I've seen this clip on Youtube here, were the box crew of Ferrari are shown all wearing protective face masks. The clip is from 1998 and I don't know if this is still the case.
I'm wondering why they do that. Surely it's not to stay anonymous, is it?
They're not facemasks but fire resistant clothing, in particular balaclavas. Refuelling an F1 car has always been a slightly risky business - moving a highly flammable liquid around at high speed in the vicinity of very hot things isn't something that would be recommended in a health and safety manual - so the pit crews, just like the drivers, wear protective clothing; their suits will be fire resistant as well. For probably the most famous recent example of what happens when it all goes wrong, see Jos Verstappen in 1994.
Even in modern F1 where mid-race refueling has been banned, pit crews are required by the FIA to wear safety equipment from head to toe during races. From helmets, safety goggles, fireproof suits and gloves, pit crews are just as geared up as the drivers during races because of safety reasons.
Unlike the drivers though, the pit crew are often only seen fully geared up during the race itself, as you'll see most of them not donning firesuits during practice sessions and qualifying. This is mostly due to the fact that cars are rarely serviced right on the pit box in these sessions, more often going in and out of the pit garages.
