Face Masks Essentially Useless - For Chinese Breathing Dirtiest Air, Moving Only Escape
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In addition there is a question of how much difference a reduction in pollutant levels at home can make because "pollutants have most of their effects at relatively low levels compared with our usual daily exposures" Hedley says. "Even for intra-uterine growth in pregnant mothers, there are very clear effects of nitrogen dioxide or PM2.5 [small particulates] at levels way below WHO guidelines. The idea that filtering would make a difference to our overall exposures and risk in Hong Kong is probably pie in the sky."
Even if filters are used by people who are more susceptible to air pollution, and who can afford it, Hedley notes this would exclude the most deprived and vulnerable people in society, who are more likely to live in older buildings where doors or windows are often open. And, with electricity consumption requiring fossil fuel burning, he wonders about the net benefit to economic health and quality of life.
There have been some studies on use of air filters for combating air pollution. In Canada, portable air cleaners reduced particulates in homes using wood stoves, and there were improvements to residents' blood vessels, suggesting the filters might decrease the risk of health effects from the wood smoke. A Danish study found that HEPA filtration removed more than half of the ultrafine, fine, and coarse particles in homes of elderly people living near highways. These results appear promising, but more research is needed to overcome the misgivings of experts such as Hedley.
Face masks are also used to combat air pollution, though Hedley considers the cheap paper masks a "waste of time". In the 1990s his research team equipped Hong Kong police with neoprene masks with carbon filters. There were some improvements in health indicators, but Hedley recalls "the masks were uncomfortable and hot and the police didn't like them - they looked like Darth Vader."
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http://www.scmp.com/lifestyle/technology/article/1136763/air-filters-and-face-masks-offer-little-protection-big-city-air