Heatwave

British houses are built to hold heat in, which is exactly wrong for this scenario and getting more wrong every decade. The card has the method. This has the medicine.

The night is what kills

The afternoon is uncomfortable. The night is dangerous.

The body recovers from heat during the cool hours. When the overnight temperature does not fall far enough, that recovery does not happen, and each day of a heatwave starts from a worse baseline than the last. That is why deaths cluster in the later days of a heat event rather than on the hottest afternoon.

Which changes what you optimise for:

• Open everything the moment the outside air is cooler than the inside, usually late evening, and get as much air through as you can overnight. Cross-ventilate: two openings on opposite sides beat one wide open window.
• Close it all again before the sun is on the house, and keep it shut all day. This feels wrong and it is right.
• Sleep low and sleep alone, if it helps. Ground floor, spread out rather than sharing a bed, minimal bedding.
• A damp sheet and a fan moving air across it is the cheapest effective night cooling there is.
• Watch the third and fourth nights hardest. Fatigue accumulates and so does risk.

Know the alert levels. Heat-health alerts run yellow, amber and red, and an amber alert is the point at which the checking-on-people part of this card stops being optional.

Exhaustion or stroke, and the line between

The card's instruction to check on neighbours twice daily is the right one. This is what you are checking for.

Check the people who will not ask. Older people often do not feel thirsty or hot in a way that prompts action, and somebody living alone can deteriorate over two days with nobody noticing. Twice daily, in person, is the card's figure and it is right.

The medicines that make it worse

This is the part that is genuinely not obvious, and it applies to a lot of British households.

Several common medicines impair the body's ability to handle heat, either by affecting sweating, fluid balance or the circulatory response.

• Diuretics, which increase fluid loss.
• Anticholinergics, which reduce sweating.
• Antipsychotics, which affect temperature regulation directly.
• Beta blockers, which blunt the circulatory response.
• Some antidepressants.

What to do with that: anybody in the household on regular medication is in a higher-risk group during a heatwave, and should be checked more actively, not less. Do not stop or alter anybody's medication on the strength of this. It is a reason to watch closely and to keep fluids up, and any actual change is a conversation with a pharmacist or GP.

Store the medicines properly too. Many are specified below 25 degrees C, and a sunny windowsill or a car glovebox in August exceeds that comfortably. See storing medicines.

What actually cools a British house

Ranked by how much difference it makes, cheapest first.

1. Shutters, blinds and curtains closed on the sunny side, all day. Blocking sunlight before it enters the glass is the single largest effect available. External shading beats internal.
2. Windows shut during the day, open at night. The instinct is the opposite and the instinct is wrong.
3. Evaporation. Wet cloth on the skin, wet towels hung in a draught, a damp sheet over a chair. Water leaving a surface takes heat with it.
4. A fan moving air across skin, which helps a person even though it does nothing to the room. Its benefit falls away in very high air temperatures, and it is not a substitute for cooling in somebody who is unwell.
5. Go low and go north. Ground floor, north-facing, and stone or tile floors if you have them.
6. Turn things off. Ovens, tumble dryers, computers and old lighting all put heat into the room. Cook outside or eat cold.
7. Salt and water together, as the card says. Water alone, in quantity, while sweating heavily, is not sufficient.

And two absolutes. Never leave a child or an animal in a parked car, not for a minute, because interiors heat within minutes. And never let anybody swim to cool off in a river, reservoir or quarry, which is a leading cause of British summer deaths and which every heatwave produces.