That passing out bit says to me that they are not drinking
enough water. I work with/volunteer with our local blood
bank, and hear often of people fainting from sheer
dehydration!! When badly dehydrated you can't even give
blood, you can't work hard in the heat, you should not even
walk much till you drink water.
When working in the heat you absolutely MUST drink lots of water. Not soda, not milk, not those "power" drinks, but plain old water.
Some of the males in my family asks us to save them empty
milk jugs, which they fill with water and freeze, then take them
on work projects. Each one has his own marked jugs and drink from them often. And makes sure they drink a certain amount in each hour. None have fainted from the heat.