I fasted for 18 days and went to sauna in most days. I broke a little sweat to expel toxins but limited my time to around 10-15 minutes per time to prevent losing too much electolytes.
Dispite that my blood potassium levels dropped to dangerously low levels during the fast. I could feel it through abnormally high pulse (100-120bpm) and a few moments of breathlessness when rising up from the chair. Even 6 days after quitting the fast my blood potassium level was 3.2 which is below safe limit so be warned!
It is safest not to go to sauna during Water Fast and let body do the toxin expelling via urine.
Also be aware that abnormal exhaustion combined with high pulse and lightheadedness / temporary vision loss when getting up from chair can be a sign of a dangerously low potassium level which may lead to accident and even death.