Where these numbers come from
In 2005 a team at Stanford parked cars on clear days with highs between 72 and 96°F and logged the inside temperature every five minutes for an hour. Three findings came out of it, and all three are counter-intuitive.
- The inside rose about 40°F above the outside air within the hour, and it did that fairly consistently whatever the starting temperature. A 72°F day gets you to about 112°F.
- 80% of the rise happened in the first thirty minutes. The damage is done early, which is why "just nipping in" is the shape of most of these cases.
- Cracking the windows did essentially nothing. 3.4°F per five minutes with them shut, 3.1°F with them open, and the same final temperature.
They also tested running the air conditioning hard before parking. It delayed the spike by about five minutes.
The mild-day trap
Because the rise is roughly the same whatever the outside temperature, a 70°F day is not the safe version of a 90°F day — it is the same event starting twenty degrees lower. Seventy outside is over a hundred inside within half an hour, and a hundred degrees in a sealed box is not survivable for a dog that cannot sweat and is breathing air hotter than it is.
Cloud cover changes this, and so does deep shade, and neither is reliable — shade moves, and it moves faster than you expect.
Which dogs this kills first
The Royal Veterinary College found flat-faced dogs at 3.1 times the odds of vehicular heatstroke compared with dogs of ordinary muzzle length. For bulldogs specifically, against a labrador, the figure was 16.6 times. If you own one, the arithmetic on this page arrives sooner than it does for everyone else.
If you find a dog in a car
- Note the registration, the make and color, and how long you have been watching.
- If there are shops nearby, ask them to page the owner over the tannoy. It is often faster than anything else.
- Call the police. In the US this is a 911 call if the dog is in distress, and dispatchers treat it as one.
- Do not break a window unless the dog is in real trouble and you have been told to. Laws protecting people who do this vary state by state, and several require you to have called the authorities first.
- If you do get the dog out, cool it before you drive it anywhere. That order matters more than people realize.