Soot may increase the incidence of diabetes even at currently acceptable levels of air pollution set by U.S. EPA, according to new research.
Scientists have found a correlation between particulate matter in the air and diabetes, and the link persists even when factors that may predispose a person to the disease, such as obesity and ethnicity, are controlled for. The study is in the October issue of the journal Diabetes Care.
Particles and liquid droplets in air that are less than the width of a strand of hair -- 2.5 micrometers or smaller -- have already been linked to a variety of health effects, including cardiovascular disease, lung cancer and asthma. The pollutants are called "fine particulate matter" and referred to as PM2.5 by EPA. They are so tiny that they can penetrate the nose and throat, and reach the lungs and heart.
The agency has declared a yearly average of 15 micrograms or less of PM2.5 in the air safe.
Now, research has shown that even in counties where PM2.5 levels are "safe," air pollution can be linked to diabetes. With every 10-microgram increase of particulate matter levels, diabetes prevalence rises by 1 percent.
This means that while a cleaner city (having about 7 micrograms of PM per cubic meter of air) may have six out of 100 people with diabetes, one with 10 micrograms more of PM would have seven diabetics, said John Pearson, a researcher at the Harvard-Massachusetts Institute of Technology Division of Health Sciences and Technology and co-author of the study.
"We drive long ways to work and drive home, eat on the go and exercise less; there are all risk factors to diabetes," said Pearson. Together with all this, we have high exposure to particulate matter while we sit on our highways, he said.
"Our current transportation infrastructure could be contributing to the diabetes epidemic," he said.
The most polluted cities in the United States can have 80 micrograms per cubic meter during rush hour. The most polluted cities in India and China can have 150 micrograms on a good day, said Sanjay Rajagopalan, professor of medicine at Ohio State University.
"This paper is very interesting because it shows an association between pollution and prevention of diabetes," he said. "The only problem with the paper is associative; you cannot, in human studies, show cause and effect very well."
While the study does not prove that particulate matter causes diabetes, it does a show link between the two, said Joel Zonszein of Montefiore Medical Center, who is not associated with the researchers.
"They do have a direct relationship effect to diabetes; the more particles, the higher the relation to diabetes," Zonszein said.
He related the real-life example of the evolution of the Cross Bronx Expressway in New York City, which cuts across residential areas. A fish market in the neighborhood of Hunts Point turns roads into a sea of vehicles each morning, and rates of asthma and diabetes have risen, he said.
Pearson and his colleagues used county-level diabetes prevalence data from the Centers for Disease Control and Prevention and matched that with the PM levels measured by the EPA between 2004 and 2005. They got 3,000 data points and eliminated from their model factors such as age, sex, ethnicity and socioeconomic status that could predispose a person to diabetes. At the end, they could say with some confidence that there is a connection between the disease and fine particulate matter.
In earlier studies, mice exposed to high amounts of PM2.5 had been shown to develop diabetes, said Rajagopalan. His work had shown that particulate matter triggers an inflammation reaction within the body that leads to diabetes. This study is an indication of a similar reaction in humans, he said.
"If you take both studies together, pollution does lead to inflammation, especially in obese and pre-diabetic people," he said.