This article doesn’t really explain why, just that it’s complicated.

That doesn’t prevent upper limits being put into place though, with scales based upon types of work, humidity, and break/water access.

For example:

• 45°C is too hot in all circumstances
• 40°C is too hot at x% humidity
• 35°C is too hot at y% humidity
• 38°C is too hot at z% humidity if manual labour but okay for office work
• Any work above 30°C can be refused if water and 5m breaks every 30m aren’t provided

But that bring us to the real reason for why, productivity may be affected and that would hurt the pockets of the wealthy.