Roxy Chick. I could name at least half a dozen of my colleagues who swear that their own and children's health improved markedly as a result of returing to their home country permanently or even just on extended holidays. And the improvement was almost instantaneous in most cases. This suggests the improvement in air quality was the main factor.
Some people are more susceptible to showing symptoms related to the air quality than others. Regardless, the bad air WILL affect ALL those living in HKG. Younger children in particular are more susceptible with their developing lungs. The Hong Kong government are burying their heads in the sand on this issue and will continue to do so for a long time to come. Donald Tsang once remarked that since Hong Kong has one of the longest life expectancy's in the world, there simply can't be a problem.
We have one of the IQ Air machines and two smaller purifiers and can reccommend the IQ Air in particular. Unfortunately they(IQ Air) are very expensive to buy. The replacement parts are also expensive, but we order our spares online and get them posted to one of the crew hotels in the USA. At least we are getting clean air for 14 or so hours a day.
I know of no one that has ordered one from USA. They are significantly cheaper than in Hong Kong, but would be difficult to get on as crew luggage unless thay can be broken down. Shipping would be expensive too. Voltage is not a problem - step down transformers are just a few dollars here.
Hope this helps.
PS. There is also a hidden danger with new renovations/furniture. Alot of the (cheap) glues used to put stuff together in China(where much of the new furniture/laminates/built -in's etc are made) have some pretty dodgy chemicals in them. They can give off some nasty fumes(e.g. formaldahydes) over a considerable length of time. Consider getting a purifier that can absorb some of these. A simple google search will yield information on all of this.