Spec sheets for pollution products are full of percentages that sound alike and mean different things. Four ideas explain almost all of them, and they are what we used to choose these picks.
How a HEPA filter catches particles smaller than its gaps
A HEPA filter is not a sieve. Its glass or synthetic fibres are spaced far wider than the particles they catch. Large particles hit fibres because they cannot follow the air around them; tiny ones zig-zag randomly and bump into fibres; mid-sized ones are caught when they brush past. The hardest size to catch is about 0.3 microns, which is why filter grades are quoted at that size: an H13 filter must stop at least 99.95% of the most penetrating particles, and H12 at least 99.5%. N95 and FFP2 masks use the same physics plus an electrostatic charge that pulls particles in, which is why they stop working as well once they are damp.
CADR and air changes: the only sizing maths you need
CADR (clean air delivery rate) is the volume of fully cleaned air a purifier delivers each hour, in m³/h. Divide it by your room’s volume and you get air changes per hour. A 150 sq ft room with a 9-foot ceiling holds about 38m³, so a 250 m³/h purifier changes its air about six and a half times an hour, while a 152 m³/h unit manages four. Manufacturers’ “covers 700 sq ft” claims usually assume one or two changes, which only maintains already-clean air. With smoke seeping in round doors and windows, five changes an hour is a sensible target.
Why neither masks nor HEPA filters stop gases
Gas molecules are thousands of times smaller than PM2.5 and do not stick to fibres, so they pass through HEPA filters and N95 masks untouched. They have to be adsorbed instead: activated carbon is riddled with microscopic pores whose surfaces hold gas molecules, and the cartridges on a respirator like the V-800 add chemical treatments that bind acid gases such as sulphur dioxide. Both work by filling up, so capacity is finite. A purifier’s thin carbon sheet can saturate within weeks of heavy use, and a respirator cartridge should be replaced as soon as you can smell smoke through it.
How a laser AQI monitor measures PM2.5
A consumer monitor draws air past a laser and counts the flashes of light scattered by passing particles, sizing each one by the brightness of its flash. That makes it fast and cheap, but it counts particles rather than weighing them, so its software converts counts to µg/m³ using assumptions about particle density. High humidity is the classic error: water droplets in fog scatter light like smoke and push readings up. That is why the most useful reading on Diwali night is the difference between indoor and outdoor, or between a room with the purifier on and one without, rather than any single absolute number.
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