Heat extremes are increasing in frequency, duration and intensity due to climate change. Their impacts include a rise in death rates, a decrease in how much of a crop is produced and power outages. Here we show that there is a lack of reporting of impacts in international organisation reports, international disaster databases. Further, heat stress, an impact of heat extremes is characterised using an index that is human-centric. With a focus on August and the Northern Hemisphere, we show that heat stress has grown in extent and is presenting a growing risk to more of the population over this millennium. This work suggests that organisations and national governments should come together to agree a protocol for how to best report heat impacts, so that we can be better prepared for them.