Dealing with the heat

Britain finally seems to have found its summer and a heatwave is predicted over the next few days. This doesn’t bode well for any of us who can’t run early morning or in the evenings when it’s cooler.

There’s no doubt heat, or more importantly humidity can affect your running. While a hot day can be unpleasant, it’s the latter that’s the greater issue because it makes it harder to keep cool. High humidity means there’s high levels of water vapour already in the air and this means the sweat / water on your skin has nowhere to go – it can’t evaporate. So it just sits there and stops you from sweating further which is a key mechanism used by the body for cooling. The heart already works harder, as evidenced by a higher heart-rate, to get more blood flowing to the skin to take away the internal heat which occurs through sweating.

If the body temperature rises too much it can be dangerous. The body usually functions at a temperature of 37-38C but add a couple of degrees to that and it begins to impair muscle function. Get to over 40-41C and you’re in danger of heat exhaustion. Heat exhaustion is problematic and if left untreated it can turn into heaststroke which can be deadly.


I think I suffered heat exhaustion once. I was running at the beach, in the middle of summer, and it was a hot sunny day. The day before I’d done the same route – a ten mile run, five miles out, five miles back and even though it had been hot, I had no issues.

On the second day, I felt fine on the outwards stretch and I seem to recall I had a slight wind in my face which helped me feel cool. Then I turned around and it was hot. Any breeze was now on my back, so I got no benefit and I was running with the sun shining on my front. Where I’d run at 8min/mile on the way out, my 6th mile slowed to 8:30 and I was beginning to feel bad. The 7th slowed further to 9:10 and I took a couple of minutes break, standing in the shade shivering behind a beach hut. My heart-rate wouldn’t drop below 115 even though I was just standing there. I resumed at an easy jog, as I still had three miles to cover to get back to the car, but when the 8th mile came in at a shuffling 9:45 I decided it would be best to walk the rest. I had no further effects that afternoon and the following day I felt fine and was back out running okay.

I’m not quite sure what happened. I’m sure I was a little fatigued from doing the ten mile run the day before although longer sessions were quite common at the time. I may not have eaten or drunk enough before setting out. I find in the hot weather, I am constantly drinking fluids and still underhydrated. One other thing may have been an issue, at one stage of the run I sped up to get past the land train that runs along the prom. This pushed my heart-rate up into the 150s and may have set off a chain of events that I would otherwise have avoided had I just been jogging casually. Whatever caused it, I believe I took the right action by cancelling the session and walking back. Fortunately I’ve never encountered it since.


I’ve always been a big believer in drinking to thirst. Most of the day I have a cup of tea to hand but I don’t force it. When I was more endurance trained and sweated less, I found cups of tea would be half finished. At the moment, with the heat I’m drinking lots of tea and I’m still seeing that I’m underhydrated when I go to the toilet. The colour of your urine is a good indicator – when it’s clear you’re hydrated, when it’s golden or even darker you need to drink more. Even though I don’t noticeably sweat during the day, I’m still losing fluid from the body – water particles will be exhaled in your breathing.

The advice used to be to drink lots before a marathon and to keep drinking throughout. But this became a risk with runners drinking over a litre per hour for 3-4 hours. It could lead to a condition called hyponatremia which can almost ‘drown’ the body’s cells and wash the important minerals out of them. The advice now is to drink to thirst.


Of course if you can do your runs in the early morning or late evening, they’re much more enjoyable. Last Sunday, I did my long run at 5:07am and was home well before 7am. It was cool from start to finish and I still had the rest of the day to enjoy the sunshine.

On other days, I’ve been running in the heat of the day but I don’t mind it. Most of the runs have been for recovery purposes which are deliberately kept slow enough that they don’t cause me to break sweat too much. There is a little but it doesn’t get too uncomfortable. I’ve actually found it’s worse when I arrive home and stop moving. The lack of air circulation causes any sweat to start building up.

The couple of workouts I’ve done in the heat I don’t mind. It’s probably because the intervals aren’t that long and you get a rest afterwards. I believe the real danger of running in the heat comes when you try to hold onto a decent pace for a long time.


If we’re lucky enough to have good weather for the rest of the summer months then it’s worth understanding that it takes around two weeks for the body to get used to the heat. There’s always going to be some degradation in performance because of it until that occurs.

Best advice I can give you is to get out and get used to it. Keep yourself hydrated by drinking to thirst and don’t try to force yourself through anything that feels untoward or out of the ordinary.