(Posted on lesswrong, reposting here. Hope I'm doing this right.)

There's a heatwave coming (or already arrived) in the UK and western Europe. Many of these places are not equipped for dealing with high temperatures and have large at risk populations* - not simply those with preexisting health conditions, but those living in accomodation grossly unsuited for high temperatures and anyone inexperienced with high temperatures who isn't properly aware of the dangers and precautions they need to take.

*(https://www.nhs.uk/live-well/seasonal-health/heatwave-how-to-cope-in-hot-weather/ for more info on who is at risk, and general advice)

Heatwaves are not just 'nice weather', not in the anthropocene, they are life threatening. People will die. I hope by writing this to nudge EAs and their local communities toward safety.

TL;DR: Sleep and hydration* are the two pillars of survival in hot weather. If you are not already, put maximum effort into getting a good night's sleep (advice below).

*(don't neglect electrolytes; consider this an excuse to move Gatorade to the 'healthy (in moderation)' column for a few days)

Sleep

Sleep is one of the most important factors in health. Most people do not get enough sleep and do not practice good sleep hygiene - are already somewhat sleep deprived or at high risk of sleep deprivation.

Too much heat and humidity are massively deletrious to sleep quality. Further, sleep deprivation puts you at greater risk of heat related illness.

What can you do?
 Sleep hygiene basics:

Establish and maintain a routine - stop eating several hours before bedtime, turn off the lights at least 2 hours before bedtime (and use programs like f.lux or redshift on your screens to reduce blue light), go to bed at the same time each night*, get up at the same time each morning, eat breakfast at the same time each morning.

*Many people find using alarms/reminders to establish a set bedtime more effective than using them to get up in the morning.

Create a better sleep enviroment - as dark as possible (Ikea sell 'adhesive' blackout blinds, I personally use an eye mask), and as quiet/peaceful as possible (ear plugs / white noise)

Keep your sleeping area clean, tidy and reserved for sleeping, and sex... and reading (but not on your laptop/phone).

Keep pen and paper near your bed; if your mind is overactive - can't stop thinking about something, or think of something you need to remember, write it down.

If your bedroom is extremely unsuitable for hot weather you might seek somewhere else to sleep for a few nights.

Hydration

Electrolytes - not just water.
Grab a sports drink (gatorade, pokari sweat etc etc) or else experiment with DIY electrolyte drinks (e.g. table salt, NoSalt, lemon/lime juice) but most have a reputation for tasting awful.

Keeping cool

Stuff

Air Conditioning (previous discussion)
Yeah, duh. Obviously. But with some caveats (see Airconditioningitis section below)

Swamp coolers (wiki) if low humidity (effectiveness/humidity)

Ceiling fans:
Underrated, out of fashion, highly efficient (https://www.lowtechmagazine.com/2014/09/circulating-fans-air-conditioning.html)

Dehumidifiers: if humidity over 50%~ then potentially valuable; won't directly cool the space, but will improve your body's ability to cool via sweating. Some air conditioners have this functionality built-in.

Do/Don't

Keep the blinds/curtains closed during the hottest part of the day (late morning, afternoon)

Try to avoid going outside during this time.

Open or close windows/doors - the better the insulation in your home, the more valuable it becomes to retain cold air and keep windows closed. YMMV, experiment.

When going outside: wear a hat, use sunscreen, consider wearing long sleeves and trousers.

Sauna - not just avoiding the heat

(Epistemic status: shaky, can't point to any conclusively good research)

The body can adapt to hotter climates, but it is a slow process that takes weeks/months. However you can kickstart/accelerate this process through the use of saunas (which are good for your health anyway) - 5-15m inside, take breaks to cool down and stay hydrated, 1hr max overall time in sauna per day.

Airconditioningitis

(Epistemic status: shaky, again. There is very little written about this in the anglosphere, but in Korea it's common knowledge.)
 

Air conditioning is a double edged sword. It can be absolutely vital to survival in hot climates, but it can also mess you up in a couple of ways:
1.) dry air leading to sore throats, runny noses etc
2.) unclean air conditioners (viruses, allergens, fuck knows)
3.) repeated transitions between very hot and very cool environments seems to mess with your body's attempts to self-regulate.

Closing remarks

I expect most of this is common knowledge to most of you, but perhaps not to your friends/family - please share this advice with those you know who might need it, please reach out to the people in your life who might need help during this time.

Please contribute any advice you have that I've missed.