“Heat illness is preventable.” So begins our public health messaging on heat risk at the New York City Department of Health and Mental Hygiene (hereafter, the Health Department). But as we go through emergency response operations for extreme heat events every summer, “heat illness is preventable” sometimes feels like an existential mantra, reminding us how much more we need to do. On behalf of our many colleagues, we share here our collective experiences and primary challenges in using applied health research findings to describe and mitigate the adverse health impacts of heat in the past decade.
One of the first questions we examined was whether the threshold for triggering a heat emergency response was appropriate. In most U.S. cities, local National Weather Service offices issue heat advisories in advance of forecast heat events. In New York City, these advisories activate the heat emergency plan. However, advisory guidelines were not derived from epidemiologic analysis of heat-dependent health effects. We found nonlinear lagged impacts of temperature on natural cause deaths at heat index levels below the threshold at the time (41°C for any duration) in a retrospective time-series analysis.1 Our agency approached the City’s Emergency Management agency and the National Weather Service to recommend lowering the threshold for triggering a heat advisory and emergency plan; in 2008 the threshold was changed to the forecast maximum heat index of 35°–37°C for at least 2 consecutive days or at least 38°C for 1 day or more.
In response to a heat emergency, New York City Emergency Management coordinates activities of over 20 city and state agencies, utility companies, and transit authorities before and during heat events based on continuing National Weather Service forecast updates. Throughout the summer season (May-September), the Health Department runs prediction models of daily syndromic surveillance of emergency department visits (based on chief complaint) and emergency medical service calls for heat-related illness with the maximum heat index and several temporal variables as predictors. We contribute synopses of the result of the observed versus predicted values to the City’s emergency response, providing situational awareness and a basis for ramping up alert messaging during severe heat waves, as needed. From a retrospective analysis2 we know that increases in heat-illness syndrome indicators predict increased heat-related, excess, nonexternal cause deaths (hereafter, excess deaths). Being able to provide situational awareness during an emergency, however, is limited in effect. We know the weather is the best predictor of health impacts,1 heat waves occur almost every year, and climate change is projected to make these events more severe and frequent in the city.3 Clearly intervention needs to occur before heat emergencies happen.
Who is dying of heat stroke and where? Following the 2006 heat wave, the Health Department developed a protocol with the Office of Chief Medical Examiner to review, after severe heat waves, heat stroke (hyperthermia) death records, which contain information beyond standard vital statistics data on circumstances surrounding deaths (e.g., presence of air conditioning). Of the 48 heat-stroke deaths that occurred between 2008 and 2011, 41 (85%) had onset at home. Of 26 heat-stroke deaths with information available on home air conditioning, none had a working air conditioner.4 Although the average number of heat-stroke deaths per year in New York City is low, the estimated average annual number of excess deaths associated with extreme heat events is nearly 10 times greater (~115 deaths).5
To determine individual- and neighborhood-level risk factors associated with excess deaths, we worked with academic researchers on a case-only analysis of heat-wave impacts. We identified individual-level modifiers of being non-Latinx Black, having congestive heart failure as underlying cause of death, dying at home, and neighborhood (census tract) risk factors of percent public assistance, percent green space (negatively associated), and surface temperature.6 To visualize neighborhood variation in heat risk, we created a Heat Vulnerability Index (Figure A), publicly available through the Health Department’s Environment and Health Data Portal.7 The City recently used this index to develop its Cool Neighborhoods initiative, which includes planting street trees in the most vulnerable neighborhoods and a pilot community resilience project.8
With neighborhood percent of residents receiving public assistance and of non-Latinx Black residents as two of the four components for the heat vulnerability index, the resulting spatial pattern of heat vulnerability mirrors the spatial pattern of other adverse health outcomes associated with the City’s pattern of residential racial segregation.9 Tracking with high poverty levels, these neighborhoods also have a lower prevalence of air conditioners (Figure B).10 An ecologic analysis in New York City found that several characteristics, including lower rates of air conditioning access, percent below poverty, and surface temperature were associated with higher mortality rates on hot days and that percent of non-Latinx Black population and household poverty were strong negative predictors of seniors’ air conditioning access.11 Similarly, an analysis of four U.S. cities reported that disproportionate mortality impacts of heat on Black residents were explained in part by the lower prevalence of air conditioning.12 Given that air conditioning reduces or eliminates indoor heat exposures, increasing air conditioning prevalence in heat-vulnerable neighborhoods is the most effective intervention to reduce heat-related morbidity and mortality.
