Is this happening with just one water bong or several different ones? It sounds kind of like an allergic reaction. A well used, but under-cleaned, vaporizer (the kind that fills the air with water vapor, not the toking kind!) can build up a nasty colony of bacteria and fungi. When this kind of vaporizer is used and it's dirty, it can cause allergic reactions. Just changing the water in a bong would not remove the the colonies- they stick to any surface and repopulate quickly. Some kinds of fungi can cause some pretty extreme reactions- ergot and 'shrooms are good examples. But all in all, you likely have the best idea- don't use water bongs. As for the other type of vaporizer- GET ONE! Your lungs will thank you. I have a little Vapor Brothers and it was worth every penny! I hardly ever smoke any more and the duff that's left over from vaping makes pretty good brownies!