This paper presents heat stress Standard ISO 7243, which is based upon the wet bulb globe temperature index (WBGT), and considers its suitability for use worldwide. The origins of the WBGT index are considered and how it is used in ISO 7243 and across the world as a simple index for monitoring and assessing hot environments. The standard (and index) has validity, reliability and usability. It is limited in application by consideration of estimating metabolic heat and the effects of clothing. Use of the standard also requires interpretation in terms of how it is used. Management systems, involving risk assessments, that take account of context and culture, are required to ensure successful use of the standard and global applicability. For use outdoors, a WBGT equation that includes solar absorptivity is recommended. A 'clothed WBGT' is proposed to account for the effects of clothing. It is concluded that as a simple assessment method, ISO 7243 has face validity and within limits is applicable worldwide.