Table –

Estimated number needed to treat (NNT) for cooling centers under various assumptions and scenarios. The NNT estimates how many people would need to visit a cooling center on a single day of extreme heat in order to prevent one heat-attributable death that day. Note that lower absolute risks imply a higher NNT. We assume that visiting a cooling center reduces the risk of heat-attributable death by 66%. See text for additional assumptions.

Scenario	Heat-Related Outcome	Absolute Risk of Outcome on Extreme Heat Day (per million people) a	NNT
Excess deaths in Maricopa County, Arizonab	Death	0.92	1,648,868
Excess deaths in US (CDC data)c	Death	0.15	9,900,990
Excess deaths in US (Weinberger et al. 2020)d	Death	3.45	439,222
Excess Extreme-Heat Related Deaths in Europee	Death	0.52	2,890,011
If Risk of Heat-Related Illness is 10x that of Heat-Related Deathf	Illness	1.53	990,099
If Risk of Heat-Related Illness is 100x that of Heat-Related Deathf	Illness	15.3	99,010
Excess emergency department visits in Californiag	Illness	25.0	60,606

^aUnless otherwise noted, we assume that any given location experiences 10 days of extreme heat per year. This assumption influences the estimation of the absolute risk of heat-attributable events per day from published reports.

^bBased on CDC data on heat-related mortality for Maricopa County for 2000 to 2019 and average population across the same time period,²⁷ and assuming ten days of extreme heat per year.

^cBased on 2019 CDC Data²⁷ and assuming a population size of 330,000,000 and ten days of extreme heat per year.

^dBased on Weinberger et. al, which estimated the number of deaths attributable to extreme heat in the 297 most populous counties in the United States (accounting for ~62% of the national population). This study defined a day of extreme heat as one in which daily maximum temperature reached or exceeded the 97.5^th percentile of the local warm season (May – September in the norther hemisphere) temperature distribution, or an average of 3.825 such days per year in any given location.²⁹

^eBased on extreme-heat fatalities data reported from the Joint Research Centre of the European Union,³⁰ and an estimated population of 515,000,000.

^fBased on the estimates from row 3.

^gIn the 2006 California heat wave, there were 16,164 excess emergency department visits over 18 days in a population of 35,950,000, yielding an event rate of 25.0 excess visits per million people per day.³¹