what difference can heat actually make to the performance of elite athletes? It

can make a really significant difference. So there was a study that was conducted around the 2019 World Athletics Championships, which were held in Doha, which is very hot and very humid. And they were measuring how the athletes performed in endurance events compared with their own personal best. And they found that almost all of the athletes that they studied overwhelmingly did worse than their personal bests. And it was between three to 20 percent worse. And the degree to which they underperformed was, they thought, correlated with how hot and how humid it was outside. And, you know, getting up to 20% reduction in performance is really, really significant. And I also think that it should be noted that this was going on despite the fact that the organisers of that event had already taken some pretty severe measures to try and mitigate against the heat and humidity that you would have holding an event like this somewhere like Doha. So the women's marathon, the start time was 11.59pm. So, you know, they're running in the middle of the night. All the events that could possibly be in a gigantic air-conditioned stadium were. And so even with that, you're seeing a really, really significant level of performance reduction. So

Rachel, we've all had the experience of being outside in the heat and finding it hard to exert ourselves. But tell me what actually happens in the body when it's very hot.

So if you want the really geeky answer, this is classic thermodynamics thing, which is that your body is converting energy sources like you get from food into kinetic energy. All of those processes create waste heat. And so when it is very hot outside, not only is your body trying to regulate its temperature against the ambient temperature, it is also having to then fight against the fact that it is generating more heat internally. There's also a lot of things that your body will do to try and cool you down, which make activities feel more strenuous. For example, your body will be sending blood to your skin because that's how heat moves towards the surface. And that means that you are not getting as much blood sent to your muscles, which you need. You are going to be sweating more, which means you're going to be getting dehydrated more quickly, all of which is going to have an impact on performance. In particular, one of the really, really dangerous scenarios is the thing called exertional heat stroke, which is basically when your body is generating too much heat that it cannot dissipate. And that can start with symptoms like confusion and evolve into very, very serious symptoms like organ failure and ultimately death. It is thought to be one of the third leading causes of sudden death amongst young American athletes. For hospitalised cases, the fatality rate is around 15%, but if untreated, it's thought to be up to 80%.