HEPA Filter?

Hepa filters are totally pointless in any case except if you live in a really old dusty and musty house or during winter months when house is closed up for months.

If you go into outside air at all it defeats the purpose any. HEPA is also way over rated for civilian use, again because it only makes a difference in a couple situations for personal uses.

In a vehicle its a complete waste. Won't do any good because your lungs and clothing already contain what your trying to filter Everytime you step outside.

The only thing maybe hepa would help with is it you live and drive is some type of heavily particulated air. Dusty, maybe some help in dense traffic (not sure how much it will filter out of air pollution from vehicle exhaust and such).

All that said, really no reason you cannot however DO NOT run the blower at the highest settings. The extra restriction (which is proven to wear out home HVAC blowers at an accelerated rate) will have the same effect on your blower motor.

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redleader74 said:

Thanks for the input folks. Yes, the obviousness of multiple large openings in a car seem a self defeating effort. Now, any thoughts on it's use or effectiveness on long road trips where you're in the car for hours on end, all windows closed?

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You will definitely get cleaner cabin air over a long trip.. but as soon as you stop for a break or fuel or a meal.. all bets are off and you start over again. This of course assumes your cabin is largely dust free to begin with... which it probably is not.. not by HEPA standards anyway.
That said... the HU and the instrument plastic dash screen are particle magnets and often are in enough sunlight to clearly see if you have particles roaming around in your cabin.. so you do have an empirical method to see if long trips and a HEPA filter are doing any good.

The nice thing is you don't have to make one yourself like you were thinking ... since that linked Bosh filter looks to be exactly like a high filtration home Air Conditioning filter.

On a related note... I use medium grade Filtrate filters in my home AC (to keep head pressure down) and I have several HEPA air units in key rooms in our home to further clean air (since I do not keep my home windows closed 100% of the time). These room scale units have automatic particle detectors in them to tell the air unit when it needs to up the air volume to scrub the room of particles. Unless there are people occupying the room... these air units rarely detect particle counts high enough to trigger a air scrub cycle. If you start moving fabrics around or doing other activities of a notable nature in the room though.. they will run a air scrub cycle sometimes... probably detecting particles disturbed and free flowing in the room air.

redleader74 said:

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.

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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.