https://phys.org/news/2018-07-minutes-dog-die-hot-car.html

Despite this, people continue to leave their dogs in cars. Between 2009 and 2018, the RSPCA had 64,443 reported incidents of animal and heat exposure in England and Wales. Around 90% of calls related to dogs in vehicles. This year the RSPCA emergency hotline received 1,123 reports of animals suffering heat exposure in just one week (June 25 to July 1 2018). That's seven calls an hour.

Perhaps this happens because many owners don't really understand what happens to a dog's body in overheating and heatstroke. If a dog's internal temperature goes above 41°C (105.8°F) it is at risk of heatstroke, which only 50% of dogs survive. Some breeds are more susceptible than others – large dogs, dogs with short faces such as bulldogs and boxers, and overweight or long-coated dogs are most at risk – but every dog has the potential to suffer from heatstroke. It doesn't have to be boiling hot for this to happen either – when it's 22°C, (71.6°F) outside, the inside of a car can easily reach 47°C within an hour(116.6°F).