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Environmental Health
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'Cancer cluster' is a term used to describe any increase in the number of cancer cases over what would be expected in a particular area during a specific period of time. Cancer actually occurs in clusters quite often. Although the tendency is to blame this type of clustering on pollutants in the air, water or soil, a variety of factors unrelated to the environment are often responsible.

Clusters are frequently a matter of statistical chance—not unlike what happens at a roulette wheel. If you play long enough, red is bound to come up six times in a row. The unusual run of one color doesn’t necessarily mean the wheel is unbalanced or that the house is cheating. Statistically, this type of run is expected to happen every now and again. Stick around long enough and you’ll probably see an equally long run of blacks.

The same is true of cancer. Although it’s highly unlikely that several men living on the same block would all develop prostate cancer within a relatively short period of time, it can—and will—happen from time to time.

Sometimes cancer clustering is explained by a problem epidemiologists refer to as “confounding.” In this instance, the number of cancers in one area may be significantly higher than in others, but the problem is not with the area itself. Rather, it happens because the people who live in the area differ from those living elsewhere—and so does their cancer risk.

For example, you’d expect to find a higher-than-average rate of lung cancer in a neighborhood where there is a heavy concentration of smokers. It may be tempting to blame the excess cancers on local smokestacks or factories, but it’s the smoking that’s probably responsible.

Southern California breast cancer rates offer another example of the effect of confounding. Breast cancer rates are higher than average in communities such as Palos Verdes, San Marino, Pacific Palisades and Beverly Hills. Cancer experts say it’s likely related to the fact that women in these communities have received more education. Better-educated women tend to have children later in life and tend to have fewer children: both factors increase their life-long exposure to estrogen and progesterone, hormones believed to increase the risk of breast cancer.

Regardless of their cause, cancer clusters have a way of doing their own damage—by creating fear. Cancer clusters also divert the public’s attention—and action—away from the truly important causes of cancer. Most cancer researchers think that toxic substances in the environment are responsible for only a small proportion of all cancers, and cancer clusters that are caused by environmental factors are believed to be extremely rare.

Any increase in cancer cases that can’t be easily explained by chance or confounding requires careful investigation, but it’s important not to be distracted by the accompanying hype and rumors. The vast majority of cancers are the result of behavioral and lifestyle factors such as tobacco use, diet and physical activity.


Cancer Clusters (Centers for Disease Control and Prevention)
Cancer Clusters (California Environmental Health Investigations Branch)
Cancer Clusters (National Cancer Institute)


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Director of Environmental Health
Liza Frias
Public Health has made reasonable efforts to provide accurate translation. However, no computerized translation is perfect and is not intended to replace traditional translation methods. If questions arise concerning the accuracy of the information, please refer to the English edition of the website, which is the official version.
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