Isn't the Hepa filter able to keep smoke out of the car? We were approaching a forest fires so we turnEd on our bio defense, but we could still smell smoke.
In theory it should. In practice, lots of details matter - crosswind being foremost among them.
Bioweapon Defense Mode works by building a small positive pressure inside the cabin, so that in a static environment any leaks will be from the inside to the outside of the car and then by drawing all the inside air through the filter stack.
However, if there is a wind against the seal area, that also creates a higher pressure region, so enough wind against a leaky seal will cause unfiltered air to push into the cabin from the upwind side.
How much is enough depends on a bunch of factors - how tight the car is in general (and thus how high the internal pressure is,) how tight the specific seal is (how much pressure differential can it withstand,) how large are the foreign particles (how much does the seal have to deform,) and probably some others that I'm not thinking of right now.
Bioweapon is a great addition for casual use, but the cars aren't really adequately sealed for dangerous environments, and you shouldn't assume Bioweapon mode will prevent anything dangerous from entering the cabin (it will greatly decrease the amount entering regardless, but it can't always keep everything out.)
