Officially, I think a place called Al Azizyah in Libya, where the hottest temperature recorded was 57.8°C, followed by Death Valley (appropriately named) in California where temperatures reached 56.7°C in 1913. According to other reports NASA recorded an incredible 71°C in the Lut desert of Iran. You also need to consider humidity. A sauna can be as hot as 200°C, but the air is only about 10% humidity. This means that it actually feels comfortable for short periods of time. But if the air was at 90% humidity, then the safe temperature is more like 110°C for short periods.