They experience extreme weakness, fatigue, nausea or headache. In extreme cases they may vomit or pass out. Treatment, move to a cool place and administer lots of liquids. With so much blood going to the periphery of the body, less is available for muscles. Strength drops and fatigue kicks in sooner than otherwise.
Accidents are more likely to happen.
For example, accident rates for heavy machine operators double when they work in hot environments. Heat stress can aggravate the effect of other toxins. Dehydration and loss of minerals through sweat decreases the body's ability to detoxify chemicals. Because the circulatory system is under strain other hazards increase. Carbon monoxide, which reduces oxygen supply to the tissues, is of particular concern.