Why? Because I have two cats, and I have allergic problems. Well, nothing I can't live with, but the worse part is my cat has feline asthma.
After google-ing for a while, it seems those with multiple filters should be better, e.g. front filter for larger particles, active carbon filter in the middle for finer particles, and HEPA filter for PM2.5 particles. HEPA is the crucial component, but it also comes with a limited lifespan, and it is expensive... You cannot wash as it would ruin its structure and makes it useless. Some people mentions you can vacuum clean it, but usually is not very effective.
Some brands have claimed their HEPA filters have 10 years lifespan... Obviously the filter would have a limited capacity for PM2.5 particles, and you can't wash it. Can it really last that long? Many brands would let you download their operation manual online. I found one famous brand has the following assumptions:
Filter life is based on smoking 5 cigarettes/per day (10 years). But according to the Specification of The Japan Electrical Manufacturer's Association JEM1467 if based on the conidtion of smoking 10 cigarettes per day, the replacement period is about 5 years.
Ok, more google-ing finds that 1 cigarette is around 12 mg of PM2.5 (I'm actually not too sure if this is correct: http://www.myhealthbeijing.com/pollution/is-pm2-5-from-air-pollution-the-same-as-from-smoking/)
The air purifier spec should tell you how much cubic meter of air it would handle per hour, the particular one I'm looking at has 150 cubic meter/hour. So let's do some math
Air flow volume: 150 cubic meter/hour
PM2.5 in the place I live: ~ 45 μg/cubic meter
150 x 45 = 6750μg = 6.75mg/hour = 0.5625 cigarette
open 24 hour/day = 13.5 cigarettes/day => 3.7 years
75% of theoretical figure ~ 2.8 years
50% of theoretical fiture ~ 1.85 years
open 12 hour/day = 6.75 cigarettes/day => 7.4 years
75% of theoretical figure ~ 5.6 years
50% of theoretical fiture ~ 3.7 years
Well, the figures doesn't seem too sad although it could quite far off the claimed lifespan under certain conditions and assumptions T_T
I couldn't help to further find out how much PM2.5 I'm inhaling per day though it's off topic...
an average person inhales ~ 0.5 cubic meter/hour at rest, 0.7 cubic meter/hour at light activity (maybe walking)
https://en.wikipedia.org/wiki/Respiratory_minute_volume
Take the average: 0.6 x 45 x 24 = 0.648mg :D
