NEW YORK - Young people who live in areas with higher levels of certain air pollutants may be more likely to have inflammatory bowel disease than those living under clearer skies, a new study suggests.
Specifically, people age 23 or younger were about twice as likely to be diagnosed with Crohn's disease if they lived in a region relatively high in nitrogen dioxide levels.
The findings, reported in the American Journal of Gastroenterology, do not prove that air pollution is a contributor to inflammatory bowel disease, the major forms of which are Crohn's disease and ulcerative colitis. In the U.S., Crohn's affects about one in 600 people in the US, and ulcerative colitis affects about one in 400, according to the National Institute of Diabetes and Digestive and Kidney Diseases.
Instead, researchers are instead calling their study a "hypothesis-generating" one that raises the possibility that air pollution could be one of the environmental factors that plays a role in IBD development.
Crohn's disease and ulcerative colitis are marked by chronic inflammation in the intestines, leading to symptoms like abdominal pain and diarrhea.
The precise causes of the conditions are unknown, but the disorders are thought to involve an immune system overreaction that injures the body's own intestinal tissue. In general, researchers suspect that a combination of genetic susceptibility along with exposure to one or more environmental factors is to blame.
Exactly what those environmental factors are remains unclear, however.
It is known that IBD rates rose during the 20th century in industrialized countries, and that rates appear to be increasing in developing nations. Such patterns suggest that factors such as diet and lifestyle changes, or environmental exposures, could be involved.
Air pollution is one possibility, which is why the current study focused on it, explained lead researcher Dr. Gilaad G. Kaplan, a gastroenterologist and assistant professor at the University of Calgary in Alberta, Canada.
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.