And this might not quite be filter related, but I find that there are certain smells, such as skunk and cattle feed lots (which I encounter on every drive up and down I-5 in CA), that no matter what you do, still make their way into the car, whether you hit recirculate a mile out (which you can anticipate if it's something like the feed lot), or just turn the fan off entirely. Makes me wonder how Tesla's "biohazard" mode in their cars would perform in these situations.
Our noses are extremely sensitive small particle detectors.
Odors are among the smallest in terms of particle size, effectively gas particles, so air scrubbers typically attack them in a different manner. A HEPA filter cannot typically do this. So Air scrubbers will use a range of approaches to scrub them from the air. Some use complex electrostatic scrubbing systems, but these are quite expensive and are typically used only in true clean room environments. For home air scrubbers, the most common approach is an activated carbon pre-filter in front of a HEPA particle filter. Activated carbon filters are pretty good at scrubbing out odors.. though they saturate and need to be replaced about 4-6 times as often as the HEPA particle filter. The room scrubbers I have in the home actually are very good at odor scrubbing.. and they use an activated carbon pre-filter to do it. There is a screen filter, followed by an activated carbon filter, followed by the HEPA filter. All three are easy to clean or replace and while the HEPA filter will last typically a year.. the carbon pre-filters need to be swapped out every three months. The screen filter simply catches stuff that would clog the other filters prematurely and is easily cleaned with a hand vacuum every few weeks.
For air circulation in a vehicle.. an activated carbon pre-filter might deal with cow and skunk smells to some degree.. but I doubt it would scrub it out completely. As you noted... even with the air circulation closed from outside air.. it is still possible to smell the funky odor as you pass the cow farms.... which indicates that there is either secondary circulation pathways in the vehicle or the particles are so small and so high volume they simply find a way through the air seals on the windows and doors as forced air moves past them (which I what I think happens, and I don't know how you mitigate that).
Regarding Tesla's approach it's basically just a HEPA filter, maybe a bit smaller particle scrub size then typical HEPA filers (which is 0.3um). It's hard to get past the usual Tesla gimmick marketing speal.. but I found this article a while back and found it to put things in an objective light.. from an actual Bioweapons expert.
https://gizmodo.com/bioweapon-experts-arent-buying-the-tesla-model-xs-biowe-1733909293 Basically.... it's a good HEPA based system, but it's not a biohazard system for anything other then bacteria. Virus and gas particles will slip right through... which is what I would expect would be the case. Since the Tesla system allegedly positively pressurizes the cabin (an old clean room trick to stop invasion of particles) it may have some benefit in that regard, but honestly as is true with all things from Musk's mouth.. it's hard to tell fact from fluff. Since there is no air compressor in the Tesla.. the air to positively pressurize the cabin has to come from outside.
