Extreme heat and the combination of high heat and humidity pose serious risks for human health. According to the National Weather Service, heat is the No.1 weather-related killer in the U.S. — more than tornados, floods, and hurricanes — and it is estimated that between 600 and 1,500 heat-related deaths occur in an average summer in the U.S.1,2 Individual heat waves can be even more deadly. The 1995 heat wave in Chicago is estimated to have led to more than 700 deaths and in excess of 1000 more hospital admissions than normal.3,4
This deadly risk is not likely to go away. By the end of the century, heat-related deaths are projected to increase by thousands to tens-of-thousands each year in the U.S.5 Those most at risk of heat-related health impacts are infants and young children, elderly over 65, those already ill, athletes, and outdoor workers2. But everyone is potentially at risk.
Elevated heat, especially along with high humidity, makes it difficult for the body to cool itself. In addition to increasing the risk of mortality, heat can cause problems throughout the body. It can range from dehydration, cramps, exhaustion, dizziness, vomiting and heat rash to more serious issues involving kidney failure, heart issues, and exacerbation of respiratory issues6,7. These heat impacts also provide a challenge to the healthcare industry with increased hospitalizations and doctors’ visits and insurance claims.
High heat also impacts other sectors and infrastructure. Stagnant air often occurs during periods of elevated heat and allows dangerous levels of air pollutants to build up. High temperatures also directly provide conditions conducive for producing harmful ground-level ozone. Periods of extreme heat can wither crops and exacerbate drought conditions greatly impacting agriculture. Blackouts often accompany heat-waves as the need for cooling puts a heavy strain on the power grid. Heat waves can also lead to harmful algal blooms and promote other bacterial growth in bodies of water and lead to degraded fish habitat, such as for species that require cooler streams and rivers.
The combination of hot temperatures and high humidity create dangerous conditions for humans. The National Weather Service defines as dangerous any day when the heat index (the combination of heat and humidity, commonly known as the “feels like temperature”) exceeds 104°F. Under these conditions, sunstroke and heat exhaustion are likely, and physical activity or being outside for long periods is risky, potentially leading to heat stroke. These dangerous heat days pose the greatest threat to kids and the elderly, and to people who don’t have easy access to air conditioning.
We analyzed 360 of the biggest U.S. cities to see how the average number of danger days is projected to increase in the coming decades. The projections draw on 29 global climate models that have been downscaled across the continental U.S. to represent local climate conditions.
The top 25 U.S. cities expected to see the most danger days by 2050 are:
The 25 U.S. cities projected to see the biggest increase in danger days over current conditions are:
Even in the absence of high humidity, extremely hot days pose a considerable health threat, particularly under prolonged exposure. Across most of the U.S. temperatures have increasingly been exceeding 90°F, 95°F, and 100°F since 1970.
Our analysis of trends in extreme heat days is based on annual counts of 90°F, 95°F, and 100°F exceedances in the country’s largest 200 cities. The hottest cities are seeing the biggest average increases in extreme heat days, in general.
The top 25 cities that have seen the biggest increase in annual average days above 90°F since 1970:
The top 10 cities that have seen the biggest increase in 100°F days are:
Since 1970, summers have been warming in 45 of the lower 48 states. In many of these states, this warming is driven largely by nighttime temperatures getting hotter.
As temperatures rise, evaporation increases, causing increased water vapor in the air. That extra moisture makes the air feel muggier, and can make it a lot more difficult to tolerate the heat because our bodies have a harder time keeping cool through perspiration. As summers warm across the country from increasing greenhouse gases, cities are also getting sticker.
Climate Central analyzed how the average summer dew point has changed in 200 major U.S. cities since 1970 (the dew point is a measure of how much moisture is in the air). We found that 87 percent of those cities have experienced an overall increase in their average summer dew point over the past 46 years, indicating that there is typically more moisture in the air on hot summer days now than there used to be.
The top 25 cities seeing the largest increase in summer air moisture since the 1970s are (several of which are among the fastest warming cities in the country):
Climate Central is an independent organization that researches and reports on climate change.
Analysis by Alyson Kenward, PhD, Jennifer Brady, James Bronzan and Todd Sanford. Read full methodology.
1. Harvard Medical School (2005), Climate Change Futures: Health, Ecological and Economic Dimensions, Cambridge, MA: The Center for Health and the Global Environment, Harvard Medical School.
2. Centers for Disease Control, Extreme Heat and Your Health, 2011
3. Palecki et al. (2001), Bulletin of the American Meteorological Society. The Nature and Impacts of the July 1999 Heat Wave in the Midwestern United States: Learning From the Lessons of 1995. 82:7, 1353.
4. Semenza et al. (1999), Am. J. Preventive Med. Excessive Hospitalizations During the July 1995 Heat Wave in Chicago. 16:4, 269.
5. U.S. Global Change Research Program (2016), The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment.
6. Becker, J.A. and L.K. Stewart (2011), American Family Physician. Heat-related illness. 83:11, 1325.