodgkin lymphoma||Rates of this disease have increased dramatically over the last few decades, although no reasons for this increase have yet been found.|
|Leukemia||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||Mesothelioma is a rare cancer affecting the linings of internal organs, most often the lungs. It is most often caused by exposure to asbestos.|
|Skin||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.|
|Liver and bile duct||Exposures to environmental pollutants such as solvents and persistent organic compounds may contribute to increased risk of these cancers.|
|Kidney and renal pelvis||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.|
Specific environmental concerns
Exposure to cigarette smoke, either through smoking or second-hand, is associated with a large number of cancers. Besides this, specific environmental concerns include:
|Environmental factors associated with cancers||Description and potential sources|
|Polychlorinated biphenols (PCBs)9|
|Polycyclic aromatic compounds5|
For more information on toxic substances, including those listed above, see the Agency for Toxic Substances and Disease Registry website.
- Stein RA, Epigenetics—the link between Infectious Diseases and Cancer. Journal of the American Medical Association, April 13, 2011. Vol 305, No. 14. 1484-1485.
- Irigaray P, Newby JA, Clapp R, Hardell L, Howard V, Montagnier L, Epstein S, Belpomme D. Lifestyle-related factors and environmental agents causing cancer: An overview. Biomedicine & Pharmacotherapy. 2007. 61;640-658.
- Lewis DR, Southwich JW, Ouellet-Hellstrom R, Rench J, and Calderon RL. Drinking Water Arsenic in Utah: A Cohort Mortality Study. Environmental Health Perspectives. 1999. 107:359-365
- Ferreccio C, González C, Milosavjlevic V, Marshall G, Sancha AM, Smith AH. Lung cancer and arsenic concentrations in drinking water in Chile. Epidemiology. 2000 Nov;11(6):673-9
- US Department of Health and Human Services, National Cancer Institute and the National Institute of Environmental Health Sciences. Cancer and the Environment. NIH Publication No. 03–2039. 2003. http://www.niehs.nih.gov/health/docs/cancer-enviro.pdf
- Barrett JH, Parslow RC, McKinney PA, Law GR, and Forman D. Nitrate in Drinking Water and the Incidence of Gastric, Esophageal, and Brain Cancer in Yorkshire, England. Cancer Causes Control. 1998. 9(2):153-9.
- Gulis G, Czomployova M, Cerhan JR. An Ecologic Study of Nitrate in Municipal Drinking Water and Cancer Incidence in Trnava District, Slovakia. Environ Res. 2002. 88(3):182-7.
- Agency for Toxic Substances and Disease Registry (ATSDR). 1999. Toxicological Profile for total petroleum hydrocarbons (TPH). Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service. http://www.atsdr.cdc.gov/tfacts123.html
- Agency for Toxic Substances and Disease Registry (ATSDR). 2000. Toxicological Profile for Polychlorinated Biphenyls (PCBs). Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service. http://www.atsdr.cdc.gov/tfacts17.pdf.
- King WD, Marrett LD. Case-control study of bladder cancer and chlorination by-products in treated water (Ontario, Canada). Cancer Causes Control. 1996. 7(6):596-604.