Heat stroke is a life threatening medical emergency in which the body's cooling systems stop working and the core temperature can rise to dangerous levels. Heat stroke can be avoided by monitoring the environmental conditions and by following simple prevention measures such as reducing physical activities, taking more breaks and drinking more water. The Heat Stroke Checkers from KEM are portable, easy to use and lightweight instruments to monitor the environmental conditions. They measure the Wet Bulb Globe Temperature (WBGT) which is is commonly used as a guidance for environmental heat stress to prevent heat stroke during physical exercise or while at work. The WBGT index is specified in ISO 7243 (Hot environments -- Estimation of the heat stress on working man, based on the WBGT-index).