The doctor should be called any time there are symptoms of heatstroke, dehydration, blurred vision (possible sun damage to the eyes), chills, fever, vomiting, or blistering associated with sun exposure.
Understanding Hot Weather and its Dangers provides additional information on identifying and preventing heat cramps, heat syncope or fainting, heat exhaustion and heatstroke.
Don't run the risk of heatstroke by strenuously running or playing sports during these hours.
Many soldiers suffered heatstroke from wearing their wool uniforms on hot days.
Have sunscreen and shade available to avoid burns or heatstroke.