Cancer results from a complex interaction of genetic and environmental factors. However, only 5 percent to 10 percent of cancers are hereditary , meaning that a person was born with a gene that makes him or her more likely to develop cancer. Therefore,90 percent to 95 percent of cancers are caused by external factors such as lifestyle choices that includes, among other things, smoking, diet, obesity and drinking alcohol. Also included in external factors are environmental causes such as exposure to radiation and chemicals, and viral or bacterial infections that can damage cells and weaken the immune system. In the Winter issue of CURE, we begin a two-part look at the environment and cancer with the hope that it will help clarify some of the complexity and confusion around this controversial issue. It seems appropriate we begin this discussion in 2011, a watershed year for the issue with voices from policy, science and advocacy bringing the discussion into the spotlight. In March, the National Resource Defense Council (NRDC) and the National Disease Cluster Alliance released a report focusing on "disease clusters" in 13 states, those places where "an unusually large number of people have been sickened by a disease such as cancer or experienced birth defects and other chronic illness." In May 75,000 names on a petition were delivered to top environmental officials in the Obama administration requesting a response to the 2009 President's Cancer Panel report, which called for a national strategy to eliminate the use of cancer-causing chemicals. In addition, two pieces of legislation focusing on cancer causing chemicals and needed government action have been placed before Congress.Determining environmental causes of cancer is quagmire of politics, science and the human factor. It pits researcher against researcher in complicated and often contradictory science. Take, for example, the May 31st announcement that the International Agency for Research on Cancer (IARC) had classified wireless cell phone use as "possibly carcinogenic to humans," placing it in group 2b of the list of carcinogens. The next day the headlines read "Cell Phones May Cause Cancer," but few of the stories explained that the classification puts cell phones in the same category as numerous chemicals ending in ene and ine as well as coffee (which has some evidence of causing cancer of the large bowel), gasoline engine exhaust and pickled vegetables. Category 1 agents (classified as carcinogenic to humans) includes 107 chemicals with recognizable names such as arsenic, asbestos and strontium-90. But category 1 also includes alcoholic beverages, leather dust, wood dust and salted fish (Chinese-style). This finding by the IARC still doesn't clarify cause and effect, says David Gorski, MD, PhD, managing editor of Science-Based Medicine and a researcher and surgical oncologist at the Barbara Ann Karmanos Cancer Institute in Detroit, who points out that epidemiological studies are particularly difficult because of the variety of factors involved as well as the design of studies. "We set statistical significance at 5 percent chance that the result could occur by random. If you do 20 studies you will get one that looks positive. And all studies have confounders, particularly epidemiological studies." This, Gorski says, is why one outcome from a study must be duplicated.This is not to minimize potential cancer causing agents, he says, some of which have been tracked down and identified, tobacco for one. What it points out is the complexity of identifying and proving something is cancer causing and the ability to read through the science speak to determine the actual message. It took 14 years for action against tobacco. The cell phone debacle came only a month after the 25th anniversary of the Chernobyl nuclear meltdown in Ukraine, which was six weeks after an earthquake damaged the nuclear reactor at the Fukushima Daiichi plant in Japan, sending radiation into the surrounding air and countryside. Scientists now know that cancer clusters followed Chernobyl and are applying this knowledge to the Japanese incident to determine who is at risk for long-term effects and how to ameliorate them. The entire issue is complicated and confusing – and increasingly important, advocates say, as more chemicals enter the environment. Whether any of the questions proposed this year around the relationship between toxic chemicals and cancer will be answered remains to be seen. Understanding the connection requires sorting through the political, scientific and human aspects of the issue, and we hope that the two features we are presenting will do just that.