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.

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Re: dehumidifiers

A standalone dehumidifier will heat the air more than sweating can cool it.  You can see that from conservation of energy and thermodynamics: evaporating water (eg. when you sweat) absorbs heat , and condensing water (eg. in a dehumidifier) releases an equal amount.  You also need to pay a bit of extra energy to run the machine and to overcome entropy.

Using an air conditioner to dehumidify doesn't have that same problem, as it vents the heat outside.

There are some circumstances where a freestanding dehumidifier would help. Suppose you live in an airtight corrugated iron hut. In shade. With 100% humidity. Your air inside is at ambient temperature. You run the dehumidifier, and the air gets hotter and dryer. The heat can easily conduct out through the walls, but the dryness stays. So soon your room is full of near ambient temperature dry air. So sweating can cool you.

I'd personally rewrite this article as a "get an AC unit, now!", with links to the cheapest brands that are worth it (probably any with an inverter), then add a few tips on using it. For example the cause of most issues is simply setting the temperature too low. In summer it's ok to put it on 25C, then "lose" the remote until autumn. Maybe lower when/where you sleep. Also it seems obvious to me, but for completeness' sake: don't install it where it blows on you often, and ideally not in the room you sleep. If you need to sleep with the door closed, then spend more than 5 minutes figuring out exactly how you want the airflow in that room. Some AC units come with a Night or Quiet mode - it's worth it in the bedroom. 

Then put the rest of the article as emergency measures until you can get an AC unit installed. 

I agree, though you do also want to be prepared in case heat-induced electrical demand leads to power outages.

Some air conditioners have this functionality built-in.

All AC units dehumidify; it's fundamental to the way they cool. They chill a small amount of air well before where you want it, and mix it in with the rest of the room air. During that chilling step, it is so cold that it can hold substantially less moisture, which condenses out. ACs evaporate this outside, or, in older models, drip it outside.

Not sure why you mention sunscreen. It’s not like the sun shines brighter during a heatwave! And it’s not like sunscreen keeps you cooler. (OK, I guess technically the mineral sunscreens, the ones that make your skin look ghostly white, would increase your skin albedo / reduce sun absorption, exactly to the extent that they make your skin looks ghostly white. But better to just wear a white hat instead!)

My understanding is that UV exposure is determined mainly by solar angle from zenith (which in turn depends on days-from-summer-solstice and hours-from-solar-noon), whether the sky is clear vs cloudy, ground elevation, how much stratospheric ozone happens to be overhead today, and whether you’re in direct sunlight (and/or have lots of line-of-sight to the blue sky). I don’t think any of these things would be different from usual during a heat wave.

If you are in a building where the thermodynamics are bad, (cooler outside than inside) then you might be spending lots of time outside. A heatwave is correlated with lack of cloud cover.

For some non-AC options if you don't manage to get one in time: Those little spray fans that also mist you with water are remarkably effective. They usually are battery-powered, so still useful in a power outage.

Also, drinking cold smoothies is surprisingly cooling -- probably any consumption of ice-cold water/stuff will get you a similar effect. I drank many of them to get through heat waves in San Francisco.

A fan and some improvisation (and old socks, plastic box, wooden spoon etc) works as a swamp cooler.

For sleeping in a heatwave, using a ChiliPad is more valuable to me than any other intervention.

I bought a dog cooling pad from Costco and put it under my fitted sheet and have slept much more soundly since. Highly recommend for those of us who run extra-hot.

"Airconditioningitis" sounds about as epistemically sound as "fan death," which is to say, not at all. 

There are indeed odd cultural beliefs about the use of fans and air conditioning in Korea, but these are urban legends. 

Same argument works against "Airheateritis", which I can say from experience causes more bloody noses (dryness) and headaches for the first few days (burning material in unused ductwork). I mention this alternate case because it's something which people more commonly note, but I can testify to some AC issues as well.