The fact that higher pollution usually means more cars, which could mean less physical activity, which might lead to higher obesity levels, resulting in higher diabetes rates were factored into this study - in other words, the scientists found a direct link between pollution levels and diabetes risk, after taking into account these variables which may occur in high pollution areas.
This is one of the first large-scale population based studies to detect an association between diabetes rates and levels of air pollution, the authors write. It corroborates previous studies which found a link between higher insulin resistance and particulate exposure among laboratory mice.
The investigators concentrated their attention on fine particulates of 0.1 to 2.5 nanometers, or PM2.5, which is commonly found in motor vehicle exhaust fumes, haze and smoke.
John Pearson and John Brownstein, PhD, of the Children's Hospital Informatics Program and team gathered data on PM2.5 pollution in every country in mainland USA (not including Alaska) from the EPA (Environmental Protection Agency) for 2004 and 2005.
They combined the EPA information with data from the US Census and the CDC (Centers for Disease Control and Prevention) to establish adult diabetes rates, as well as adjusting for obesity, physical activity, geographical location, population density and ethnicity - known risk factors for diabetes.
We wanted to do everything possible to reduce confounding and ensure the validity of our findings.
Throughout their investigation, they detected a consistent and compelling link between the prevalence of diabetes and concentrations of PM2.5 (pollution levels). They found that diabetes prevalence went up 1% for every 10 microgram per cubic meter rise in PM2.5 exposure. This occurred in all areas in both years, 2004 and 2005.
We didn't have data on individual exposure, so we can't prove causality, and we can't know exactly the mechanism of these peoples' diabetes," acknowledges Brownstein. "But pollution came across as a significant predictor in all our models.
They even found that counties which had PM2.5 exposure levels well within the EPA limits had diabetes prevalence rates varying by 20% - i.e., counties with the highest PM2.5 exposure levels within the EPA limits had 20% higher diabetes prevalence than those with the lowest PM2.5 exposures.
From a policy perspective, the findings suggest that the current EPA limits on exposure may not be adequate to prevent negative public health outcomes from particulate matter exposure.
Study co-author, Allison Goldfine, MD, head of clinical research at the Joslin Diabetes Center, said:
Many environmental factors may contribute to the epidemic of diabetes in the United States and worldwide. While a lot of attention has correctly been attributed to caloric excess and sedentary behaviors, additional factors may provide novel approaches to diabetes prevention.
The scientists say further research is necessary, especially ones that examine the inflammatory mechanisms in diabetes and what exactly PM2.5 does in all this.
We would like to access better individual-level data on diabetes and exposure," adds Brownstein. "We also have an interest in investigating this finding internationally where standards may be less stringent.
The study authors concluded:
Our results suggest PM2.5 may contribute to increased diabetes prevalence in the adult U.S. population. These findings add to the growing evidence that air pollution is a risk factor for diabetes.
"Association Between Fine Particulate Matter and Diabetes Prevalence in the U.S."
John F. Pearson, BS, Chethan Bachireddy, BS, Sangameswaran Shyamprasad, Allison B. Goldfine, MD, John S. Brownstein, PHD
Diabetes Care October 2010 vol. 33 no. 10 2196-2201. Published ahead of print.
Written by Christian Nordqvist