Making sure there’s no gaps around your HVAC air filter is a good start. Air should only flow through the filter. I like to put duct tape around the sides of of it, forming a seal around the grill so that no air leaks around the sides. Buy one with a MERV rating of 9-12 to minimize dust and pollution. Don’t go any higher than that (and avoid HEPA) unless you don’t mind a higher power bill and slightly more stress on your system.

During the times of the year when you’re not running A/C or heat, change the fan setting from “Auto” to “Circulate” so that it clicks on for a few minutes every hour or so. This will help keep the air clean.

If your air quality is really bad, you can always suppliment your HVAC system with a dedicated air purifier. You don’t need anything fancy or expensive; a box fan with 4 HEPA filters taped to it is among the cheapest and most effective ways to clean the air.