No one thinks about this certain source of indoor air pollutants because it is the exact opposite of the indoors.
If the outdoor air around your home or building isn’t healthy, your indoor air will be affected. If your HVAC outdoor intake is pulling in air near a cooling tower, generator, plumbing vents, garbage etc.…that air is going to be filled with contaminants which is now freely flowing through your house. Even a loading dock upwind can contaminate the air your outdoor intake pulls from.
ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) has a standard to use when putting in HVAC systems. The standard recommends 25 feet between the intake and a cooling tower and a minimum of 15 feet to the nearest dumpster.
So, make sure the outdoor air is clean to help you control the indoor air.