As someone from a hot country that's regularly on fire, here's some tips:

WATER IS YOUR FRIEND! WATER! IS! YOUR! FRIEND! You can transfer SO much heat into this bad boy! You cannot cool down without water!

Wrists under the cold tap. Splash your face and the back of your neck. Fan yourself.

In some countries you can buy a little handeld fan with a water sprayer.

Damp tea towel around the neck. Stick an ice pack in there on hotter days.

Half fill a water bottle with water, stick in freezer. If you use a bottle with a straw, make sure it's lying on its side with the straw side up and out of the water. When frozen top up the rest of the way with tap water and off you go.

Desperate to cool off? Wet T-shirt. Sit in front of a fan. This will nuke it, just don't get hypothermia and don't fall asleep like this.

Cold showers are also your friend in summer. Some people get psyched up by these. Personally, I sleep like a baby, so I'm good to have them before bed. Just keep in mind that it takes a bit of time for the cool to circulate, so your body will tell you that you're colder than you actually are. I find that when I have cold showers I need to step out of the spray when I think I'm cold... I'll just wait, and thirty seconds later the temperature has evened out and I actually need to step under again. Rinse and repeat until you maintain coolness even after stepping out for a bit.

If you can't do cold showers, turn the cold shower on anyway and just stick your arms under. When they're cold, lift your arms up above your head. The sensation of cool blood draining into your body is fucking weird and kinda unpleasant but less unpleasant than being hot.

Feet in a tub of water with ice. Blood naturally flows to your extremities when hot, so take advantage of this. If you don't have a tub of ice water, sticking a wet rag on your feet in front of the fan works too, it's the less powerful version of the wet T-shirt.

Drinks lots of water but make sure that water has electrolytes as well. Stay in the shade.

Keep air circulating. Fans don't actually cool rooms down, they just help transfer heat from your body to the moisture on your skin or the air via evaporative cooling.

Block north facing windows early in the morning so the sun doesn't get in. If you're in the northern hemisphere, this is opposite for you. Keep in mind that if your home is brick, the bricks will still heat up and slowly release heat into your home even after the sun goes down so this will only do so much.

If it's hotter inside than outside, close all your windows but two, making sure they're on opposite sides of the house/unit you're in. Point a fan out of one window, making sure that the doors between the rooms with the open windows are all open. This will help create a mini pressure system in your home, pulling cooler air in and pushing the hotter air out via the fan. Bonus points if you can get that fan high up where the hot air rises; even within a single room the top is much hotter than the air by the floor. Adjust the amount of open windows based on how many fans you have, but generally you want more windows with fans open than windows without fans to keep the pressure correct.

Obviously, use your common sense for these. Not everything WILL work for you, just use the stuff that does and adjust what needs to be adjusted. Some of these will be impossible to use in the workplace but others you can still use. Others are best used at home. If humidity impacts your ability to use any of these, get a dehumidifier if that's an option, or use more ice instead of evaporation.

Putting it on your forehead feels good but does little for cooling down your circulatory system.

Understand your home and if its architectural features still function well. There is not much you can do if you live in an apartment/flat but a house may have features that can help or hinder keeping cool... and features that don't work anymore. For example, maybe you do live in a 'breezy' bungalow but the roof has been renovated and there's no longer a light-blocking overhang, or it's missing the ceramic roof it once had. Maybe your home is built to retain heat in the winter and previously relied on a tall fence or hedge to shade a north wall for summer which has since been since taken out. Not all of these can be addressed by quick fixes but it can still be helpful to understand-- including when your safety means leaving for the day because personal effort to cool the residence might be futile.

Stay on the first floor if you live in a multi-story house. Again, this may not be helpful for those who live in an apartment/flat. But you want to be as close to the ground as you can and away from rising heat. If you have access to a basement that is sunk into the ground, you may want to relocate there in extreme conditions.

Close your windows during the day if possible. ESPECIALLY if you live in an urban or highly developed area. You do not want the hot air from outside to get sucked into your relatively cool, shaded house interior. The effect is much worse if there is a lot of paving and especially asphalt around your house, which many normally-colder areas use for its flexibility and resistance to cracking in wintertime. That stuff absorbs 80-90 percent of sunlight and radiates it as heat. WAIT UNTIL AFTER SUNSET to open your windows, try to prioritize cross-airflow and if you have a second floor, do it up there. If you live in a flat and can't do cross airflow, this is a good time to use any box fans you can find to blow air out of the window as stated by robotslenderman above.

If bugs are a problem when you open your windows and you have no window screens, there is no shame in going to a hardware store for cheap screen (that is rated for mosquitos) and taping it up temporarily. If you use the 'open two windows' tip that may work very well because you'll only have to screen two windows. Climate change also means a change in the range of insects.

Some people have double layers of curtains and the 'conventional' wisdom is to put a decorative/sheer curtain on the outside and the blackout curtain on the inside, but reverse it for now. You want the sun to hit the blackout curtain and not enter your house, and if the blackout curtain gets hot the second layer of curtains might be able to create an insulating layer of air to prevent it from heating the room up. If your non-blackout curtains are too sheer to do this, there is also no shame in using a spare sheet, blanket, tarp, or even trash bags.

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