Workers and the union representing them say working in extreme heat is a major safety issue. UPS says air conditioning would be "ineffective."
As a sizzling heatwave hits the UK, we ask an expert how the hottest day yet of 2022 will affect our behaviour.
Around the world, high temperatures have killed dozens, sent thousands to the hospital, and led to angry protests calling for stable electricity supplies.
Pakistan saw a respite from the heat on Thursday with the temperature at 93.2 degrees Fahrenheit in Karachi, down from 113 degrees recording on Sunday, but heat stroke victims were still arriving at local hospitals.