Activated carbon filters: what they remove and when they stop
An activated carbon filter adsorbs some gases and odors, misses others and fills up without warning. What EPA and test data say about carbon in air purifiers.
Independent guides to home air purifiers
Air purifiers, explained honestly
Plain guides to how home air purifiers work, HEPA and carbon filters, sizing by CADR, wildfire smoke and allergens, and what these machines can and cannot do, with the evidence and its limits.
Guides by subject
Six sections: basics, filters, sizing, smoke and allergens, ozone and other technologies, and choosing by room.
Basics
What a portable air cleaner physically does, what the evidence shows, how to choose one, where to put it, how long to run it, and what to do before buying anything.
2 guides
Filters
What HEPA and activated carbon actually remove, what a MERV number means, and how and when to replace filters.
3 guides
Sizing
Reading CADR, matching a unit to a room with worked arithmetic, and what noise and electricity use really depend on.
2 guides
Smoke and allergens
What a purifier can and cannot do about wildfire smoke, tobacco smoke, allergens, odors, VOCs, mold and airborne viruses, with the limits of the evidence stated plainly.
2 guides
Ozone and tech
Why EPA warns against ozone generators, what ionizers, UV and electrostatic precipitators do, and what CARB certification does and does not mean.
1 guides
By room
Bedrooms and children's rooms, and when a whole-house system makes more sense than a portable unit.
0 guidesNew guides
What size air purifier for your room: the math, worked out
Air purifier room size in one sum: a smoke CADR of two-thirds of the floor area. Worked examples, air changes per hour, high ceilings, smoke and open plans.
Air purifiers and allergies: what they can and cannot do
An air purifier for allergies can lower some airborne allergens. What trials measured, how small the effects were, and what usually matters more at home.
CADR ratings explained: what the three numbers measure
CADR is the clean air a portable purifier delivers, in cfm, tested on smoke, dust and pollen at top speed. What it measures, what it misses, how to read it.
How to choose an air purifier: a step-by-step decision
How to choose an air purifier: name the problem, size CADR to the room, check the filter, noise at your real setting and ozone, and know when not to buy.
HEPA filters: what the term means and what it does not
A HEPA filter is defined by 99.97% capture at 0.3 microns, yet EPA says consumer HEPA has no widely accepted definition. What the label can tell you.
How do air purifiers work? Fan, filter and repeated passes
How do air purifiers work? A fan pulls room air through a filter and returns it, again and again. What each method removes, and what an air purifier cannot do.
How air purifiers work and whether they help
What a portable air cleaner physically does, what the evidence shows, how to choose one, where to put it, how long to run it, and what to do before buying anything.
How to choose an air purifier: a step-by-step decision
How to choose an air purifier: name the problem, size CADR to the room, check the filter, noise at your real setting and ozone, and know when not to buy.
How do air purifiers work? Fan, filter and repeated passes
How do air purifiers work? A fan pulls room air through a filter and returns it, again and again. What each method removes, and what an air purifier cannot do.
HEPA, carbon and MERV filters
What HEPA and activated carbon actually remove, what a MERV number means, and how and when to replace filters.
Activated carbon filters: what they remove and when they stop
An activated carbon filter adsorbs some gases and odors, misses others and fills up without warning. What EPA and test data say about carbon in air purifiers.
HEPA filters: what the term means and what it does not
A HEPA filter is defined by 99.97% capture at 0.3 microns, yet EPA says consumer HEPA has no widely accepted definition. What the label can tell you.
When and how to replace air purifier filters
Air purifier filter replacement follows the maker's interval, not a universal number. What a loaded filter does, why carbon gives no warning, how to swap it.
CADR, room size, noise and energy
Reading CADR, matching a unit to a room with worked arithmetic, and what noise and electricity use really depend on.
What size air purifier for your room: the math, worked out
Air purifier room size in one sum: a smoke CADR of two-thirds of the floor area. Worked examples, air changes per hour, high ceilings, smoke and open plans.
CADR ratings explained: what the three numbers measure
CADR is the clean air a portable purifier delivers, in cfm, tested on smoke, dust and pollen at top speed. What it measures, what it misses, how to read it.
Smoke, allergens, odors, mold and viruses
What a purifier can and cannot do about wildfire smoke, tobacco smoke, allergens, odors, VOCs, mold and airborne viruses, with the limits of the evidence stated plainly.
Air purifiers and allergies: what they can and cannot do
An air purifier for allergies can lower some airborne allergens. What trials measured, how small the effects were, and what usually matters more at home.
Air purifier for wildfire smoke: sizing, setup and limits
How to choose an air purifier for wildfire smoke: EPA and AHAM sizing, a clean room, running it on high, what carbon adds, and what the data does not show.
Ozone generators and other technologies
Why EPA warns against ozone generators, what ionizers, UV and electrostatic precipitators do, and what CARB certification does and does not mean.
Ozone generators sold as air cleaners: why to avoid them
An ozone generator makes a lung irritant on purpose. What EPA, California, FDA and OSHA say, the levels measured in homes, and what to do if you own one.