Re: Are air purifiers good for allergies and asthma. Which is best unit?

Portable air purifiers may help as long as you remain confined to your 
room. Most portable air purifiers are not efficient enough (in terms 
of air exchange) to do much good in any place except a small room. If 
you decide to get one, make sure that they use ULPA or HEPA filters. 
Check the rate of air exchange before making a decision to purchase, 
and make sure you change the filter at the recommended interval.

If you need to reduce allergens in an entire house, HEPA filters can 
be integrated into central heating and air-conditioning systems to 
reduce airborne-particles associated with allergies, particularly 
allergies to pollen, animals, mites and mold. However, the unit only 
works when the fan is running, which would require the system to be 
left on continuously.