Where was Heat Abnormally High in 2023?
Figure: Temperature is calculated from NOAA’s nClimGrid-Daily v1-0-0, a 5km gridded dataset aggregated into counties for the contiguous U.S. For each day from April 1st to September 30th, a county’s temperature in 2023 is compared against its climatological normal from 1991–2020. Temperatures above the 95th percentile are considered abnormally hot for the region. Thick lines on the map indicate HHS regional boundaries:
Region 1: CT, ME, MA, NE, RI, VT
Region 2: NJ, NY, PR, VI
Region 3: DE, DC, MD, PA, VA, WV
Region 4: AL, FL, GA, KY, MS, NC, SC, TN
Region 5: IL, IN, MI, MN, OH, WI
Region 6: AR, LA, NM, OK, TX
Region 7: IA, KS, MO, NE
Region 8: CO, MT, ND, SD, UT, WY
Region 9: AZ, CA, HI, NV, AS, MP, FSM, GU, MH, PW
Region 10: AK, ID, OR, WA
2023 was the 5th-warmest year on record for the contiguous U.S. (average annual temperature 2.4°F above average). Texas, Louisiana, Mississippi, New Hampshire and Massachusetts each ranked warmest year on record while Maine, Vermont, Connecticut, Maryland, Virginia and Florida each ranked second warmest. The map above depicts the number of summer days in 2023 when a county’s maximum temperature exceeded its 95th percentile, thus indicating an abnormally hot day. Much of HHS Region 6, especially Texas and Louisiana, experienced more than 30 days of abnormally hot temperatures in 2023.
Heat-Related Illness in 2023: A Case Study of Maricopa County, AZ
Figure: Table and plot of heat-associated deaths in Maricopa County in 2023 from the Department of Public Health’s latest report. Heat-caused deaths include cases where heat is listed as a direct cause of death on the death certificate, heat-related deaths include cases where heat is listed as contributing, and under investigation includes cases where the medical examiner suspects a heat-associated death.
Maricopa County, AZ is a highly populated county that experiences some of the U.S.’s most extreme heat. In 2023, there were over 100 days when the temperature was above 100° F. It also experiences high rates of heat-related illnesses. For example, it ranks in the 99th percentile of U.S. counties for their number of heat-related Emergency Medical Services (EMS) activations according to the National EMS Information System’s (NEMSIS) Heat-Related EMS Activation Surveillance Dashboard. In 2022, Maricopa suffered a record 425 confirmed heat-related deaths, after experiencing 339 confirmed in 2021. Through November 7, 2023, they had already confirmed 579 heat-associated deaths. Additional analysis gives insight into risk factors: 25% of the deaths occurred indoors, with air conditioning present but not functioning for 109 out of 146 of these cases. 45% of the deaths occurred among individuals experiencing homelessness.
This high number of heat-related deaths has prompted Maricopa County to take many actions including:
- Partnering with the Maricopa County Association of Governments, municipalities, universities, and community and faith-based organizations to form the Heat Relief Network to establish cooling centers and hydration stations;
- Providing surveillance via the county’s Department of Public Health to track heat-related illness and death in order to support heat relief planning and provide guidance for residents to protect themselves;
- Expanding access to heat relief by increasing evening and weekend hours and funding 2-1-1 Arizona to assist in finding transportation to the nearest Heat Relief location;
- Increasing funding to the HVAC repair and replacement fund; and
- Partnering with the state of Arizona to sign up for heat warning alerts.
Heat Affects Health in Many Ways
Warmer temperatures increase the risk for a diverse range of health risks. For example:
- An increased risk of hospitalization for heart disease.
- Heat exhaustion, which can lead to heat stroke if not treated, can cause critical illness, brain injury,
and even death.
- Worsening asthma and chronic obstructive pulmonary disease (COPD) as heat increases the production of ground-level ozone.
- Dehydration, which can lead to kidney injury and blood pressure problems. Some kidney damage can become irreversible with repeated or untreated injury.
- Violence, crime, and suicide may increase with temperature, adding to the rates of depression and anxiety already associated with climate change
- Some medications increase the risk of heat-related illness. These include diuretic medicines (sometimes called “water pills”), antihistamine medicines (including many allergy medicines), and many antipsychotic medicines used to treat a variety of psychiatric and neurologic illnesses. Please review this list of common psychiatric medications that can impair the body’s normal ability to cool itself, as well as this list of tips for coping with hot weather for people who take medications.
