A new study by E.G. Knox of the University of Birmingham (UK) claims children who were born near "emissions hotspots" (bus stations, train stations, and other transportation hubs) are at greater risk for childhood cancers than those who were born farther away.
Because it takes many years to progress from exposure to illness to death from cancer, researchers sometimes look to past cases to determine links between exposure and illness (rather than identifying a group at risk and following them over many years, which is costly and time-consuming). Knox's study takes a unique approach to this, comparing children living between 1966 and 1980 to emissions maps from 2001. It doesn't take a lot of imagination or medical expertise to see that emissions patterns from 2001 could differ drastically from 1966, when the first of Knox's study subjects were "exposed" -- this is perhaps the most drastic shortcoming of the study. In addition, Knox only relies on place of birth and death to determine exposure, not accounting for the possibility that mothers and children may have spent significant periods of time in other places.
In his study, Knox compared the frequency of childhood cancers among the different groups of people; some lived relatively close to transportation hubs and others lived further away. Unfortunately, this type of study is vulnerable to confounding factors -- it is impossible to construct a study like this in which all factors are equal except for the one being examined. Normally, researches use statistical methods to adjust for possible differences, but Knox makes no effort to control for obvious possible confounders such as education, income, access to medical care, and smoking and diet habits of parents (such as consumption of fruits and vegetables). Presumably, socioeconomic factors are key determinants in where people chose to live, and it is curious that Knox did not consider that these very same factors might also influence cancer risk.
While the Knox emissions study is epidemiologically creative, it is just as epidemiologically unsound. Herman Kattlove, spokesman for the American Cancer Society, has stated that the new study falls far short of proving a link between emissions and childhood cancers.
Source: Knox, E.G. Childhood cancers and atmospheric carcinogens. Journal of Epidemiology and Community Health. Vol 59, 101-105.
Mara Burney is a research intern at the American Council on Science and Health.