DIY fresh air intake
I use a charcoal filter ahead of the regular filter. My wife has a nose that can smell anything even a football field away. They work great, I buy mine at Home Depot or lowes for around $10.00. Look for the WEB Absorber, ask an attendant in the heating/ ac section. It is a cut to size and comes in  20x25 size. I only had to cut the plastic piece once and reuse with each change. Mine last 6 months.