If you live in any of these 13 Indian cities, you know you need lungs of steel when you step out. But what you may not know is that even at home, with the windows and doors closed, you still face indoor air pollution: dust mites, mould, volatile organic compounds (VOCs) emanating from paints, polishes and aerosols, mosquito coil fumes, even smoke from incense sticks and dhoop.

So when you can’t fling open a window to air out your home (because the outside air is even worse) what can you do?

We spoke to experts to give you the lowdown on the effectiveness, affordability and other considerations of common air purifier options.

Plants Can Fight Indoor Air Pollution

A 1989 NASA study is among the most-cited evidence of plants’ ability to act as an indoor air purifier. It inspired Kamal Meattle to turn his 50,000-square foot office into a green fortress, where the air outside is first filtered through an in-house technology, and further purified by plants. The CEO of Paharpur Business Centre in New Delhi-turned-green space advocate researched and installed more than 1200 plants to offset the effects of pollution for 300 office workers.

However, the NASA study itself is not without controversy; critics say the sealed environment in which the experiment was conducted is not achievable in real life. Also, the number of plants required to make a true impact on indoor air pollution would likely not fit into an average Indian apartment (one expert estimates that about 680 plants would be required to replicate the NASA results in a 1,500 square foot space). The humidity that would result is yet another practical challenge.

Yet the NASA study is not the only one to point toward the efficacy of plants: In 2009, researchers at the University of Pennsylvania found that money plant, mother-in-law’s tongue and spider plant help reduce indoor ozone, a respiratory irritant.

Meattle’s efforts have been studied, too. The Central Pollution Control Board, in collaboration with Kolkata’s Chittaranjan National Cancer Institute, found in 2008 that non-smokers employed at Paharpur Business Centre experienced fewer cases of impaired lung function, hypertension, headaches and eye irritation than Delhi’s other non-smoking office-goers. The study concluded that staying in “a cleaner indoor environment for 8-10 hours a day reduces the prevalence and magnitude of health impairments associated with chronic exposures to air pollution.”

Conclusion: Plants are a low-cost way to clean the air in your home – but their value may be greater for peace of mind than anything else.

Air Purifiers Can Fight Indoor Air Pollution, Too

Meattle’s efforts combined plants alongside air-filtering technology. He’s hardly alone; since the 2014 report from the World Health Organization revealed many Indian metros among the world’s most polluted cities, the air purifier market in India has boomed. But the industry is still nascent and relatively unregulated, making it difficult to sift fact from puffery.

“There is no independent agency in the country to certify the effectiveness of air purifiers,” says Pritee Shah, chief general manager, Consumer Education and Research Centre, Ahmedabad. “So in the absence of any regulatory mechanism, several brands are making unverified claims.”

Certifications from international bodies like Association of Home Appliance Manufacturers (AHAM), or the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) can tell a consumer if a brand is legitimate – but not whether the filter has the technology needed to battle India’s indoor air pollution.

“A multitude of technologies—ozone-generating, ionizing, odour-removing, UV, etc — floods the market, but an air purifier with a combination of pre-filters, carbon and HEPA (high-efficiency particle air) filters is what you should be looking for,” says Barun Aggarwal, director of BreatheEasy, a Delhi-based indoor air quality solution provider.

This is because Indian indoor air pollution is filled with particulate matter 2.5 microns (PM2.5 and PM 1) and fewer in diameter, which are too small to be filtered by nostril hair, Meattle says. HEPA filters can remove these particles with at least a 90% success rate, but they are expensive, with an ongoing cost as filters must be replaced every six months to one year.

The other important feature to look out for is CADR, or clean-air delivery rate, for it defines the volume of air your purifier can clean. The higher the CADR number the better, says Aggarwal. However, “it should allow a minimum of three to four air changes per hour, which is determined by dividing the volume of air cleaned by the volume of the space it is meant for.” But this comes with a caveat, too: For HEPA purifiers fitted with a fan, CADR is determined at top fan speed, which means you’d need to run it at top speed – which can mean a lot of noise. Reports suggest choosing a purifier designed for an area larger than yours for better, quieter results.

Whatever you do, steer clear of ionizing purifiers, which can create ozone as a byproduct and pollute your home more, says Meattle.

Conclusion: A high-CADR air purifier with a pre-filter, and carbon and HEPA (high-efficiency particle air) filters – if used correctly – might be worth the significant investment if you live in a very polluted city. But that’s a lot of ifs, so make sure you do your homework before purchasing one.

Other ‘Air Purifiers’ Don’t Work At All

Beeswax candles and salt lamps are sometimes touted as natural air cleaners, but the truth is, they actually add to your home’s indoor air pollution.

“Anything that burns will take up oxygen and emit carbon dioxide, so burning candles will not really help improve indoor air quality,” explains Meattle. “In fact, they will add VOCs and PM 2.5 and PM 1 particles (to the air).”

Conclusion: It’s money wasted.

Health Benefits of Using An Air Purifier

It’s difficult to make a definitive judgment on the positive effects of air purification, whether plant-based or technological. Evidence of indoor air pollutants’ adverse effects on health is ample, and while the success of air filter technology in easing asthma symptoms has been well-documented, few studies have connected the dots to say categorically that air purifiers lead to overall better health. And it’s unlikely they ever will, according to Jens Hammes, CEO, IQAir International, a Swiss-based air purification solutions company.

Hammes describes an imaginary clinical trial that must take into account objective and subjective evaluations of health, along with correct and full-time usage, as well as the amount of time participants spend in the room with the air purifier; in short, a complicated and very expensive test unlikely to ever happen.

Pulmonologists interviewed for this article also said the dearth of scientific studies makes it impossible to conclusively recommend air purification of any kind. Some, in fact, said the unavoidable exposure to outdoor pollution negates the effects of breathing clean air at home.

Aggarwal says the anecdotal evidence he comes across as part of his job – which includes referrals from top cardiovascular surgeons – is proof enough for him that there is some benefit to a reduction in the amount of polluted air we breathe indoors.

“Would a pulmonologist want his patient to smoke? No,” he says. “But if the choice is between 20 and 10 cigarettes, I believe he would ask the patient to cut down.”