A medical expert explains that the human body can begin to acclimate to extreme heat in as little as seven to 14 days. Some Valley residents spending time outdoors in the heat say they’ve learned how to handle the high temperatures. However, Bhuyan says those changes only provide limited protection — especially during extreme heat. While residents braving the July sun say they are used to 100-plus-degree days, health professionals emphasize that no one is immune to extreme heat. And whether someone has lived in Arizona their entire life or just moved here, Bhuyan says caution is still important during extreme heat.