Hazardous heat exposure among incarcerated people in the United States

Climate change is predicted to increase the frequency of potentially hazardous heat conditions across the United States, putting the incarcerated population of 2 million at risk for heat-related health conditions. We evaluate the exposure to potentially hazardous heat for 4,078 continental US carceral facilities during 1982–2020. Results show that the number of hot days per year increased during 1982–2020 for 1,739 carceral facilities, primarily located in the southern United States. State-run carceral facilities in Texas and Florida accounted for 52% of total exposure, despite holding 12% of all incarcerated people. This highlights the urgency for enhanced infrastructure, health system interventions and treatment of incarcerated people, especially under climate change.


Brief Communication
https://doi.org/10.1038/s41893-024-01293-y The majority of carceral facilities in the southern United States have experienced a rapid increase in potentially hazardous heat exposure since the 1980s and are located in states that do not have mandatory conditioning access for state-run institutions 6,7 .Whereas physically this rapid increase in heat exposure is a result of anthropogenic climate change, land-cover and land-use change, including an urban heat island effect caused by the materials used to construct carceral facilities 3 , this geographic disparity also reflects state-level criminal justice policies, as southern states have the highest imprisonment rates in the United States (though not necessarily highest jailing rates) 13 and the inherent differential effects of climate change.Throughout the country, including in the Northeast and Midwest, many locations with carceral facilities also experienced an increasing number of days WBGT exceeded 28 °C compared with other locations.This continuing intensification limits the effectiveness of heat-mitigation plans (if they exist at all) at non-air-conditioned facilities 11 .
That we found carceral facilities are systematically exposed to an increasing number of potentially hazardous heat days compared with other areas of the United States is plausible for several reasons.First, carceral facilities are often built where there is availability of low-cost land and limited resistance of local communities 14 .In many states, areas that meet these criteria are in sparsely populated desert or swampy environments 5 .Zoning laws in urban environments and security issues also favour construction in isolated, desert-like areas 14 .
annually compared to locations without carceral facilities (Fig. 2a).However, there was a considerable amount of variation by year, with a maximal disparity of 9.8 more days at carceral facilities than locations without carceral facilities in 1998 and a minimal disparity of 3.5 days in 1994.Arizona, California and Nevada ranked as the top three states with the greatest exposure disparities (Fig. 2a).Carceral facilities in Arizona experienced 13.1 more days per year than the rest of the state and 40.9 more days compared to the entire continental United States during 1982-2020 on average.Statistics comparing the characteristics of incarcerated and non-incarcerated people are found in Supplementary Tables 1 and 2.
In 2018, 915,627 people in the United States, 45% of the estimated total incarcerated population, were housed in 1,739 carceral facilities with an annual increase in the number of days per year WBGT max exceeded 28 °C during 1982-2020 (Fig. 2b).These facilities are primarily located in the southern United States, which faced the greatest number of potentially hazardous heat days per year since 1982 (Fig. 2b).Carceral facilities in Florida experienced on average 22.1 more days in 2020 compared to 1982, the greatest increase in humid heat days for all continental states, consistent with previous work finding that the largest relative increases in heat stress are expected at latitudes closer to the equator 12 .The greatest overall increase relative to the state was for Webb County Jail, Texas, with 58.7 more days than the rest of Texas in 2020 compared with 1982 (Fig. 2c).We also present results from Figs. 1 and 2 with alternative thresholds of 26 °C and 30 °C (Supplementary Figs.3-6).

Brief Communication
https://doi.org/10.1038/s41893-024-01293-y The lack of disparity we identify in Florida is an exception probably due to the North-South climate gradient, with a relative dearth of carceral facilities in the most hot-humid, but economically wealthy and densely populated, southern tip.We found that the top four most-exposed states to potentially hazardous heat days per year were Texas, Florida, Arizona and Louisiana, all of which do not provide universal air conditioning to their prisons 7 , potentially creating a double burden of increased exposure and vulnerability.Incarcerated people have few options to reduce the impact of hazardous heat 3,7,9 , and these marginalized communities are often disproportionately susceptible to the effect of heat exposure given pre-existing health conditions.An estimated 43% of the state prison population has a previous mental health diagnosis 15 , and people on psychotropic medications are at increased risk for heat illness 16 .Exposure to elevated heat can also cause both acute health effects, such as heat stroke or mortality, and long-term damage.For example, chronic dehydration strains kidney function and those with chronic heat exposure have been shown to have higher rates of kidney disease 17 .Such vulnerabilities are especially relevant given restrictive prison policies with respect to drinking water and other potential heat-adaptation tools 3 .
Though there have been recent declines, the incarcerated population of the United States has increased by 500% over the past four decades 18 .People of colour are overrepresented in carceral facilities and compose an estimated two-thirds of the total incarcerated population.The prison population is also ageing, with one in seven serving life in prison 19 , potentially resulting in greater overall heat vulnerability to those incarcerated.Structural racism manifests in persistently higher proportions and rates of incarcerated people being people of colour 20 .Acknowledging and accounting for the role structural racism plays in incarceration is critical to understand both key vulnerabilities to heat and contextualizing solutions to heat exposure.Appropriate preparation for periods of elevated heat is also critical.For example, seasonal forecasts could help facilities prepare for summer heatwaves to reduce the impacts of hazardous conditions for incarcerated communities.
Our work highlights how incarcerated populations in the United States are systematically exposed to potentially hazardous heat with the greatest exposure and rates of increase concentrated in state-run institutions.Federal, state and local laws mandating safe temperature ranges, enhanced social and physical infrastructure and health system interventions could mitigate the effect of hazardous heat.Underlying this is the need for a fundamental overhaul to the perception and