Figure: Image of the EMS HeatTracker showing county-level rates of heat-related EMS activations from July 29, 2023 – August 11, 2023. View the EMS HeatTracker: https://nemsis.org/heat-related-ems-activation-surveillance-dashboard/
OCCHE and the National Highway Traffic Safety Administration (NHTSA) have launched a new resource called the Heat-Related EMS Activation Surveillance Dashboard, or the “EMS HeatTracker” for short. This first-of-its-kind tool maps EMS responses to heat-related illness across the country to help local decision makers and communities prioritize resources and interventions to prevent heat-related illness and save lives.
The EMS HeatTracker highlights jurisdictions (including all 50 U.S. States, Puerto Rico, and D.C.) and counties with the highest rates of heat-related EMS activations and allows for county- and jurisdiction-level comparisons to national averages in three categories in the prior rolling 30- and 14-day periods:
- population rate of heat-related EMS activations within a community;
- average EMS time in transit to reach a patient; and
- the percent of patients who are transported to a medical facility for further treatment.
The tool also provides national-level information on the number of heat-related EMS activations and the number of heat-related deaths among patients who were alive when EMS officials arrived on the scene. Demographic information is also available at the national level, including the age, race, gender, and urbanicity (i.e., urban, suburban, rural, and frontier) of patients.
The information displayed on the EMS HeatTracker is updated every Monday morning with a two-week lag behind real time.
Figure: Heat map of all 50 U.S. states and District of Columbia. States with the largest increase in heat-related illness diagnoses over the assessment period are red, with less dramatic increases represented in shades of orange.
A recent publication from the U.S. Department of Veterans Affairs, Stanford University, University of Iowa, and CDC found that Veterans have been diagnosed with heat-related illnesses (HRIs), such as heat exhaustion and heat stroke, in all 50 states and that the rate of diagnosis has increased over time. The assessment utilized the Veterans Health Administration’s national electronic health record database to identify HRIs diagnosed from January 1, 2002, through December 31, 2019, and found that there were 33,114 documented cases of HRIs, which impacted 28,039 unique patients. The Veterans Health Administration is the largest integrated healthcare system in the United States, with 1,298 facilities serving over 9 million enrolled Veterans.
In addition to a statistically significant increase in the incidence of HRIs over time, the results raised important health equity concerns. Of note, Black and American Indian/Alaska Native Veterans were more likely to be diagnosed with HRIs. Veterans with existing medical conditions, including common comorbidities, also saw a greater increase in HRIs over time. For example, in 2022 ~50% of HRI diagnoses impacted Veterans with hypertension, but this increased to ~70% in 2019. The results demonstrated that there has been an increase in diagnosed HRIs among Veterans for nearly all U.S. states with a disproportionate increase of HRIs in California, Florida, and Texas. There were also notable increases in HRI diagnoses in other states such as Missouri, Arkansas, Virginia, Ohio, and New York. However, the rates for the Veteran homeless population were increasing in the first half of the assessment period, but then decreased in the second half of the assessment. This change in the trend occurred alongside the development and expansion of Veteran homeless programs, which suggests specific interventions can decrease the extent of heat related illnesses.
From August 31 through September 9, 2022, a record-breaking heat wave occurred across California. Temperature records were set in approximately 1,500 locales and excessive heat warnings were issued for much of the state. During this 10-day heat wave, an analysis by the California Department of Public Health (CDPH) found a 5% increase in deaths in the state – 395 more deaths than would be expected. The highest increases in deaths were seen among people aged 25-64, people who identify as Hispanic or Latino, and people from the South Coast region, including Los Angeles and neighboring counties. While we know from previous analyses that older adults and the very young are vulnerable to negative health impacts from extreme heat, CDPH notes that it is important for heat interventions to also consider the vulnerability of working-age adults, who may work in hot conditions or have other types of over-exposure to heat. CDPH believes the finding for the South Coast region may reflect where fewer residences have air conditioning, and where people are less acclimated to heat than inland residents.
This analysis highlights that we can likely expect heat impacts to extend into September in coming years. California has invested $404 million towards addressing extreme heat impacts, guided by the state’s Extreme Heat Action Plan. CDPH hopes that these findings will help guide future public health prevention, response, and resilience efforts as this plan is implemented.
