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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.