A recent study suggests that breathing dirty air takes a heavy toll on gut bacteria, boosting risk of obesity, diabetes, gastrointestinal disorders and other chronic illnesses. The study, published online in the journal Environment International, found that young adults exposed to higher levels of ozone showing less microbial diversity and more of certain species associated with obesity and disease.The study comes at a time when air quality in many U.S. cities is worsening after decades of improvement. In December, the Environmental Protection Agency downgraded the Denver metro and North Front Range regions to "serious nonattainment" status for failing to meet national ozone standards. Regions of eight other states, including some in California, Texas, Illinois, Connecticut, Indiana, New Jersey, New York and Wisconsin, were also penalized for high ozone. Worldwide, according to research published this month, air pollution kills 8.8 million people annually -- more than smoking or war.