I know with my immune compromised guys I used Honeywell HEPA filters to keep the air clean, and humidifiers to keep the air moist. We've also used nebulizers on our cats to help their congestion. If you're thinking what I was the first time I heard that (how the heck does a cat hold an inhaler tube and breathe in?! I'm so blonde some times), we put them in their carriers, put the hose through the grate, cover the entire carrier with a towel and start the nebulizer. It works really well for cats with allergies because you can add their liquid meds right to the bottled water (not normal drinking water, but the other stuff), and when it mists in the carrier and they breathe it in, they get their meds. Works wonderfully.

Also helps to take them in the bathroom when you shower as well. My mom and dad have my one girl Lucy since 1999 because she reacted so terribly to Bethlehem, PA air (city grime and the Steel factory crap), she sneezed blood, and no antibiotics helped. She is now living happily on their farm in the Poconos, and gets an allergy shot every other month or so.