Heat waves and extensive exposure to the sun can be dangerous. People are not necessarily aware of the dangers of heat because the symptoms can "sneak up on you". Heat stroke is a life threatening condition that can cause death or extensive damage to your internal organs.
I was personally told a story of a bicycler who was riding in an event. He was getting hot and wanted to stop and told his friends to go on. When he didn't show up, his friends went back from him and found him collapsed. He now has permanent organ damage. All this simply because he did not drink enough fluids and ignored the symptoms of heat stroke.
To avoid head related injuries the Red Cross has put together a Heat Wave Safety Checklist. Going over this information at a toolbox talk would certainly help workers to stay safe in the hot weather.