Whilst most modern portable spas cannot be heated past 40°C (104°F) using their own heating systems, in theory, the water could be heated past this point by introducing warmer water to the pool or through external heating methods. You should never do this, though. Bathing in water any warmer than 40°C (104°F) can bring on several serious health issues, including:  Burns Nausea Dehydration Heart attacks/swelling Exhaustion  Heat cramps