A petition calling for it to be illegal in Guernsey to walk a dog in temperatures of 20 Celsius and above, and for it to be illegal to leave dogs unattended in vehicles in temperatures of 20 Celsius and above. No dog has ever died from missing one walk but dogs do die from one walk on a hot day. 

Temperatures above 20°C can be too hot to walk a dog. Touch the pavement with your hand; if you are unable to keep your hand there for more than 5 seconds, it is too hot for your dog’s paws. 

Sand is another surface that can get unbearably hot in the sun and potentially burn your dog’s paws, so it is best to avoid it on hot days. Pavements and sand can become up to 50°C higher than the air temperature.

Heatstroke can be fatal, with over 1 in 10 affected dogs dying. Exercise-related heatstroke is the most common type, accounting for 75% of cases in the UK. Avoid heavy exercise and keep your dog’s walks brief in hot weather.

Sources: 
https://www.uk.pedigree.com/dog-advice/caring-for-your-dog/walking-dogs-in-hot-weather​

https://www.bellaandduke.com/dogs/expert-advice/dog-health/when-is-it-too-hot-to-walk-a-dog/

Please also see the RSPCA advice about dogs left in cars: https://www.rspca.org.uk/adviceandwelfare/pets/dogs/health/dogsinhotcars