Air Purifier?

We have an 8 yr old with severe asthma. We seem to be spending a lot of time in and out of hospital but not getting any control. We just wait for the next emergency dash to the Children's hospital. It's frustrating and worrying as it's just not settling.

Last week we were on holiday in Wales, a great week, but his asthma was great too, we hardly used the blue inhaler at all, unlike here at home where it's used every day and several times a day (even several times in an hour). We were wondering whether an air purifier might be of benefit and wondered whether anyone here has any experience or advice?

Thanks