Essential Health Outcomes and Data Points for Informing Extreme Heat Work

As climate change intensifies, cities are increasingly confronted with the public health challenges posed by extreme heat. In most areas of the United States, extreme heat is defined as a period of two days or more with high heat and humidity with temperatures over 90 degrees Fahrenheit. There are significant health impacts that accompany heat waves and consistent high heat days, particularly for sensitive populations. To address these challenges, it's essential to understand health outcomes and associated data points to inform policy development. For example, using geographic information system (GIS) data and hospital data to identify hot spots within a city that need additional cooling centers.

This article explores useful metrics for understanding and quantifying health impacts from heat as well as case studies from cities across the United States. By identifying crucial data points and understanding the health outcomes that each can reveal, cities can craft targeted interventions to improve health outcomes. To summarize these metrics, a comprehensive guide organized by type and review frequency is provided.

Public Health: Emergency Outcomes Data

One of the most direct and severe consequences of increasing temperatures nationwide is the increase in heat-related illnesses and deaths. In fact, extreme heat is one of the leading weather related causes of death nationwide. Additionally, the inability to properly cool down can lead to a range of heat-related conditions such as heat stroke, heat cramps, heat exhaustion, and the exacerbation of existing health problems.

To effectively track and respond to these concerns, cities can utilize several key data sources. First, emergency department (ED) visits and hospitalizations provide real time indicators of extreme heat’s impact on public health. In Arizona, for example, this data has been developed into a dashboard, available to the public, detailing the number of heat-related illness ED visits and demographic details for every county in Arizona. The CDC also has a national level map of ED visits due to heat-related illnesses. As health officials monitor and track the number of patients seeking medical attention for heat-related symptoms and illnesses, they can quickly identify trends, prepare for ED surges, and respond to emerging crises.

Extreme heat can also significantly impact cardiovascular and respiratory health, often exacerbating existing conditions and placing additional stress on vulnerable individuals. One key data source for tracking these health outcomes is hospital admissions for heart attacks and strokes. During heat waves, cities often see a spike in these admissions and use this information to implement target interventions for high-risk populations and to allocate healthcare resources more effectively.

Public Health: Mortality Data

Mortality data is another important metric. It offers a more comprehensive view of heat’s deadliest effects and can help inform longer-term prevention strategies. However, it is important to note that mortality data often comes from a relatively small sample size, which makes it difficult to draw broad conclusions or trends from the data alone. Additionally, heat-related deaths are frequently underreported or misclassified, requiring significant meta-analysis of death certificate records to determine if deaths are heat-related due to the fact that often, they are not coded as such. For instance, Maricopa County in Arizona conducted an analysis of heat related deaths in 2023 that included a rigorous screening process of Maricopa County death certificates by Maricopa County Department of Public Health (MCDPH) staff. In their analysis, they listed key phrases that were associated with a heat-related death such as, “Heat exposure; Environ; Exhaustion; Sun; Heat Stress; Heat Stroke; Hyperthermia.” MCDPH have also continued to provide heat-related mortality data in an online interactive dashboard on a weekly basis during the heat season in an effort to keep the community, including policy makers and health care providers, better informed.

Public Health: Chronic Respiratory Stress

Extreme heat often coincides with poor air quality, as high temperatures can trap pollutants and increase ground-level ozone formation. Poor air quality can exacerbate respiratory conditions and increase the risk of cardiovascular disease. For instance, in Philadelphia, where public health officials have found that certain areas like the 19140 zip code have high rates of childhood asthma, heat is often a trigger for residents in these areas. Monitoring chronic respiratory stress indicators over time, such as asthma-related emergency department visits and deaths, can provide valuable insights into long-term trends and the effectiveness of interventions. Many cities are already implementing such tracking systems. For example, the New York State Health Department provides a dashboard reporting 44 asthma-related indicators, offering a model for other departments to follow. By collecting and analyzing this comprehensive data against temperature data, cities can develop more targeted and effective response strategies, including alerting families that asthma can worsen in extreme heat as well as cold or preparing for acute flares of chronic conditions in schools or during athletic practices or recreation time.

Air quality index (AQI) data can also be used to supplement the health outcomes data. By tracking AQI alongside temperature data, cities can better understand the compounded risks their residents face during heat waves and develop strategies that take both into account.

