A multiday heat wave is bringing high temperatures and health risks from the Midwest to the Northeast, prompting warnings and emergency measures.

A heat wave will intensify along the East Coast this week as tropical humidity levels and 100-degree temperatures reach big cities from Boston to Washington. On Sunday, humidity levels were higher in parts of the Northeast and southern Canada than in Florida, breaking records in some places.

Tens of millions of people across the Midwest and East are enduring another sweltering day of dangerously hot temperatures as a sprawling June heat wave grips much of the U.S. Most of the northeastern