that requires the heart to pump faster and harder radiation and evaporation summer heat increases the risk of stroke and heart attacks Harvard Health Radiation Harvard Health twice as much blood per minute two ways trying to disperse some of its own internal heat actually a strain on the heart, Harvard Health that requires the heart to pump faster and harder cardiovascular heart failure two ways evaporation of sweat Harvard Health Harvard Health Harvard Health two ways two ways hard on the heart Harvard Health Harvard Health Harvard Health Harvard Health Harvard Medical School Harvard Health twice as much blood per minute force our hearts to work harder heart to beat faster and pump harder