Air filter machine is not very useful. You should maintain clean of your house. Remove all dust collecting item eg. hairy dolls. Try your baby how to swim if the physical status is fit. Give adequate expouse to sunlight well balance diet. Avoid allergy foods e.g. cold drink, spicy food etc.
I just bought a "Sharp" Air Fresher two weeks ago, the price is about $2,210. My daughter have asthma, so every night I switch on until the next day, I am not sure it is work or not (still trying), but I asked the doctor and he told me it has a little bit helpful for my daughter.