Spanish authorities are investigating the death of a street sweeper who collapsed this week in the intense heat wave gripping Europe. Italy has banned outdoor work during the hottest hours. Schools in France have been shuttered. Even the top of the Eiffel Tower is closed.

As Europe puts its citizens on alert for record temperatures, the increasing frequency of extreme heat is raising an urgent question: How hot is too hot to work?

The French government issued a directive Tuesday strengthening rules for businesses to keep employees cool, as officials warned that working in extreme heat “can increase the risk of workplace accidents, including serious or fatal ones.”

Under the rules, French businesses must ensure they are lowering heat risk for all workers. In offices, that means modifying workspaces near sunny windows or providing more ventilation and water. People toiling in construction, farming and other outdoor activities have the right to demand shorter work hours and heat-protective equipment.