Kings of Dust and Bone
Perditions of Athas
The heat on Athas commonly exceeds 100 degrees Fahrenheit, but most creatures have learned to adapt to such extremes. No ill effects are suffered until the heat reaches 120 degrees or more. Thereafter, apply the exhaustion check rules on page 110 of the DMG. In the Tyr region, this temperature is common during the hour of noon, but uncommon for other hours of the day.
Food and Water
On Athas, deprivation is a more potent danger due to the intense environment and the rigors of the burned world.
A level of exhaustion is applied on a character’s first day without food. Thereafter, apply the normal period of starvation before additional levels of exhaustion are gained. (PHB page 185.)
Athasian days are always hot, meaning two gallons of water are always required. A third gallon is necessary if the character endured a forced march that day or otherwise toiled at some vigorous outdoor task.
Because the land is so barren, food and water derived from successful Wisdom (Survival) checks and the Wanderer background feature (even multiple instances of them within a party) only ever provides enough sustenance to count each character as having had half rations and one gallon of water daily. The remaining requirements must be made up with carried provisions.