However, there's also positive pressure inside my BMW when the HVAC system is running. This is one of the reasons that the windows slide down ~1cm when you open the door and slide back up after you close it. It makes it easier to close the door. Even with that, it takes a considerable amount more force to close the door when the HVAC system is running.

The HVAC system in the BMW was probably not designed with the intent of stopping bioweapons, but what I'm alluding to is that I'd be curious to see the same air quality tests performed on other cars which include in-cabin air filters.

Maybe they all work that well?

I don't think the BMW has a HEPA filter though. However, it should be good enough. I found the following online (though not from the best of sources):

----- Cabin air filters are very efficient and have electrostatically charged fibers that can trap particles smaller than 1 micron in size. Most quality cabin air filters will stop 100% of all particles that are 3 microns or larger in size and 95-99% of particles in the 1-3 micron size range. ----

http://www.underhoodservice.com/tech-tip-cabin-air-filter-q-...