Brief Communication
https://doi.org/10.1038/s41893-024-01293-ytreatment of incarcerated people in environmental public health policy and regulatory action.Further work is critical to comprehensively characterize the vulnerability of the United States incarcerated population to heat and how heat impacts health, to build reliable and validated datasets of cooling mechanisms in prisons and jails, to directly measure indoor temperatures in prisons and jails and to deploy adaptation measures to mitigate the worst impacts of climate-related stressors.Doing so is critical to environmental justice, particularly for incarcerated people with limited social and political agency.

Methods
We assigned daily WBGT max estimates to 4,078 carceral facility locations for the United States during 1982-2020.WBGT max is constructed from high-resolution (4 km) daily maximum 2 m air temperatures (T max ) and maximum vapour pressure deficit (VPD max ) from the Parameter-elevation Regressions on Independent Slopes Model (PRISM) dataset 21 .T max and VPD max are used to construct daily maximum heat index (HI max ) following the US National Weather Service's procedure 22 , which is converted to indoor, or shaded, WBGT max using a quadratic transform that assumes fixed wind speeds (0.5 m s −1 ) and no radiated heat (daily WBGT max estimates in Supplementary Information).Facility location and population data are from Homeland Infrastructure Foundation-Level Data (HIFLD), produced by the Department of Homeland Security 5 .We evaluated PRISM-derived WBGT max against ECMWF Reanalysis v5 (ERA5)-and Hadley Centre Integrated Surface Database (HadISD)-derived WBGT max in Supplementary Figs. 7 and 8.
We then define potentially hazardous heat frequency as the number of days per year where the maximum WBGT max exceeded 28 °C, the threshold used by the US National Institute for Occupational Safety and Health for acclimated populations to limit heat exposure under moderate workloads (234-349 W) 10 , and it is used widely in environmental epidemiological research 23,24 .Exposure during 2016-2020 is measured by multiplying the number of incarcerated people housed at each carceral facility in 2018 by the average number of days WBGT max exceeded 28 °C per year during 2016-2020.Annual disparity between incarcerated and locations without carceral facilities is measured by taking the population-weighted difference between the number of days WBGT max exceeded 28 °C at the location of a facility and the rest of the state.Population weighting fairly reflects the experience of a population to heat stress.To measure the annual rate of change in annual heat exposure, we fit linear regressions to the count of days WBGT max exceeded 28 °C per year for each facility.A more detailed explanation of methods is in 'Calculating humid heat exposure and trajectories of change metrics' in Supplementary Information.

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Fig. 1 |
Fig. 1 | Mean annual exposure during 2016-2020 to potentially hazardous heat in carceral facilities within the continental United States.a,b, For each carceral facility (N = 4,078), metrics of potentially hazardous heat measured by: the number of person-days WBGT max exceeded 28 °C for incarcerated people by

Fig. 2 |
Fig. 2 | Trends in annual exposure during 1982-2020 to potentially hazardous heat in carceral facilities within the continental United States.a, Populationweighted difference between the annual number of days WBGT max exceeded 28 °C at the location of carceral facilities versus all other locations in the continental United States during 1982-2020, overall and stratified by state, ordered by average population-weighted difference.b,c, The total change in the number of days WBGT max exceeded 28 °C per year for each carceral facility in the continental United States during 1982-2020 (b) and the total change in disparity in number of days WBGT max exceeded 28 °C per year for each carceral facility in the continental United States, compared with the rest of the state the carceral facility is located, during 1982-2020 (c).
and populated carceral facilities were includedReproducibilityAll code to reproduce this work, as well as underlying daily WBGTmax for each carceral facility during 1982 -2020 and analytical products used here, are freely available via GitHub.