Public Health: Cognitive and Behavioral Impacts

Extreme heat has also been found to negatively impact learning and sleep, with significant consequences for community health and productivity. Studies have shown that for every 1°F increase in average temperature, there is a 1% decline in learning outcomes. Additionally, high nighttime temperatures can deteriorate sleep quality and make it harder to fall asleep, leading to increased fatigue and reduced cognitive function the following day. These can result in decreased work productivity, lower academic performance in students, and even worsening of psychiatric and neurological disorders.

To mitigate the adverse effects on youth health and well-being, Miami-Dade County has partnered with the Miami-Dade County Public Schools (M-DCPS) by creating a “Cool Schools Initiative.” This program aims to plant trees to increase the number of cool outdoor spaces for students, prioritizing schools with the most vulnerable student populations and therefore the greatest need.

Demographic Data: Vulnerable Population Impacts

Extreme heat events do not affect all members of a community equally. Certain groups, including the elderly, young children, individuals with chronic health conditions, homeless, and low-income communities, are disproportionately vulnerable to the health risks associated with high temperatures. Therefore, data on demographic distribution serves as a foundational resource for identifying vulnerable populations within a city. This information allows officials to map out areas with high concentrations of elderly residents, young children, people experiencing homelessness, or low-income households. Cities like New York City have also overlaid this demographic data with temperature maps, in order to create a heat vulnerability index (HVI) and pinpoint neighborhoods that may require additional resources or interventions. For example, the city’s Department of Parks and Recreation is using the Index to guide tree planting, with a focus on the two highest-risk areas (HVI-4 and HVI-5 neighborhoods).

The Social Vulnerability Index (SVI), developed by the CDC and Agency for Toxic Substances and Disease Registry, is another valuable tool. This index considers various socioeconomic factors such as poverty, lack of vehicle access, and crowded housing to determine a community's capacity to prepare for and respond to hazardous events like extreme heat. The SVI has already been used to understand access and barriers to cooling centers in Maricopa and Yuma Counties, predict the effects of social vulnerability on emergency medical services in San Antonio, and show that “individuals with rheumatic conditions living in areas with high and heat vulnerability had significantly greater odds of recurrent hospitalizations.”  

Place-Based Data: Urban Heat Island Effect and Infrastructure

The urban heat island effect significantly exacerbates the impact of extreme heat events in cities. This phenomenon is characterized by higher temperatures in urban areas compared to surrounding rural regions, and is caused by factors such as dense building materials and reduced greenery. Additionally, specific neighborhoods or spots within the greater urban area can experience even higher temperatures.

Satellite thermal imaging of urban areas provides valuable data for mapping and analyzing urban heat islands. For example, researchers at Arizona State University used Landsat satellite data in order to determine changes in surface temperature in Phoenix, Arizona. These images allow city planners and health officials to identify hotspots within urban areas, often correlating with areas of high building density, limited green space, or historical redlining practices. By tracking changes in thermal patterns over time, cities can measure and assess the effectiveness of heat mitigation strategies, and target cooling interventions to the most affected areas.

Analyzing green space and tree canopy coverage data can offer insights into natural cooling mechanisms within the city. Trees and vegetation provide shade, reduce surface temperatures through evapotranspiration, and improve air quality. Cities can use GIS to map existing green spaces and identify areas lacking in vegetation, and identify shade areas or parts of the urban infrastructure that could benefit from adaptations to the built environment (e.g. shade sails, overhangs, etc.). 

Additionally, power outage reports can also serve as a critical indicator of infrastructure strain during extreme heat events. As temperatures rise, increased use of air conditioning and other cooling systems can overload electrical grids, leading to blackouts. These outages can pose serious health risks, particularly for vulnerable populations reliant on medical equipment. By monitoring power outage frequency, duration, and location during heat waves, cities can identify weaknesses in their energy infrastructure and prioritize upgrades or emergency response measures.

The following guide synthesizes the information above and provides a format for identifying: where the data discussed previously can be found and what priority level each type should be considered. This matrix is a starting point that can, and should, be tailored for each individual city. Additionally, it can be paired with our Starter Guide for Mayors: Addressing Extreme Heat in Your Cities.

Extreme Heat Matrix Guide