The National Weather Service developed the heat index to help people identify days when the risk for a heat illness is higher than normal. During a heat wave, the heat index is excessive for many days in a row. Everyone has an increased risk for a heat-related illness during a heat wave.

A heat-related illness can be more serious for:

• Older adults, who may not notice excessive heat, do not sweat as effectively, or do not feel thirsty.
• Infants and children, who can't transfer heat very well.
• People with chronic medical conditions, such as heart disease, mental illness, or being overweight.
• People with developmental problems.

Other things that affect a person's risk for a heat-related illness during a heat wave include:

• Living in cities, because heat is trapped by tall buildings and air pollutants.
• Living alone.
• Working outdoors.
• Exercising outside.
• Not having cooling devices, such as fans or air-conditioning.