Yet even if we could universally provide air conditioners, we might not be able to eliminate heat-related mortality. A 2011 telephone survey of a representative sample of New York City adults found that some seniors or those in fair or poor health never/rarely used it on hot days. Disliking air conditioning and not feeling hot were identified as major reasons for not using air conditioners, in addition to the cost of running them.13 Air-conditioning access must be coupled with outreach to those unaware of the danger of high indoor temperatures to increase use of the intervention. Indoor temperatures without air conditioning can be substantially higher (e.g., > 10°C) than outdoors.14 Further, a recent New York City study found that indoor temperatures in non–air conditioned residences remained high for days after a heat wave, even at night, due to buildings’ thermal inertia.15
As the Health Department has presented evidence supporting equitable access to air conditioning for those whose health depends on it, we have faced some resistance. At scientific and public health meetings, colleagues ask about resulting energy demand that could increase the chance of power outages; chemical refrigerants that contribute to global warming; increased air pollution from generating additional energy to power air conditioners that can contribute to climate change; and the waste heat that could further contribute to the urban heat island problem. Although all these concerns are valid in isolation, we have realized that many professionals in public health and climate science are unaware that people are dying today—cooking to death in their own homes—from the lack of air conditioning. “Adaptation” to heat as the climate changes may be discussed in the abstract, but there is a concrete biologic limit to how much heat humans can tolerate.16
Skeptics of air conditioning also must consider implementation realities and scale when estimating potential negative environmental impacts. Energy load, air pollution and waste heat would be substantial issues should any municipality double the air conditioning prevalence—that is, 50–100%—in the short-term. But the air conditioning prevalence is already nearly 90% in many U.S. cities17—88% in New York City in 2014.10 However, inequities persist, with 30% of residents without air conditioning in the highest poverty neighborhoods versus 1% in the lowest. Given the racial and economic disparity in adverse health impacts from heat, closing this relatively narrow gap across neighborhoods in New York City (and other U.S. cities) is paramount juxtaposed against the associated, incremental increase in energy usage. Increases in energy use also can be somewhat offset by reductions in wasteful air conditioner (AC) use to super-cool businesses and office buildings, and new air conditioning technologies can reduce greenhouse gasses.18
The global perspective is more daunting. According to a recent report by the International Energy Agency, only 8% of the 2.8 billion people living in the hottest parts of the world possess air conditioners, and the energy needed for space cooling is expected to triple by 2050,19 highlighting the need to develop highly energy-efficient societies. The U.S. took five decades to increase air conditioning prevalence from 10% (before 1960) to current levels. An analysis of the heat impacts on New York City mortality from 1900 to 2006 found a substantial decline in risk from the 1970s to 2000s,20 and the increase in air conditioning explained substantial declines in mortality impacts of extreme heat between 1900 and 1959 and 1960 and 2004 in a nationwide study.21 Another U.S. study of 105 cities from 1987 to 2005 also observed a decline in heat impacts on deaths, which was not materially explained by the incremental increase in air conditioning prevalence during the period.22 However, before the study period, air conditioning prevalence was already over 60% nationwide and higher in warmer cities. The U.S. story provides evidence of potential global gains in reducing excess deaths owing to heat by increasing air conditioning access in hot areas as quickly as feasible.
Extending widespread access to cooling in other countries will likely present different challenges and require alternative solutions than in the U.S. For instance, countries without reliable electric grids could consider investment in solar technologies.23 But solutions must be found: People in some areas of the world are projected to be exposed to intolerable temperatures within this century under high-emission scenarios.24 Equitable coverage worldwide also requires the U.S. and other countries with higher air conditioning prevalence to prioritize responsible energy use and efficiency across sectors, and financial investment, to compensate for this life-saving adaptation to climate change.
The Health Department’s application of epidemiologic research led us to a lower heat advisory threshold, development of heat illness syndromic surveillance, and identification of individual- and neighborhood-level risk factors associated with adverse heat impacts. But most importantly, we have identified areas of intervention for local government that can tangibly minimize the adverse health impacts of heat in the near future:
- In collaboration with community organizations, we must reach out to vulnerable populations to (1) facilitate access to and financing to pay for using air conditioning; and (2) educate on the danger of heat and life-saving need for air conditioning in hot temperatures.
- Federal, state and local public assistance to counteract energy insecurity25 through need-based benefits (i.e., Home Energy Assistance Program in the U.S.) must be optimally balanced to reflect the changing climate between heating costs in the winter and cooling costs in the summer. In addition, local governments need to evaluate gaps in coverage and distribution barriers for cooling benefits and their impact on high-risk residents.
- Epidemiologic research, and return-on-investment analyses for health care payers, can identify highest risk patients for whom air conditioners should be considered life-sustaining medical equipment, providing evidence to support air conditioning and energy cost coverage through federal insurance programs (e.g., U.S. Medicaid) for low-income people.
- Responsible energy use must become the culture. We need to raise awareness about inequitable energy use and access to safe temperatures at home across race and social class. It must become unacceptable for indoor temperatures to be set low so some can wear suits at work, while others are dying in overheated homes in part due to energy usage concern. We can encourage—and model—setting thermostats in public spaces in the upper range of normal comfort zone during warm months (e.g., 26°C26) to minimize the energy impact of space cooling. Equitable access to air conditioning should also become a part of larger conversations around sustainability, factoring in the need for this adaptation as efficiency measures are developed, ranging from smart thermostats to development of better air conditioning technologies, such as solar-thermal chillers,24 to setting criteria for green buildings.
Pursuing these solutions will stretch the traditional capacity and scope of health departments and epidemiologists, but complex problems require interdisciplinary efforts. Healthy indoor temperatures should not be a privilege that excludes vulnerable and low-income populations, especially when its consequence is illness and death.
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