This information is good enough to stand as it's own post. Having M.S. the heat for me carries it's own stress that has taken me out of action more than once. On the table below the far right column is my own guideline and at the bottom is how I need to manage a whole day of actives. Yes it sucks having limitations of what I can and can't do but it's better then ending up in a Ambulance and or a Hospital! Been there done that more than once and don't want to do that again. Each person can set their own limit.
Wet Bulb Global Temperature (heat) Index (Standard*)
Category Temperature work/rest work/rest
1 78-81.9 40/20 30/30
2 (green) 82-84.9 30/30 20/40
3 (yellow) 85-87.9 30/30 10/50
4 (red) 88-89.9 20/40 5/55
5 (black) 90-or greater 10/50 0/60
For each hour worked using this chart, I decrease work by 10 minutes and increase rest by 20 minutes. Anything over 4 hours increase rest by one hour for the rest of day. If temperatures drop down below index category 1 it's a judgement call based on how well I stuck to the day plan.
*Standard is set by a person who is physically fit and in good general health within the age range of 18 to 23 years old.