The Basics

Air purifiers typically have three layers of filtration media: a pre-filter for large debris such as dust and hair, an activated carbon filter for odors and , and a particle filter (usually ) for very small particles. They're meant to be run 24/7, usually with one unit covering a single room. Please note that buying an air purifier is not a total replacement for vacuuming and dusting. You'll still need to do those things, but probably less so.

Things to Avoid

UV Light

Some companies use lights to kill bacteria and viruses that enter into the air purifier's filter. You can read about UV light's effectiveness, or lack thereof, and . In short, the amount of time needed to kill those viruses and bacteria is longer than the time they're typically exposed to it in these air purifiers. Killing them is also not actually required -- trapping them inside the particle filter essentially gives the same end result.

Ionizers

Ionizers release negatively charged ions into the air. Some airborne particles become attracted to these, latch onto them, and the combined result becomes heavy enough to sink to the ground. Unfortunately this process produces ozone as a byproduct, which can be harmful for humans to breathe in. Note that some vendors use marketing names like "PlasmaWave" (which is technically a bipolar ionizer) to avoid the stigma of ionizers and their health risks.

Avoid any units with either of these technologies unless they can be disabled.

Proprietary Filters

We also recommend only buying units with HEPA filters, not other proprietary particle filters. BlueAir is one popular company that does not use the HEPA standard.

Room Size

Each unit listed below includes the area which the manufacturer claims it can cover. Sometimes these numbers are inaccurate. For example, there may be fine print that states a unit can only perform one air change per hour in such a room size, or the unit has to be in the middle of the room, or the ceiling can only be so high, etc. Please only use the advertised number as a general idea of how much space it can cover. For large spaces, it's to buy multiple smaller units than a single larger unit, assuming there are no other specific requirements. Doing so will provide multiple points of filtration.

Cleaning / Replacement Considerations

Each unit has different cleaning and filter replacement schedules. Some have filters that last several years, while others require manual cleaning and buying of replacements every few months. While one unit may appear substantially more expensive than another, the cost of replacement filters and the time needed to clean them should be taken into consideration too. The higher initial cost sometimes makes up for the long-term cost.

Amount of Carbon

The amount of activated carbon determines whether any given air purifier can practically filter out smells, smoke, and VOCs. Most low-end units include a very small amount that won't actually make a difference. Carbon typically saturates faster than HEPA filters, so the ones with a small amount of it become entirely useless for gas filtration within a short period of time.

Recommended Purifiers

(when odor / smoke / VOC removal is NOT a concern)

(when odor / smoke / VOC removal IS a concern)

(when odor / smoke / VOC removal is the MAIN concern)

The hidden costs of air purifiers

Considering the costs of electricity and filters, an air purifier might cost you hundreds of US dollars per year to run. See, for example, .

(If you do use an air purifier, consider reducing its settings to "Low" or "Off" mode from 4 p.m. to 9 p.m., or whenever electricity is the most expensive where you live. You can do a Google search to find out the when it may be cheapest to purify your air.)

What to do

Before you consider buying an air purifier, please consider doing all of the following:

If the outdoor air is clean, consider opening your windows

• If the outdoor air is cleaner than your indoor air, open all your home's windows whenever the weather is nice.

  • On any day when your local air quality index is 50 or less, the outdoor air is almost surely cleaner than your indoor air.

• Make sure you have window screens, to avoid Zika, West Nile virus, and other insect-borne diseases.

• During the cooling season, even if it's too hot to open windows during the daytime, it might be cool enough to leave all your windows open overnight.

Buy a better furnace/AC filter

Please buy a better filter for your home furnace/AC system. It's usually best to try this before you buy an air purifier.

If you want to reduce particulates (e.g. dust, pollen, allergens, and viruses):

• I personally would choose a filter with a MERV 13 rating (or ), at least 4" thick, if your furnace/AC system can accept such a thick filter. Make sure to choose a brand which is , and to replace it regularly. MERV 13 filters are not hugely expensive. If you can't afford one, please leave us a comment describing your situation, and ask us for advice.

• If possible, it's best to buy a filter in person from a trustworthy retail store in your country, or from a trustworthy HVAC technician. Watch out for MERV fraud, and choose a brand carefully; please see . Consumer Reports and other websites have tested various makes and models of furnace/AC filters. If you're looking for a store which carries HVAC filters, you can try: a hardware store, or a home-improvement store such as Home Depot, or Walmart's hardware department. Do not trust the FilterBuy "MERV 13" filters sold on Amazon.

If you want to reduce odors, such as the smell of freshly-cooked fish:

• I suspect that a 1" furnace filter with a thin activated carbon layer, such as "3M Filtrete Odor Reduction" or "Arm & Hammer", might not really be helpful for odor reduction.

• Try to stop the smell at source. For example, if your downstairs neighbors own an illegal pet skunk, talk with them and/or phone the building manager.

• Open all your windows, if the weather's nice; please see above.

• You could try to live with the smell, and to otherwise ignore it.

• You could wear a respirator mask. For example, in America, you could wear the 3M 6502QL mask plus 3M 60926 air-purifying cartridges. In Europe, you could wear 3M's equivalent European products. The aforementioned respirator mask will muffle your speech quite severely.

• If the weather doesn't allow you to open windows, consider installing a heat recovery ventilator or energy recovery ventilator, if you don't yet have one.

• Or you could buy a specialized air purifier which is truly competent at odor removal. Please see the last two tables in .

Set your thermostat to "On", not to "Auto", "Circ", or "Circulate"

• During the heating season, do not set your thermostat's fan setting to "Auto", "Circ", or "Circulate". Instead, set it to "On". If you set it to "On", the blower fan will run 24 hours per day, and therefore will push the maximum possible amount of air through the filter. If you set it to "Auto", "Circ", or "Circulate", it will run less often.

• During the cooling season, please see the comments below this post. See also by Allison Bailes, who's collected some data on the matter.

Replace your filter regularly

• Because the blower fan is always running, replace your filter , in order to protect your HVAC system.

Now you might not need an air purifier after all

The above steps, alone, might improve your home air quality enough that a standalone air purifier turns out to be unnecessary.

Visualizing the results

You can use the CDC's to see how well various combinations of furnace/AC filters, open windows, and portable air purifiers will remove viruses from your home.

Note

Consumer Reports writes that 1-inch filters "can be tricky to shop for, according to many HVAC professionals — because 1-inch filters with high MERV ratings are especially likely to be too restrictive for some HVAC systems. ...

"If you want to be certain about how much airflow your HVAC system requires to function properly, you’ll need to measure the static pressure. (Basically, that’s the resistance to airflow in your ducts, based on a handful of factors.)

"A qualified technician with basic tools of the trade can take the measurement in a couple of minutes—for example, when they’re already at your home performing a system tuneup—and advise you on how to pick filters that will work well with your equipment.

"If you can’t take a measurement, experts sometimes recommend sticking with a MERV 8 pleated filter as a safe-harbor option. Filters with this rating allow plenty of airflow, so they’ll be safe for most systems. They also don’t cost much, and they catch a lot more debris than basic fiberglass filters can."

Alternatively, you can look for a MERV 13 filter which is at least four inches thick. Such a filter should always be okay to use, as long as you have a filter cabinet which is large enough to hold it.

Conclusion

I thank and for this post.