Can ethical businesses bring a breath of fresh air to Asia's polluted cities?

Air pollution kills an estimated seven million people worldwide every year.

Ethical businesses are developing new ways to tackle air pollution - a major public health threat in developing nations - by making affordable air purifiers for the most vulnerable people in the worst hit countries like China and India.

Nine out of 10 people breathe air containing high levels of pollutants, and air pollution kills an estimated seven million people worldwide every year, according to the World Health Organization (WHO).

People living in urban areas in low- and middle-income countries are worst-affected, with the health impact of air pollution linked to strokes, heart and pulmonary disease, lung cancer and acute respiratory infections, WHO said.

"Purifiers are essential health protective measures in places like Delhi and Beijing," said Thomas Talhelm, founder of Smart Air, a social enterprise that sells indoor air purifiers in countries such as China, Mongolia, Pakistan and India.

"Yet companies convince people that purifiers need to be expensive, and that prevents people from protecting their health. It's morally outrageous," said Talhelm, an associate professor of behavioural science at the University of Chicago.

With 15 of the world's 20 most polluted cities in India, many businesses there have rushed to market air purifiers that are usually priced at hundreds of dollars - well beyond the reach of the average resident.

Social enterprises - companies which address environmental and social problems while making a profit - are aiming to help.

Last year, Smart Air crowdfunded what it described as "the world's most cost-effective" air purifier, priced at about $100.

Air purifiers "are just fans and filters, with companies claiming that they have patented, proprietary technology that allows them to charge higher prices," said Talhelm, who first developed a do-it-yourself purifier kit for China in 2013.

The Indian government's National Clean Air Programme aims to reduce deadly air pollution by 20%-30% over the next five years.

But air pollution is a low priority for most cash-strapped city agencies, said Amol Chaphekar, founder of StrataEnviro in India, a social enterprise which makes outdoor purifiers for public spaces such as bus stations and near traffic lights.

The purifiers filter out pollutants such as dust, diesel fumes and deadly particles, and reduce air pollution within a 60-foot (18-metre) radius, Chaphekar said.

He has forged alliances with companies and non-profits, and sold advertising display rights to help pay for the purifiers.

"The social enterprise model works well, as we are able to attract investors and advertisers, and we do not need to depend on the government except for the installation space," he said.

Social enterprises are even into face masks: at the Consumer Electronics Show in Las Vegas last year, Brooklyn-based startup Ao Air displayed a transparent face mask with battery-powered fans.

The social enterprise model is particularly important for purifiers because the industry "is out of whack with reality", Talhelm told the Thomson Reuters Foundation on Tuesday.

When Smart Air launched in China, the most popular purifier sold for about $2,000, yet he built one for about $30, he said.

"This industry is clearly divorced from reality - in a way that harms people. That's where social enterprise is most needed," he said.