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Cancer and the Environment

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:

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:

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:

Meanwhile, the number and quantities of compounds that could cause cancer-at least in theory-in the global environment have been increasing.

Reasons for concern. For more information on specific chemicals, see the panel below.
Breast
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 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.