It's time to upgrade my furnace and AC, and as a lifelong asthma and allergy sufferer, I'm always looking for ways to improve my indoor air quality.  I currently run standalone HEPA filters in my main rooms, and a 4" standard media filter on the furnace. 

From what I've gathered so far I would want to avoid any type of electronic air cleaner that ionizes the air, as that can trigger asthma symptoms.