Heat Waves in the United States: Mortality Risk

during Heat Waves and Effect Modification by

Heat Wave Characteristics in 43 U.S.

Communities

By: G. Brooke Anderson and Michelle L. Bell

What is a heat wave? In this study, a heat wave was defined as: ≥ 2 days

with temperature ≥ 95th percentile for the community for 1 May through 30 September.

Effect Modification? Intensity

Duration

Time in Season What day in the season did the heat wave occur?

Was that heat wave the first of the season?

Method:

Bayesian Hierarchical Models were constructed to estimate effects of heat wave characteristics on mortality rates for: Individual Heat Waves

Communities

Regions: Northeast, Midwest and Southern U.S.

The Nation

Daily apparent temperatures also incorporated

“Catastrophic Heat Wave” Model

Association between heat wave effects (percent mortality increase during the heat wave vs. non-

heat wave days) and average daily temperature during each heat wave

What they found: Mortality risk increased:

3.74% during heat wave

2.49% for every 1°F increase in heat wave intensity

0.38% for every 1-day increase in heat wave duration

5.04% during first heat wave of the season

2.65% during later heat waves

Regional Trends in Effect Modification: Effect modification was most pronounced in Northeast

2.50% increase in mortality rate per extra day of heat wave

Midwest region less affected by 1st in season heat wave

Larger, more intense heat waves in the South Mortality effects were smaller

Threshold temperatures higher in the South

Some southern cities showed no mortality increase Charlotte, NC., Oklahoma City, OK., Dallas/Fort Worth, TX.

Catastrophic Heat Wave observed: Chicago: July 12 – 16, 1995

Mortality risk increased 133.9%

Model predicted increase of 18.9%

Milwaukee: July 13-15, 1995 Mortality risk increased 93.0%

Model predicted increase of 10.3%

Heat wave characteristic modification could NOT explain severe mortality risk increase

Why do mortality rates increase during a heat wave? Intensity & Duration

Increase strain on cardiovascular system

Timing in Season Mortality Displacement

Acclimatization

Better practices later in the summer

Why did mortality rates differ between communities? Acclimatization of people in warmer communities

Different levels of exposure (A/C use, house type, clothing)

Different community-level responses

Different demographics (more elderly, sick or less-healthy)

Geographical, meteorological, pollution factors

Generally: The cooler the region, the greater the heat wave effect

To Conclude: Heat wave intensity, duration and

frequency are expected to increase due to climate change.

This study investigates heat wave characteristics that might contribute to an increase in mortality rates, and that should be noted for public health purposes.