Bacteria from dog waste may be the dominant source of airborne bacteria in the wintertime air over Cleveland and Detroit, according to a new study by researchers at the University of Colorado.

The scientists were studying the bacterial communities found in 100 air samples previously collected by Colorado State University for an earlier study when they found that the samples from Cleveland and Detroit contained significant amounts of fecal bacteria, the most likely source of which is dog poop.

The research was published in the journal Applied Environmental Microbiology.

Scientists have known that bacteria exists in the atmosphere and that they can have detrimental effects on human health, triggering allergic asthma and seasonal allergies. But researchers are just beginning to understand how diverse airborne bacteria can be.