Exposure to air pollution may take a heavy toll on gut bacteria, boosting the risk of obesity, diabetes, gastrointestinal disorders and other chronic illnesses, according to a study.

The research, published in the journal Environment International, is the first to link air pollution to changes in the structure and function of the human gut microbiome -- the collection of trillions of microorganisms residing within us.

The researchers from the University of Colorado Boulder in the US found the gaseous pollutant ozone -- which forms when emissions from vehicles are exposed to sunlight -- is particularly hazardous.

Young adults exposed to higher levels of ozone show less microbial diversity and more of certain species associated with obesity and disease, they said.

"We know from previous research that air pollutants can have a whole host of adverse health effects," said Tanya Alderete, an assistant professor at the University of Colorado Boulder.

"The takeaway from this paper is that some of those effects might be due to changes in the gut," Alderete said.

Worldwide, according to research published this month, air pollution kills 8.8 million people annually -- more than smoking or war, the researchers noted.

While much attention has been paid to respiratory health, Alderete's previous studies have shown pollution can also impair the body's ability to regulate blood sugar and influence risk for obesity.

To investigate just what might be going on inside the gut, the researchers used cutting-edge whole-genome sequencing to analyse faecal samples from 101 young adults in Southern California, US.

They looked at data from air-monitoring stations near the subjects' addresses to calculate their previous-year exposure to ozone particulate matter, and nitrous oxide.

Of all the pollutants measured, ozone had the greatest impact on the gut by far, accounting for about 11 per cent of the variation seen between study subjects -- more of an impact than gender, ethnicity or even diet.

Those with higher exposure to ozone also had less variety of bacteria living in their gut, the researchers said.