LONDON (GUARDIAN) - First it was clean eating; now it is clean breathing.

Sales of air purifiers are soaring, with the global market expected to be worth £6.2 billion (S$11.4 billion) by 2024.

The trend is in response to the rise in asthma and allergies linked to poor air quality inside buildings. Research says it can be up to five times more polluted than air outside , with chemicals from cleaning products, aerosols and perfume rivalling diesel fumes as causes of contamination .

Tech companies are racing to come up with ways to clean the air around people.

The San Francisco startup behind Molekule , for example, promises its nanotechnology will "actually destroy" pollutants "at the molecular level".

BetterAir's Biodify , meanwhile, is the world's first probiotic air purifier, using healthy bacteria known as Bacillus subtilis to create a "protective shield of microflora".

But can tech really help? "There is no silver bullet because every home is different," said Mr Douglas Booker, chief executive of NAQTS, a social enterprise seeking to improve awareness of indoor air quality through its monitoring technology.

"But we spend 92 per cent of our time indoors so it's important we're aware of ways to reduce indoor air pollution, such as opening windows, using extractor fans when cooking and never smoking indoors," he added.

There is also evidence that plants can reduce levels of toxic compounds - some have been used to filter air inside the International Space Station.

Mr Freddie Blackett, founder of online plant shop Patch, recommends aloe vera, which can improve sleep, and Epipremnum aureum, which removes formaldehyde and benzene from the air.