Thirty-year average (1980–2009) of monthly average wet bulb globe temperature (WBGT). In the afternoons indoors or in full shade for the hottest month (e.g. August for USA and Egypt, April for India, January for Australia) in each part of the world (0.5 × 0.5° grid cells). The yellow regions (WBGT 25–27) indicate where workers performing heavy labour are affected by hot climate conditions; the orange (WBGT 28–30) and red (WBGT 31 +) regions indicate where workers performing moderate or light work are affected (Hyatt et al. unpublished).