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California Environmental Health Tracking Program

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Last Edited: 9/30/2010

What is Cancer

Cancer is the general name of a large group of diseases characterized by cells that grow out of control and have the potential to spread to other parts of the body.  If left untreated, many forms of cancer lead to serious illness and death.

What causes cancer?

Cancer cells develop when there is damage to a cell’s DNA or an alteration to its functioning.  DNA is the genetic material in the cell that directs the cell’s activities.  Damage to the DNA can occur for many reasons.  Sometimes alterations to DNA that can predispose a person to getting cancer can be passed down from parent to offspring.  In this case, the risk of getting cancer is inherited. DNA can also be damaged by external factors or things in the environment, like tobacco smoke, chemicals, viruses, and exposure to too much sunlight.  Some cancers are thought to develop when the body’s ability to deal with damaged or otherwise altered cells has been impaired by changes in hormones, metabolism, or immune conditions.


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How does cancer spread?

Cancer cells keep growing and dividing instead of dying and being replaced, like normal cells. Cancer cells can travel through the blood stream or the lymph system to other parts of the body and replace normal tissue. This is called metastasis. 

For many cancer types, cancer cells form into a lump or mass, called a tumor.   Other cancer types, like leukemia, do not form tumors, and involve the blood and blood-forming organs. 

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What are risk factors for cancer?

Risk factors for cancer include:

  • Diet
  • Exercise
  • Tobacco use
  • Sunlight exposure
  • Family history of cancer
  • Certain viruses
  • Environmental pollution
  • Occupation

Although diet and tobacco use are responsible for the largest proportion of cancer cases, 35% and 30%, respectively, evidence continues to emerge suggesting that the environment plays a role in contributing to cancer.1


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What are the symptoms of cancer?

There are multiple types of cancer and each cancer is unique in characteristics and symptoms.  However, there are common symptoms that, when present, may warrant an evaluation for cancer:

  • Persistent fatigue
  • Losing 10 or more pounds without dieting or intending to lose weight
  • Fever
  • Changes to the skin, such as darkening, abnormal hair growth, reddening, itchiness
  • Pain


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How is cancer diagnosed?

Cancer can be detected through a positive screening test or by experiencing symptoms.  Diagnostic tests can confirm whether the patient has cancer.  These diagnostic tests can include an examination of the suspected cancerous tissue, also known as a biopsy.    


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What is a cancer cluster?

Generally speaking, a “cancer cluster” is defined as a greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a period of time.  This is done by looking at the number and rates of cancer cases that are occurring (observed) and comparing it to the number and rates that would be expected in the same population.  An apparent excess of cancer may occur due to (1) chance, (2) miscalculation of the expected number of cancer cases (e.g., not considering a risk factor within the population at risk), (3) differences in the way the cancer was identified between observed cases and expected cases, (4) known causes of cancer (e.g., smoking), and (5) unknown cause(s) of cancer.

It is important to note that the term “cluster” tends to imply different things to different people, which leads to a large amount of confusion.  For example:


  • The term is most often used when the increase in cancer incidence is unexplained or contested.  In this sense, if an explanation such as an excess of elderly residents in a community were documented, the increase may no longer be considered a cluster.
  • Others label cancer increases as clusters only when an environmental cause is known or suspected.  This definition may or may not conflict with the one described above.
  • Some commentators assume that the term can only be applied to small areas such as a county or part of a county, regardless of its cause.  In this case, larger regions (for example “Southern California” or “The Panhandle of Florida”) are not considered clusters even if they experience a documented excess of cancer cases.

For each of the above definitions, the appropriate methods for statistical analysis and documentation of the cluster are very different.  Interestingly, other diseases and conditions such as preterm birth and asthma are rarely labeled “clusters” even though the above situations as applied to them occur frequently.

Most frequently, questions from citizens about possible clusters are discussed with local or state health department staff and are addressed satisfactorily with education about cancers and cancer risk factors.  However, some situations will require further evaluation to understand what may be going on.  The follow-up evaluation may consist of a routine review of data in the cancer registry, or could require extensive investigation that may take years to complete.  Because cancers are complex diseases, however, in many instances, no clear cause of a true cluster is ever found.


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1. 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.