The Wet Bulb Globe Temperature (WBGT) is a measure of heat stress on the human body, taking into account the effect of temperature, relative humidity, wind speed, and solar radiation on humans. Sweating can usually cool the body down to a stable internal temperature, but when the humidity is high, the air can become so saturated with moisture that the evaporation of sweat slows, hindering the body’s ability to cool itself.
Those who work or exercise in direct sunlight are particularly vulnerable to heat stress when the WBGT is high. Monitor the WBGT using the National Weather Service one-week WBGT forecast and take a break in an air conditioned building if you notice that you are sweating excessively while working or exercising outside.
Between 1998 and 2022, a total of 937 children died due to heatstroke when left in a vehicle alone, with an average number of 37 deaths each year (see noheatstroke.org for more information). More than half of the deaths (54%) are children under 2 years of age. All of these fatalities were preventable. In 10 minutes, a car can heat up by as much as 20 degrees Fahrenheit and become life threatening for a child trapped inside. Rolling down a window does little to keep a vehicle cool. Since the body temperature of a child rises three to five times faster than that of an adult, a hot vehicle can become dangerous within a short period of time.
Among the reported fatalities, 53% of children were forgotten by their caregivers, 25% gained access to the car on their own, and 20% were knowingly left in the car by the caregiver. Three tips to remember to prevent children from dying in a hot car: never leave a child in a car unattended, make it a habit to look in the back seat every time you exit, and always lock the car, and put the keys out of reach.
According to the National Oceanic and Atmospheric Administration, May 2023 was the world’s third-warmest May on record and North America’s warmest May on record. Due in part to North America’s record- setting high temperatures, several hundred wildfires broke out across Canada in May, burning over 6 million acres and causing widespread air quality deterioration across much of Canada and the United States. Elsewhere in the world, Antarctic sea ice extent hit a record low in May and tropical cyclone Mocha made a devastating landfall as a Category 4 Cyclone in Myanmar on May 14. Earth’s ocean surface temperatures also set a record high for the second month in a row.
Although early season extreme heat events are less common than those that occur later in the summer, they may be more deadly. As the summer progresses, our bodies get used to the higher temperatures and become more efficient at cooling, by increasing sweating rates, for example. When temperatures are very high early in the season, before our bodies have a chance to adjust, the risks of heat stress and heat stroke may be higher.
One study of heat waves in 43 U.S. cities found that the first heat wave in a community generally had greater impacts on mortality than heat waves that were not the first in the season. Heat-related mortality risk early in the heat season is even greater for those who are more vulnerable to the impacts of heat, such as young children, older adults, pregnant people, or those with chronic medical conditions that make them more susceptible to heat-related illness.
Because of the ways climate change increases temperature and temperature variability, these early season extreme heat events may become more common. Whenever temperatures are high, and particularly during extreme heat events in the early heat season, it is important to take precautions to protect yourself and those around you.
During June–July 2021, the western U.S. experienced a record-breaking heat wave that lasted for several days. Estimated heat-related deaths and illnesses demonstrate the tragic toll of the heat wave on public health. Comparing the health records from June 26–July 10 between 2021 and 2020, heat-related deaths increased from 2 to 145 in Washington, 0 to 119 in Oregon, and 12 to 25 in California. These estimates were provided by the California Department of Public Health, Oregon Health Authority, and Washington State Department of Health. For context, the CDC estimates an average of 702 heat-related deaths per year for the entire U.S. (based on 2004–2018 data).
An article from the Administration for Strategic Preparedness and Response Technical Resources, Assistance Center, and Information Exchange features health care stakeholders sharing how lessons learned during the 2021 heat dome event and robust regional and local collaboration and communications during the pandemic facilitated connections during the heat wave in the summer of 2022.
In April 2022, Agency for Healthcare Research and Quality used the Healthcare Cost and Utilization Project (HCUP) 2016–2019 to estimate county-level population rates of emergency department (ED) visits with a diagnosis directly indicating heat exposure. The analysis was limited to records of ED visits, regardless of hospital admission, at community hospitals, excluding rehabilitation and long-term acute care facilities, with any-listed diagnosis directly indicating heat exposure. This analysis includes ED data from 2,550 counties in 39 States and the District of Columbia, representing 85 percent of the population and 81 percent of all counties in the United States in 2019. Among the 1,122 rural counties, 152 (13.5%) had heat-related ED visit population rates of 85 or more per 100,000 population (i.e., 90th percentile of population rates). In contrast, among the 344 large metropolitan counties, 8 (2.3%) had heat-related ED visit population rates in the 90th percentile. The report highlights that a larger proportion of rural than large metropolitan counties experience a high rate of heat-related illness, although there are more heat-related ED visits in large metropolitan areas (n=135,585 ED visits) than in rural areas (n=30,115 ED visits).
