That’s a relief. Your home is heaving with bacteria – but more of the bugs you breathe in come from tap water than from the toilet.

Marzia Miletto and Steven Lindow at the University of California, Berkeley, have mapped the bacteria circulating in 29 homes in California. Taking samples from kitchen surfaces, shower heads, bathroom tiles, carpets, toilets, pets and the people who lived there, the team found that one of the most common sources of airborne microbes in these homes was outside air.

The supply of air from outside is a good thing, says Miletto. Fresh air brings in new bacteria, and gets rid of some of the potentially harmful ones you’ve been culturing. “Opening the windows clears out the bacteria in your home,” Miletto says.


In warm California – where many people had open windows – outdoor air accounted for 16.5 per cent of the bugs, while floors and carpets contributed nearly 20 per cent – probably because walking across a floor or hoovering stirs up these bacteria into the air.

Flushed out

Microbiologists have wondered whether flushing our toilets might be spreading faecal bacteria into the air around our homes, but thankfully that doesn’t seem to be the case. Only 0.4 per cent of those in the air were traced back to the toilet bowl – far less than the almost 9 per cent that came from tap water.

Amy Pruden at Virginia Tech in Blacksburg thinks tap water has been overlooked as a source of bacteria. “We drink a lot of it, we shower in it, and when we use a tap, water is aerosolised and we breathe it in,” she says. “There are a lot more bacteria in drinking water than people think.”

These bacteria are likely to mostly be beneficial for us. Exposure to a diverse array has been linked to better health, says Jack Gilbert at Argonne National Laboratory in Illinois.

Given how much time many of us spend indoors, it is important to understand the microbes in our homes and how they might influence our health, says Pruden. She is investigating how the plumbing of a building might affect the bacteria in tap water. “We hope to tailor the water heater temperature, pipe orientation and water use frequency to exclude pathogens like Legionella… and select ‘good’ bacteria,” she says.

Journal reference: Microbiome, DOI: 10.1186/s40168-015-0128-z

Image credit: Donald Iain Smith/Getty