A recent study has linked breathing polluted air to weight gain. The study suggested that breathing dirty air can take a heavy toll on gut bacteria and increase risk of obesity, diabetes, gastrointestinal disorders and other chronic illnesses. The finding was published online in the journal Environment International. Of all the pollutants measured, the researchers found that ozone had the greatest impact on the gut. Higher exposure to ozone also led to lower bacteria diversity in the gut, which is linked with obesity and Type 2 diabetes. Study participants with higher exposure to ozone also had a greater abundance of a specific species called Bacteroides caecimuris. Some studies have associated high levels of Bacteroides with obesity. A member of the research team explained that ozone changes the environment of the gut to favour some bacteria over others. This may lead to health consequences. You may put on weight if you eat and drink more calories than you burn. But an underlying health condition may also cause unintentional weight gain. Here are 5 medical conditions that can cause weight gain.