I believe it's because pumping filtered air from the outside faster than it can escape from the cabin creates the positive internal pressure.

Modern cars have a cabin air outlet. It's usually hidden in the trunk or the back of the cab (on trucks), and vents somewhere like behind the plastic bumper. There are rubber flaps on them to ensure one-way air travel and keep water out. Perhaps Tesla's has a motor on theirs to keep it closed against the increased cabin pressure.

From a 2015 Honda Accord, it's #35: http://imgur.com/QsJexwI

Side-effect of this part - manufacturers tune them so that the doors produce a solid sound when closed.

Just by pumping air inside through filters a positive internal pressure is automatically created. How much pressure will be created is proportional to the pump's flow rate and inversely proportional to the car leakage constant ( I think it's something like leakage ~ k * P.I.P., which would make P.I.P. ~ pump flow/k ), but some positive internal pressure is a given even with those outlets.

How is that a bioweapon defense then? Do HEPA filters work against bioweapons?

Probably depends on the bioweapon. For instance, I'd expect it to be effective against anthrax spores, which was the most recent bioweapons scare we had.

You're still using outside air, but by maintaining positive pressure you ensure that it's only coming in through the filter, rather than a mix of air intake and leaky joints.

This will be used 99% for skunks.

Not sure about particular type of filter, but CBRN countermeasures found on tanks, APCs and such are also basically air filtration with positive internal pressure.

I think that is tongue in cheek.

It absolutely is, but that doesn't mean it wouldn't work in principle.

A HEPA filter is used in a lot of labs where biological substances are handled, even potentially hazardous ones. It should filter many types of biological agent out of the incoming air, and the positive pressure will keep such things entering other ways.

I think the reason they put it in is so people would talk about it and, look, we are. It is a marketing gimmick that likely doesn't require any additional hardware in the car (the air pump and HEPA filter are already in it, this just turns the pump up and adds a new dash button).

I'll give Tesla the nod for one thing: They sure know how to market. Between the wing doors, the bio-protection mode (which will be on the nightly news), and insane mode they're very savvy.

Pretty sure this is the real reason Musk wanted a superior air filter - playa dust storm protection: https://farm2.staticflickr.com/1057/1338020035_678b988a6e_o....

Fans, which force air through the filters into the cabin.

It could heat the air on the inside, so that it expands :-)

This kills the passenger.

They are probably storing compressed air in the cabin. You start the bioweapon-defense-mode, the fan raises the pressure in the cabin ( that uses outside air) and then shuts off. (Or they store a little bit of air somewhere, perhaps for the breaks, and use that.)