It doesn't need to feel like a scorching day for a car to become dangerous for a dog inside it, and "I'll only be five minutes" is the sentence attached to nearly every case the RSPCA deals with. Here's what actually happens inside a parked car, and what to do if you see a dog in one.
How fast a car actually heats up
The RSPCA's figures are stark: on a day that's 22 degrees Celsius outside, a car can reach an unbearable 47 degrees inside within an hour. That's not a heatwave figure, that's a fairly ordinary UK spring or summer day. Cracking a window or parking in what looks like shade doesn't meaningfully change this. A car heats up like a greenhouse regardless of how warm it feels outside once the sun is on it.
Recognising heatstroke
The RSPCA lists the signs to watch for: heavy or noisy panting, drooling that's thicker than normal, red gums and tongue, weakness or trouble standing, vomiting or diarrhoea, and in severe cases collapse or seizures. Brachycephalic breeds, older dogs, and dogs with existing health conditions are at higher risk and can deteriorate faster than a fit, younger dog.
If you think a dog has heatstroke
The RSPCA's own guidance is "cool first, transport second." Get the dog into shade immediately, then cool them gradually with water poured over the neck, stomach and thighs (not the head), using a fan or airflow to help, before transporting them to a vet in a cooled, ventilated car, calling ahead so the practice knows you're coming.
If you see a dog alone in a hot car
If the dog is showing signs of heatstroke, the RSPCA's advice is to call 999. If the dog seems alright for now, stay nearby, note the car's registration and any pay and display ticket, ask staff at nearby shops to make an announcement, and call the RSPCA's cruelty line on 0300 1234 999 for guidance. Breaking a window is legally risky, it can count as criminal damage, and the RSPCA specifically advises telling the police what you intend to do, photographing the situation and getting a witness first, rather than acting alone and hoping it's justified after the fact.
The simplest fix
None of this needs to happen in the first place. If you can't take your dog in with you, don't take them in the car at all, even for what feels like a two-minute stop. That's the whole rule, and it holds regardless of how mild the day feels when you park up.