Air sanitizers and purifiers?

Anybody have any experience with a particular air sanitizer that you would recommend or like to warn me about?
My son is having some respiratory problems and I'm looking into sanitizers to hopefully help alleviate the problem, at least in the house. There are tons of them out there and I'm really just looking for some input about which ones actually do a good job and which ones are just expensive pieces of plastic with a fan and a light...

I would look first (or also) at what in your house could be a contributing factor... get to the source of the problem. It could be something as simple as putting in quality high efficiency allergen-rated filters in your HVAC unit(s). Or, since mold is the most common passive residential indoor air contaminant that can cause acute respiratory problems (as opposed to active contaminants like pet dander or tobacco smoke), are there any potential sources of water problems or water intrusion?... a plumbing leak, leaking window or door installations, building veneer or roof leaks, a faulty HVAC unit or moldy ductwork, a moist crawlspace, etc. These issues can be there even though there is no evidence of them (like visibly water damaged drywall or floors).

As for air purifiers, they can help, but won't solve the problem. Stay away from the ones that produce ozone ("ionic"). The best ones are expensive and have a combination of mechanical filtration and non-ozone producing electrostatic precipitators. Here's a read on how to select an air purifier: http://www.epa.gov/iaq/pubs/residair.html