I found interesting video clip in YouTube. A guy with air pollution detector(small white cube thing), tested Model S's air filter. Even without HEPA filter, MS is doing its job - it's just slower than HEPA filter. He also tested on Hyundai Sonata, he said he just changed air filter two months ago. And sadly, it did not work very well. Unfortunately, YouTube clip is not English, but you can understand context by reading center screen and numbers on its detector.