Indoor air quality consistently tests lower than outdoor air quality. Mold, dust (much of which is dead skin cells), VOC's from paint and other household chemicals, bacteria, pet dander, carbon monoxide, and other gasses are among the indoor air pollutants that combine with outdoor air pollutants whenever a door or window is opened and are trapped inside when ever said door or window is closed. The air filter on your HVAC alone does not remove all indoor air pollutants. Everyone should have a standalone or integrated air purification system to work in concert with the HVAC system to keep your indoor air clean and healthy. Breathe easier with an air purification system.