Re: air purifier?
Asthma is generally caused by allergies, so it is likely you have some that you don't know about, typically the things you inhale like dust, pollens, molds, animal dander, etc.
I think the air purifiers are helpful and the best are the ones with an actual filter you can take out and wash and reuse. Get one that filters microparticles. I have asthma and use one and am always amazed at the amount of dust and pollen is on the filter when I clean it. I don't have animals, keep my house clean, but the pollen is thick where I live. It is particularly helpful to run it all night while you sleep. National Jewish Hospital in Denver is a wonderful resource for asthmatics as far as information on ways to control allergies/asthma. You can find them on the internet and call toll free and talk to a nurse.
You might consider getting some allergy testing done to find out what you are sensitive to, so you can avoid those things that are avoidable.