That is not how humans work. People will continue as normal until severe symptoms emerge. That is why an 80-90% preventable cause of death, cornary artery disease, is the leading killer in the developed world. People do not eat the diet and obtain the exercise necessary to prevent it even though they know how.
In addition, most people do not have the wealth or luxury of not going to work to seek shelter from heat. They have to feed themselves and their families. They will stay in the heat until it is too late.