With Hong Kong recording its smoggiest day of the year two Sundays ago, perhaps you were persuaded to not only stay home but also invest in an indoor air purifier.
As the air quality index climbs, so do sales of air purifiers in China. According to Euromonitor International, air purifier volume sales in China rose 87 per cent from 2012 to 2013.
Nowhere in the world is the number of air pollution-related deaths climbing as quickly as it is in Asia, especially Southeast Asia. A 2012 study by the World Health Organisation found that, globally, seven million deaths were attributable to the combined effects of household and ambient air pollution.
Southeast Asian and Western Pacific regions - of which China is part - bear the brunt of those numbers, with 2.8 and 2.3 million deaths respectively.
In Hong Kong, the Hedley Environmental Index, a real-time health information system developed by the University of Hong Kong's school of public health, shows a city map that is often dotted with red during the day, indicating the WHO level of permissible short-term exposure to fine particulates and other pollutants is more regularly "very bad" than it is green ("acceptable"), or even yellow ("not good").
So can a household air purifier protect your health indoors? It's easy to get your hands on one in Hong Kong - at a wide range of prices.
A new, moderately sized model that can purify up to 48 square metres (or about 515 sq ft) can cost more than HK$4,000, while a used machine can cost as little as HK$290 on classified sites such as AsiaXpat.
There are several factors to consider when buying an air purifier, including the volume or space to be cleaned, external weather and environmental conditions of the home, and what types of allergens or pollutants are being removed.
"There are many types of air purifiers - some of them sterilise air or use ultraviolet or other heating systems to kill germs," says Dr Fanny Ko Wai-san, a specialist in respiratory medicine and president of the Hong Kong Thoracic Society.
While purifiers with special heating systems may be sufficient for killing germs, they may not be adequate in removing fine or respirable particulates that can exacerbate allergies or asthma.
That's not including toxic gases such as nitrogen dioxide, which comes from gas-burning stovetops, or sulphur dioxide, emitted from cars and ships.
Despite becoming increasingly popular, home air purifiers aren't essential for the average household in Hong Kong, Ko says.
"Most people live without one, and without any problem," she says. "It will not lead to a cure for allergies or asthma. Some patients in previous studies show they have the air purifier, but don't show any signs of improvement. You can install one and find it provides no benefits."
A number of variables determine an air purifier's effectiveness.
"Sometimes, indoors can be very moist and, in those situations, an air purifier can be helpful," Ko says. "But if you open the windows, there's almost no point because the pollution comes in. Can it really help or translate into improvement? It's hard to say."
Ko says air purifiers could be considered for places such as hospitals that need to prevent infection, or work settings in industrial areas with extreme levels of particulate matter.
"In general, however, we do not recommend installing air purifiers unless the asthma is difficult to control. In the end, it's a personal preference over a scientific reason," she says.
Their use might be justified for those with acute allergies or asthma. But even in those cases, most specialists prescribe medicine as a treatment.
Studies and tests show mixed results of how well these machines can purify indoor air from toxins, germs and particulate matter.
Indoor air purifiers are advertised as safe household products for health-conscious people, but some purifiers produce ozone. Those that work by charging airborne particles and electrostatically attracting them to metal electrodes emit ozone as a by-product of ionisation.
Emissions may be a few milligrams of ozone an hour, about as much as a dry-process photocopier emits during continuous operation. In a small, poorly ventilated room, this could create an ozone level that exceeds public health standards, according to a study by the University of California, Irvine, published in 2006 in the Journal of the Air & Waste Management Association.
Ozone can damage the lungs, causing chest pain, coughing, shortness of breath and throat irritation. It can exacerbate chronic respiratory conditions such as asthma and compromise the body's ability to fight respiratory infections.
In 2011, the Hong Kong Consumer Council tested 10 air purifiers priced from HK$629 to HK$4,000, and found that they all provided a less usable area of purified air than they claimed. In seven of the larger models, the shortfall ranged from 12 per cent to 67 per cent. In the three small models, the shortfall was up to 90 per cent.
The study also found the four models provided insufficient protection from electrical leakage, while the plastic parts of some models were insufficiently heat and flame retardant.
Ko says many household cleaning supplies, office equipment such as copying machines, pesticides and painting materials can contain and emit harmful volatile organic compounds (VOCs) that can have short- and long-term effects that range from eye, nose or throat irritation to asthma, and may even cause some cancers.
According to the US Environmental Protection Agency, all of these products can release organic compounds while being used, and to some degree, when they are stored.
"Choose furniture that doesn't have VOCs," Ko says. "Those may be the source of emissions in the home, but an air purifier may not be tackling the source. Clean your carpets, too as they can produce allergens and house a lot of dust.
"A dehumidifier is a home essential. It can help with preventing moisture and mould in the home," Ko says.