Asthma, for me, brought with it a fear of death. It is a miserable thing to live with and having been a severely asthmatic child is what interested me in air purifiers back in 1967. I had a bunch of them. During Junior High I had an old HEPA filter/purifier, an ionizer (which turned the wall black), and an old carbon filter machine all running at the same time in my bedroom. My allergies included horse hair, dog hair, dust, mold, pollen (and probably about 100 other things I didn't know of). I took shots each week and medications to try and prevent attacks.
Did these air purifiers for asthma really help? At that time I honestly didn't know whether the relief was psychological or physiological. It was likely a bit of both.
HEPA - I mention HEPA first simply because most allergists will recommend a HEPA asthma air purifier first for allergy-induced asthma. HEPA, a cloth-like filter which must be replaced every so often, is best for capturing dust, pollen, dirt in the air, and airborne particles.If you have an allergy-based asthma and know you are allergic to these type of substances then start with HEPA. But don't stop there. If you're allergic to (or "also allergic to") mold HEPA will not do the job.
UV - Ultraviolet light will kill germs, bacteria, mold and mildew, and viruses. If you are not allergic to these then you might just go for HEPA. However asthmatics often have a weakened immune system and these substances definitely tax the immune system (This is true for all forms of asthma in my experience).
PreFilter - A Pre-filter is like a screen door, usually washable, and captures hair and dust. HEPA captures these too, but having a washable filter in front will save the life of the HEPA filter. If you have dog or cat allergies which create asthma then this would be a must along with HEPA.
Carbon - Carbon (and/or Zeolite) filters are treated charcoal minerals which have millions of tiny cell pockets. These pockets are effective at trapping and holding gas, odor, and toxic chemical fumes. If you are chemically sensitive, and /or chemical fumes bring on asthma, Carbon would be added to your list.
Electrostatic - Also called "truman cells", electric grids, electronic cleaners, or ionic plates. These type of air purifiers capture particles using positively and negatively charged "washable plates" or grids. Oreck is one example, also Friedrich, and many others. Most do not have HEPA filters. The washable part is great (and great for marketing), but the effectiveness is not. Good for smoke and some pollution. Not good for dust, hair, pollen, mold, mildew, and germs.
Tio2 - Used in conjunction with UV it magnifies the effectiveness of the virus, germ, and bacteria killing of ultraviolet light.
1) Getting my liver cleansed of toxins which were causing me to create too many histamines.
2) Special treatments to the brain which had to do with the mind-body causes of allergies. Many allergies (and hence asthma attacks) are caused by patterns in the brain which cause energy blockages in the nervous system.
3) Building up my adrenal glands which relieved the muscle spasms of asthma.
The video below talks more about the work I had to do to get rid of allergies and asthma.
The Comparison Chart on the site may be of help.