OSHA is in full swing with warnings and precautions to follow on avoiding heat stress and making sure you can continue to work.  OSHA has published fact sheet/poster type files that you can print out and hang up as a reminder to all your employees.  There is :

One for general safety and protection (Click Here)

One for working in landscaping, and (Click Here)

One for construction safety. (Click Here)

Take the initiative, hang these up, it just might save you some trouble, or even save a life.