I’m not an F1 driver but I race karts, and I know a thing or two about those kind of situations. With the bathroom situation, you sweat so much that during the race your body has nothing else to expel. However if the need arises, we are told to let it out, because any concentration that it takes to hold it in is concentration wasted towards driving. This can cause an error, and can result in a crash, which is slightly more expensive than a new suit. Sneezing, I have only sneezed once during a race and that was during the formation lap. I almost missed the fact that everyone slowed down to a crawl and almost wrecked it. Sneezing and coughing are rather dangerous while driving because again, you can cause an accident due to a loss of concentration. Usually though, we’re so focused on driving that sneezing barely ever occurs. And as was said above, our bodies are focused on taking in oxygen, rather than letting it out. I’m sure that F1 drivers do the same, since it’s probably more important to keep concentration when going 180 through a corner than it is doing 70.