Heat Waves in the U.S. Can Be Deadly and Are Getting Worse

Microclimates occur within large cities like San Francisco, New York and San Diego, and heat waves can exist and not exist in different areas within those cities, putting vulnerable residents at risk of death.

by Jim McKay / May 22, 2019
Children play in Salmon Street Springs as temperatures hovered in the mid-80s in Portland, Oregon on May 13th 2014. Alex Milan Tracy/Sipa USA

What weather-related threat causes the most deaths in the United States each year?

If you thought floods, you were wrong. It’s heat.

Heat waves can occur just about anywhere in the United States and can even happen in different neighborhoods within cities. Definitions of a heat wave can vary. The World Meteorological Organization’s definition of a heat wave is five or more straight days of heat that exceeds the daily average temperature by 9 degrees Fahrenheit.

It’s difficult to determine exactly how many heat-related deaths occur each year in the United States because some are caused indirectly by heat or exacerbated by heat and not listed as having been caused by heat on the death certificate.

According to the National Resources Defense Council, heat waves kill an average of 1,300 Americans and send more than 65,000 to the emergency rooms each year. Tarik Benmarhnia, a public health researcher at the University of San Diego, called that a vast underestimate.

Phoenix had a record-setting 172 heat-related deaths in 2017, an increase from the then-record 150 in 2016 and 85 in 2015.

A heat wave can cause death directly and indirectly and can happen almost anywhere in the U.S.  Not surprisingly, minorities and the poor suffer more than their more affluent counterparts.
A European heat wave killed more than 70,000 in 2003 and 10,000 in Russia in 2010. The 1995 heat wave in Chicago killed 739 in a five-day span.

“Heat is the most dangerous of events, even if it doesn’t seem so,” Benmarhnia said.

He cited the horrific smoke from the recent Camp Fire in Northern California, which plagued much of the state. He said that happens once or twice a year in a certain region, whereas heat waves can happen all over the country during the warmer months.

“Sometimes there is co-morbidity,” Benmarhnia said. “You could have heat stroke but die from infection or other causes exacerbated by the heat.” Heat can cause indirect morbidity, exacerbating conditions like diabetes, cardiovascular problems, dementia and other disease.

Heat kills when the body’s ambient temperature reaches a point where thermoregulation overwhelms the cardiovascular system. When the body’s ambient temperature begins to elevate, blood vessels dilate near the skin to transfer heat from the body to the skin. The higher the temperature or the longer the heat wave, the harder the cardiovascular system has to work to regulate temperature.

Excessive heat can directly cause heat exhaustion and then heat stroke, which can lead to death. This is the direct form of morbidity associated with heat and affects those who work outside or are otherwise outside for an extended period and are exposed to excessive heat.

There is no one definition of a heat wave, and it can vary from place to place in the United States and even regionally, where urban planning can mean the difference between 7 degrees in one community compared to a neighboring one.

“What is a typical temperature in Phoenix, Ariz., may be a heat wave in Seattle, Wash., Benmarhnia said. “There is spatial variability even within a city. There is a lot of variability about how people are going to die or not going to die, even in San Diego.”

Benmarhnia said there are microclimates within large cities like New York, San Francisco and San Diego that are influenced by variation on construction of homes and green spaces that can make a big difference in temperature. “That may explain why if you live in a neighborhood with a lot of green space, you may be less vulnerable as opposed to no green space, no trees, where the local temperature may be really high.”

Vulnerable populations can exist in cities where neighboring populations are protected from the heat because of better technology and urban planning. The poor and minorities are more vulnerable generally because of where they live. “Living in a poor neighborhood with no green space, you’re also going to be exposed to high levels of pollutions, which may lead to asthma or other co-morbidities that may make you more vulnerable to a heat event,” Benmarhnia said.

The situation could worsen as the climate changes, although adaptation to heat can and does take place.

Right now, 30 percent of the world’s population lives in areas where heat kills. The planet is expected to warm another 1 degree Celsius by 2100 if we halt greenhouse gases and by 3.7 degrees if we don’t.

According to the Natural Resources Defense Council, the 1,300 deaths per year average could grow to 13,000 by 2040 at the current rate.

“We expect climate change to make the heat waves even worse than now and happen more often,” Benmarhnia said. “We will have more extremes, which means heat waves, however we define them, will happen more often and will last longer and probably be earlier in the season.”

At the same time, he said, adaptations can occur with heat warnings, better urban planning and improving green spaces.