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Air Purifier Knowledge FAQ

How do air purifiers work?

They draw air from a given space into the unit and then have it pass through several layers of filtering devices within the unit and then have it recycled and released back into the room, through a vent from the unit, as clean or purified air.

What are the best air purifiers?

This often depends on specific concerns you want to address. Below are core applications and the best air purifiers for the job:

Best air purifiers for bacteria and viruses

Best air purifiers for mold

Best air purifiers for pets

Best air purifiers for salons

Best air purifiers for smoke

Best air purifiers for dental offices

Best air purifiers for multiple chemical sensitivities

Best air purifiers using UV light

Do air purifiers help with allergies or asthma?

See our Allergy and Asthma Care Blog

Do air purifiers remove dust?

See our How To Remove Dust Blog

Are air purifiers worth it?

See our Are Air Purifiers Worth it Blog

How much do air purifiers cost to operate?

To determine electricity cost per month:

Machine Wattage * rate/killowatt hour x Hours/Day x 30 x 0.001 —— Monthly Cost

Wattage = the wattage draw of each machine (can be found on each model's specification sheet)

Charge/kilowatt hour = Can be found on your electric bill

Hours/Day = How long the unit runs per day (24 max)

Days/Month = 30 average

x 0.001 = Converts from watts to kilowatts

So, if you have a 60 Watt unit that cost 12 cents per kilo-watt hour and you ran 24/7 then:

X = 60      Y = .12      Z = (24*30*.001) or .72

X * Y * Z = ( 60 * .12 * .72) = $5.18 per month

So, given Z assumes non-stop use or 24/7, then just find X and Y and do the multiplication and that would be your worst-case bill. 

Where should air purifiers be placed?

See our Where to Place Air Purifier Blog

How loud are air purifiers?

A quiet air cleaner, like all other sound-emitting objects, has their sound/noise levels measured in decibels [dB], usually multiplied by an 'A' factor which, put simply, accounts for the effect of human hearing on the sound. Thus, when you assess an air purifier's noise levels, look for dB(A) on the product's specifications.

The dB(A) scale is logarithmic. That is, it goes up in powers of ten, rather than in single units. Thus an air purifier marked as 70dB(A) is actually ten times noisier than one marked as 60dB(A).

See our Air Purifier Noise Level Reference


How many air changes per hour (ACH) is good?

Any number is better than nothing. But, ACH can actually be a cause for confusion because it is meaningless without context. So, it must be associated with Cubic Feet per Minute (CFM) and Room Size to be meaningful. 

To determine the air changes per hour for a machine at any setting:

The CFM rate of the machine (at low, medium, or high) x 60 (minutes/hour) divided by cubic feet of room —— air changes per hour

Example: A room with the dimensions 10 feet wide, 24 feet long, and 10 feet high has an area of 2,400 cubic feet and the air purifier supports 200 CFM.

(200 CFM * 60 minutes/hour) = 12,000 cubic feet per hour

12,000 / 2,400 = 5 air changes per hour  

See our CADR and ACH and SQFT Reference Table

What size air purifier should I get?

Determine the square footage of the space you want to cover then just multiply that by your ceiling height to get cubic feet.


Your room is 400 sq. ft. with 10 ft. ceilings or (400 * 10) = 4.000 cubic feet.

If you want 4 ACH (See above FAQ) then 4,000 * 4 = 16,000 cubic feet / hour

CFM = 16,000 / 60 min per hour = 267

You can also follow the 2/3rds Rule based on square feet if you want 4 ACH. So, we have:

400 sq. ft. * (2/3 or .667) = 267 CFM

Typically, air purifiers will either give you a Cubic Feet per Minute (CFM) or a Clear Air Delivery Rate (CADR) number. CADR would theoretically be the same as CFM if the efficiency were 100%, which is never the case.

Formula: CADR = [(ACH x L x W x H) / 60] CFM * Efficiency

Now back to our example above and the need for 267 CFM. If the assumption is that all the air is actually purified then what we would actually need is 267 CADR.

See our CADR and ACH and SQFT Reference Table 

What is activated carbon?

Reference How Does Activated Carbon Work 

Why do HEPA filters work?

Reference HEPA Filters Explained 

What is the difference between HEPA and ULPA?

See our HEPA and ULPA Blog

How does ionization work?

Reference What Is Ionization

How does PCO or photocatalytic oxidation work?

Reference What is Photocatalytic Oxidation