The way to get air purifiers to really work well is to install them at the air intake, i.e. in the windows, or where the central air intake is, so all incoming air passes through them. I use indoor air purifiers too, but not as a substitute for ones at the intake. Note that tires and diesel fumes are prominent neighborhood sources of harmful particulates.

It is not expensive to run intake fans in the spring and fall seasons when active heating or cooling are not required.

That helps for pollution that comes from outside (traffic, pollen, wood smoke), but most of the microplastics are generated by moving/wearing synthetic textiles inside the home.

Positive pressure systems are great, love 'em, but there's a quantitative mismatch in this case. Above ~1 ACH your HVAC costs will go through the roof (even with heat/humidity recovery), but for effective filtration you need 6-8 ACH to catch the larger dust before your lungs do.