I was in the Marine Corps and was indoctrinated in the flag system described on this site.
http://www.zunis.org/how_hot_is_hot_...fe_is_safe.htm
It's all about being able to regulate your body temp. in the heat.
How Safe Is Safe?A Few Dry Bulb Temperatures For Your Consideration:
[TABLE]
[TR]
[TD]212°
[/TD]
[TD="width: 844"]Boiling point of water
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff8000"]200°-
175°
[/TD]
[TD="width: 838"]Temperature in a Finnish Sauna
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff0000"]136°
[/TD]
[TD="width: 838"]Highest temperature ever recorded at the Earth's surface (Libya)
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff8000"]135°
[/TD]
[TD="width: 838"]Highest US temperature (Death Valley)
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff0000"]130°
[/TD]
[TD="width: 540"]Temperature reported during 1-97 Australian Open Tennis Tournament
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff8000"]122°
[/TD]
[TD="width: 838"]Highest European temperature (Spain)
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff0000"]118°
[/TD]
[TD="width: 838"]Temperature reported during 7-95 Tennis Tournament, Washington D.C
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff8000"]108°
[/TD]
[TD="width: 838"]Human core temperature which cannot be sustained during exercise
[/TD]
[/TR]
[TR]
[TD="width: 61, bgcolor: #ff0000"]107°
[/TD]
[TD="width: 838"]Human core temperature maintained at rest for 1 hour during whole
body heating for treating certain cancers and AIDS
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff8000"]105°-
102°
[/TD]
[TD="width: 740"]Core temperature sustainable by top endurance athletes during exercise
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff0000"]104°-
101.5°
[/TD]
[TD="width: 740"]Core temperature at which most athletes collapse
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff8000"]104°
[/TD]
[TD="width: 740"]Temperature at which the Australian Open Tennis Tournament allows
postponement of matches
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff0000"]100°
[/TD]
[TD="width: 740"]Temperature at which US Marine Corps recruit physical training was
previously halted
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff8000"]90°
[/TD]
[TD="width: 740"]Temperature at which the Women's Tennis Association allows a
10-minute break between 2nd and 3rd sets
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff0000"]88°
[/TD]
[TD="width: 740"]1996 US Olympic Trials, Women's 10 Km Walk, 6/18 (33%) of athletes
required medical attention for heat-related problems
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff8000"]88.0°- 73.4°
[/TD]
[TD="width: 740"]1996 Olympics, Men's Marathon, 14/124 (11%) of runners required
medical attention for heat-related problems
[/TD]
[/TR]
[TR]
[TD="width: 7, bgcolor: #ff0000"]77.5-
70.3°
[/TD]
[TD="width: 740"]1996 Olympics, Women's Marathon, 21/86 (24%) of runners required
medical attention for heat-related problems
[/TD]
[/TR]
[/TABLE]