New research shows polluted city air could be even more of a health risk than we thought, because it's been found to carry traces of drug-resistant bacteria that can't be treated by common antibiotics.

Scientists are now cautioning that city smog might be spreading the genetic material that makes viruses untreatable, and at this stage, it's not clear how much damage this could do in the world's most polluted cities.

Researchers from the University of Gothenburg in Sweden analysed 864 samples of DNA taken from humans, animals, and environments worldwide, looking for genes linked to antibiotic-resistant bacteria.

The results showed that samples taken from the Chinese capital Beijing – known for its extremely bad pollution – turned up a high level of antibiotic resistance genes (ARGs).

"We studied only a small number of air samples, so to generalise, we need to examine the air from more places," explains lead researcher Joakim Larsson. "But the air samples we did analyse showed a wide mix of different resistance genes."

The research doesn't show whether the bacteria in Beijing's smog is actually alive – which would significantly increase the threat – but Larsson says it's "reasonable to believe that there is a mixture of live and dead bacteria, based on experience from other studies of air".

Particularly worrying is the fact that the study turned up genes resistant to carbapenems – the 'last resort' drugs given to counter difficult bacterial infections that haven't responded to other treatments.

While we don't know how serious the threat is just yet, the researchers write in their paper that there might be "vast sources" of unknown resistance genes hiding in polluted environments. 

"This may be a more important means of transmission than previously thought," says Larsson.

As we treat our illnesses with antibiotics, bacteria and viruses modify themselves to become hardened against them, creating new microorganisms known as superbugs.

Right now, bacteria is evolving faster than our antibiotics can be adjusted, which is why it's vital that we understand more about how it spreads and reaches new groups of people.

If we could no longer use antibiotics in treatment, it would mean routine infections would become dangerous and potentially fatal, so scientists are scrambling to find alternative ways to fight these superbugs.

Larsson and his team, previously known for their work linking the discharge of antibiotic-contaminated water to increased bacterial resistance in India, are now turning their attention to sewage treatment plants in Europe to investigate the threat further.

Treatment plant employees will be given samplers to measure the composition of the air, while their gut bacteria, and that of nearby residents, will be studied too, to look for a link between the plants and airborne bacteria.

If we can understand more about how resistant bacteria spreads, we'll stand a better chance of trying to beat it – meaning it's yet another reason to work on cleaning up our cities.

The findings are published in Microbiome.