Using a UK database of electronic medical records, he and his colleagues identified 367 children and adults diagnosed with Crohn's disease and 591 diagnosed with ulcerative colitis between 2005 and 2008. The researchers matched each of those people to five IBD-free individuals the same age and sex.
They then used air-quality data from government monitors to assess the average yearly levels of three air pollutants in the study subjects' residential areas.
The pollutants included nitrogen dioxide, which is produced largely by vehicles and is highest in urban, high-traffic areas; sulfur dioxide, which is produced through industrial processes, including the burning of coal and oil; and particulate matter, fine particles emitted via car exhaust, as well as power plants and other industrial sources.
Overall, Kaplan's team found no association between IBD and the three air pollutants across the study group as a whole.
However, young people — those age 23 or younger — were about twice as likely to be diagnosed with Crohn's disease if they lived in a region in the top 60 percent of nitrogen dioxide levels, versus the bottom 20 percent.
Similarly, people age 25 or younger were twice as likely to have ulcerative colitis if they lived in areas with higher sulfur dioxide levels. However, there was no evidence of a “dose-response” relationship — that is, the risk of ulcerative colitis climbing steadily as sulfur dioxide levels rose.
That lack of a dose-response, Kaplan told Reuters Health, “makes us a little more cautious about that finding.”
Indeed, he urged caution in interpreting the findings as a whole. While he and his colleagues tried to account for other factors — such as study subjects' smoking habits and socioeconomic status — they cannot rule out the possibility that something other than air pollution itself accounts for their findings.
“This is an interesting association,” Kaplan said. But, he added, the findings do not prove cause-and-effect.
As for why air pollution would affect IBD risk, Kaplan said he could only speculate, based on research into other health conditions, including heart and lung disease. Studies indicate that air pollutants can trigger inflammation in the body; that, Kaplan explained, raises the possibility that in genetically predisposed people, air pollution may trigger an inflammatory response in the intestines that leads to IBD.
Since the current study found a relationship between pollutants and IBD only in young people, the findings also raise the question of whether children and teenagers are particularly susceptible to any effects of air pollution on the risk of the digestive disorders.
Much more research is needed, Kaplan said — both larger population studies and research in animals to see how exposure to various air pollutants might affect intestinal health.
He added that no one is proposing that air pollution is the environmental cause of IBD; if it does turn out to be a factor, he said, it will likely be one of many players.
But if air pollution is confirmed as a risk factor, there would be important implications, Kaplan said, since air quality is something that can be modified.More