The Geography of Cancer Risk

Chris Walker * February 3, 2014

Notes: County-level data not available for Kansas.


Cancer stubbornly remains a condition that modern science has yet to fully conquer. In America it’s now the #2 leading cause of death, under heart disease. By one measure, doctors have made great strides in cancer treatment in the last couple decades. The national mortality rate from cancer peaked in 1991, at 215.1 cancer deaths per year per 100,000, and has since come down to 171.8 as of 2010. Still, new cancer diagnoses remain high and one recent report estimates that the lifetime risk of the average American developing cancer is 40.8%.

Cancer seems to be the most egalitarian of illnesses, affecting both young and old, rich and poor. Due to cancer’s complexity, one can only speak of its causes in terms of risks. One possibly surprising risk, geography, is the focus of the map above. The National Cancer Institute provides a web portal to download cancer incidence and mortality data down to the county level, and for the most part this is the data that is displayed on the map. For Washington, Ohio, Minnesota, and Virginia I obtained comparable data from those states’ cancer registers. Color corresponds to the incidence of new cancer cases over a five-year period, controlled for population size. Darker colors indicate a higher frequency of new cancer diagnoses.

The map defies easy explanation. Cancer rates are not obviously correlated with population density, for instance, as is evident when one notices the dark counties in sparsely-populated Wyoming. Nor is geographic risk explained by differences in pollution levels. Cities in Central and Southern California top the American Lung Association’s list of most polluted cities, with Los Angeles at #1, but one still has a higher chance of developing cancer amidst the much cleaner air of upstate New York. Poverty levels don’t do much to explain geographic cancer risk either.

There are several surprising clusters in the map, for example the darker regions (higher cancer risk) in Northwest Washington state, Montana, Minnesota, Illinois, Louisiana, Kentucky, and much of the mid-Atlantic states and New England. On the other hand the Western portion of Texas and much of the South West have relatively low cancer risk.

Even if two counties have the same high cancer risk, they may be high for different reasons. That’s partially because populations around the United States aren’t homogenous, and therefore genetically-induced cancer risk varies by geography. Different types of cancers may be more common in certain areas than others. There may also be a number of particular environmental factors not readily observable in the data—nearby chemical plants, or a long history of industrial activity, for example. The big takeaway should be this: the blend of environmental factors contributing to cancer risk in your geography are most likely unique, and it’s important to investigate what those factors are, along with your family history and individual factors, so that you can take appropriate preventative measures toward reducing your risk.

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