The promise of an air purifier is an enticing one: An appliance designed to cleanse the air in your home, getting rid of all the impurities including odors, smoke, dust and pet dander. Given the fact that indoor air can have levels of certain pollutants up to five times higher than outdoor air, we get it. Air purifiers can indeed neutralize some of the threat posed by air pollution and by indoor activities. In reality, though, not all air purifiers necessarily live up to their marketing hype.
How do air purifiers work?
Air purifiers usually consist of a filter, or multiple filters, and a fan that sucks in and circulates air. As air moves through the filter, pollutants and particles are captured, and the clean air is pushed back out into the living space. Typically, filters are made of paper, fiber (often fiberglass) or mesh, and they require regular replacement to maintain efficiency.
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How frequently you will have to change filters varies based upon the purifier type and usage. Some filters are reusable and washable, but they require meticulous maintenance, so you don't usually find them on the most effective air purifiers. Reusable filters are generally good at removing larger particles from the air, like dust mites and pollen. You'll also find UV (ultraviolet light) filters on the market, which often claim to destroy biological impurities like mold or bacteria, but many require higher wattage and greater exposure to be effective (not to mention some bacteria is UV-resistant).
That means, in addition to the purchase price of an air purifier, you should also factor in operating costs and filter replacement costs. Operational costs can easily amount to $50 annually, since you should be running air purifiers near constantly to garner the benefits. Filter replacements can run upwards of $100 a year all told.
Some air purifiers use ionizers to help attract particles like static — negative ions bond to dust and allergens and make them settle out of the air. If you're interested in buying an air cleaner that uses ionizers, make sure it does not produce dangerous levels of ozone (a gas made up of three oxygen atoms that is often marketed as helping break down pollutants), because ozone could be a lung irritant and further aggravate asthma conditions. Usually the air purifiers with ozone will have that listed on the packaging or in the marketing descriptions. Currently, our recommendation until additional testing and more robust industry standards are in place is for people with units with plasma/ionization to use their machines with those functions off. This is due to the fact that there is the potential for unknown harmful consequences, coupled with additional energy usage and a negligible or non-existent increase in purification.
What are air purifiers supposed to filter out — and do they actually do it?
Most filters on the market are designed to capture particles like dust, smoke and pollen, but they don’t catch gases like VOCs (volatile organic compounds) or radon that may accumulate from adhesives, paints or cleaning products. That would require an absorbent, like activated carbon. In fact, the Environmental Protection Agency (EPA) warns that the functionality of air purifiers is limited in terms of filtering out gases and that you must frequently replace filters for optimal functionality, usually about every three or so months. Allergens that are embedded into furniture or flooring are also not captured by purifiers.
Additionally, the effectiveness of air purifiers in real-world situations likely won’t mimic those of controlled conditions in a lab (which is what those "99% effectiveness" claims are referring to!). The location, installation, flow rate and run time for all will vary, as will the conditions in the space. In addition, there are other things happening in your home that may effect the efficacy like ventilation (open or closed windows) and new particles are constantly emerging, so the air might not be as filtered as the claims may have you believe. And to remove allergens, bacteria or viruses that have settled on surfaces, you need to use disinfectant cleaners and/or effective vacuums.
If you are concerned about mold, we’d recommend buying a dehumidifier or humidifier to help maintain the appropriate moisture levels in your home and stave off mold growth issues. Air purifiers do not prevent mold growth, so it is necessary to eliminate the source of moisture that is allowing it to grow.
Can air purifiers filter the outdoor air that enters your home?
Some models may be able to target bad air that creeps into your apartment or house, especially if you live in an area affected by pollution or natural disaster. Most people shouldn't be worried about exposure to temporary pollutants like smoke or exhaust in the air outside your home, as they dissipate over time, explains Ryan Roten, D.O., an emergency medicine doctor with Redlands Community Hospital in California.
"In the short term, people will have asthma-like symptoms, primarily, or symptoms closer to allergies or sinusitis, including stuffy nose and a bit of a cough," says Dr. Roten, who has been treating patients with underlying respiratory illnesses as mass wildfires rage along the West Coast and air quality reaches new lows. "If the smoke is dense enough, you might have some headaches due to carbon dioxide, and those with issues like asthma or COPD will have it worse in the moment."
Sometimes, non-organic air pollutants — like the VOCs we mentioned previously — can originate from outside your home. "There are all sorts of scenarios in structure fires where large doses of smoke inhalation may lead to cyanide toxicity. But that would largely need to be someone who was standing directly in or near the fire: Those people are brought to emergency rooms immediately," Dr. Roten explains. "Generally, outside pollution or smoke or temporary bad air isn't a constant concern for bystanders."
But the right kind of purifier can address any environmental air qualities in your locale. Using wildfires as an example, Dr. Roten adds that a HEPA filter-equipped purifier is your best bet: "Anything that has a true HEPA filter in it is probably adequate enough to filter out most of the large particles that would be concerning," he says. "Most of the smoky smell will also be addressed as well."
What is a HEPA filter?
HEPA is an acronym for High Efficiency Particulate Air. HEPA filters capture variously sized particles within a multi-layered netting usually made out of very fine fiberglass threads (much thinner than a strand of human hair!) with varying sized gaps. The filter is composed of a dense sheet of small fibers pleated and sealed in a metal or plastic frame.
The air purifier's fan draws air into the filter and particulates are captured in the filter. The larger particles (ones bigger than the fibers) are captured via impaction (the particle crashes into the fiber), mid-sized particles are captured by interception (particle touches the fiber and is captured), and ultra-fine particles are captured by diffusion (while zig-zagging the particle will eventually hit and stick to the fiber).
Can air purifiers capture the coronavirus?
Air purifiers that utilize HEPA filters can capture particulates the size of the coronavirus. However, the actual efficacy of it at preventing someone from getting the virus is still unknown, as the rate of transmission may be faster than the air purifier can capture the particulates. Therefore, we continue to recommend adhering to the CDC's advice regarding best methods for reducing transmission risk and avoiding exposure to the virus as best as possible.
So… should I buy an air purifier?
Before you do, know that an air purifier is not a cure-all. There is very little medical evidence to support that air purifiers directly help improve your health or alleviate allergies and respiratory symptoms. That’s due in part to the fact that it is difficult to separate the effects of known air-quality pollutants in your home from other environmental and genetic factors. (For instance, how are the furnishings and ventilation in your home affecting you in addition to any indoor pollutants?) But if you are an allergy or asthma sufferer, an air purifier with a HEPA filter may be helpful for you as it will be good at removing fine airborne particles.
What should I look for in an air purifier?
What are other ways I can improve the air quality in my home?
The best advice is to address the source of indoor air pollution and to ventilate your home. If you are looking to supplement the work of your air purifier or see if you can get by without one, we recommended trying these steps to help reduce indoor air irritants:
Chief Technologist & Executive Technical Director
Rachel Rothman (she/her) is the chief technologist and executive technical director at the Good Housekeeping Institute, where she oversees testing methodology, implementation and reporting for all GH Labs. She also manages GH’s growing research division and the analysis of applicants for the GH Seal and all other testing emblems. During her 15 years at Good Housekeeping, Rachel has had the opportunity to evaluate thousands of products, including toys and cars for GH’s annual awards programs and countless innovative breakthroughs in consumer tech and home improvement.
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