i asked my doctor about this and she said the only real risk is overheating and that she thought it extremely unlikely to happen where we live over winter (armidale). i was also told spas where a little more dangerous but only if you have the heating element turned up to high and start with the water really hot in the first place.

i couldn't live without my bath. we have such a small tub and are in such a cold climate, that i would argue its impossible to overheat at all in our bath - every ten minutes i have to empty half the water out and put in hot water.