Resources for People at High Risk of Heat-Related Health Problems
Certain populations with limited resources may have restricted access to information on heat illness prevention, cool indoor environments, and government programs that provide critical support. Find more resources on heat illness prevention from Heat.gov and CDC websites.
Occupations that require strenuous work outdoors pose a high risk for heat-related illness. This includes construction workers, farmers, agricultural workers, delivery workers, athletes, landscapers, and others. Learn more about the dangers of working in heat. Employer responsibilities and resources for safety are also available through the Occupational Safety and Health Administration (OSHA) Heat Illness Prevention campaign.
- The Heat Safety Tool provides real-time heat index and hourly forecasts, specific to your location, as well as occupational safety and health recommendations from OSHA and the National Institute for Occupational Safety and Health (NIOSH).
- The National Institute of Environmental Health Sciences (NIEHS) Worker Training Program has heat safety and health training for at-risk workers.
- The Health Resource Services Administration (HRSA) funds National Training and Technical Assistance Partners — Farmworker Justice and Migrant Clinicians Network that helps clinicians prevent and treat heat-related illness among agricultural workers.
Image source: https://www.osha.gov/heat
A new resource from the National Integrated Heat Health Information System and the White House Extreme Heat Interagency Working Group highlights federal funding opportunities that are relevant to heat made available through the Inflation Reduction Act and the Bipartisan Infrastructure Law. These opportunities are open for applications from state, local, territorial, and Tribal governments; nonprofit organizations; manufacturers; and more. The webpage will be updated weekly as new funding opportunities become available.
- The Low Income Home Energy Assistance Program (LIHEAP) and the Weatherization Assistance Program (WAP) help keep families safe and healthy through initiatives that assist families with energy costs. To inquire about LIHEAP assistance, call the National Energy Assistance Referral (NEAR) hotline at
- HHS has issued guidance that for the first time expands how LIHEAP can promote the delivery of efficient air conditioning equipment, community cooling centers, and more.
- The National Institute on Aging resource Hot Weather Safety for Older Adults offers background information on heat stroke, who is at risk, lowering your risk, and best practices.
- Medicare Advantage (MA) plans may provide Special Supplemental Benefits for the Chronically Ill (SSBCI) with equipment and services that improve indoor air temperatures and quality (such as portable air conditioners) to chronically ill patients.
This CDC report on Heat Response Plans reviews steps emergency managers and health officials can take to develop and implement measures to protect their communities. Spikes in energy demand should be expected during summer months as air conditioning use increases. The combination of sagging power lines (copper expands as it heats up, thus increasing impedance and reducing throughput) and increased energy demands can cause power failures that make certain populations more vulnerable when the risk is highest. The HHS emPOWER program collects and shares de-identified Medicare data to help response agencies take action to protect the health of Medicare beneficiaries who depend on vulnerable electrical medical equipment.
Real-time information on health impacts from extreme heat can also help decision-makers implement strategies to reduce risk. CDC’s Heat and Health Tracker provides regular updates on the rate of heat-related Emergency Department visits (organized by HHS regions) and observed temperature.
If you are a local organization planning to open a cooling shelter, consider referring to CDC guidance on how to maintain a safe shelter during a heat wave.
Check out SAMHSA’s Tips for People Who Take Medication: Coping with Hot Weather for more information on how higher temperatures may impact your health if you are taking certain medications and steps to build resilience to climate impacts, as well as SAMHSA’s Climate Change and Health Equity site for more information on the behavioral health impacts of climate change, preparing for a disaster, and resources for disaster planning and climate change education.
A heat stroke is a medical emergency, and rapid recognition and aggressive early treatment are essential to reduce morbidity and mortality (as illustrated below).
See https://pubmed.ncbi.nlm.nih.gov/33856299/ for the full algorithm.
OCCHE’s Referral Guide summarizes resources that can address patients’ social determinants of health and mitigate health harms related to climate change. These resources include social services and assistance programs to which patients can be referred, as well as references for anticipatory guidance and counseling to help patients prepare for potential hazards.