Bad air pollution and extreme heat each raise health risks, but they're worse combined. Brian van der Brug/Los Angeles Times via Getty Images
On the morning news, you see the weather forecast is for high heat, and there is an “excessive heat watch” for later in the week. You were hoping the weather would cool down, but yet another heat wave is threatening human health and increasing the chance of wildfires. On top of these warm days and nights, air quality data has been showing unhealthy levels of pollution.
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Sound familiar? This scenario is increasingly the new normal in many parts of the world.
High heat and air pollution are each problematic for human health, particularly for vulnerable populations such as older adults. But what happens when they hit at the same time?
We examined over 1.5 million deaths from 2014 to 2020 registered in California — a state prone to summer heat waves and air pollution from wildfires — to find out.
Deaths spike when both risks are high
The number of deaths rose both on hot days and on days with high levels of fine particulate air pollution, known as PM2.5. But on days when an area was hit with a double whammy of both high heat and high air pollution, the effects were much higher than for each condition alone.
The risk of death on those extra-hot and polluted days was about three times greater than the effect of either high heat or high air pollution alone.
The more extreme the temperatures and pollution, the higher the risk. During the top 10% of hottest and most polluted days, the risk of death increased by 4% compared to days without extremes. During the top 1%, it increased by 21%; and among older adults over age 75, the risk of death increased by more than a third on those days.
Why risks are higher when both hit at once
There are several ways the combined exposure to extreme heat and particulate air pollution can harm human health.
Oxidative stress is the most common biological pathway linked with particulate air pollution and heat exposure. Oxidative stress is an imbalance between production of highly reactive molecules known as reactive oxygen species, or ROS, and the body’s ability to remove them. It’s been linked with lung diseases, among other illnesses.
Antioxidants help clean up these molecules, but particulate air pollution and heat disrupt this balance through excessive metabolic ROS production and lowered antioxidant activity.
Our research also showed that the effects of particulate air pollution and heat extremes were larger when high nighttime temperature and pollution occurred together. High nighttime temperatures can interfere with normal sleep and potentially contribute to chronic health conditions such as heart disease and obesity, and disrupt how the body regulates temperature.
Older adults may be more susceptible to effects of extreme heat and air pollution exposure, in part because this stress comes on top of age-related chronic health conditions like heart disease, high blood pressure, diabetes or chronic lung disease. Impaired body temperature regulation in response to heat can also occur with aging. And older adults may be less mobile and therefore less able to get to cooling centers or to medical care and be less able to afford air conditioning.
A future of high temperatures and air pollution
This isn’t just a California problem. Climate change will increase exposure to high heat and air pollution in many parts of the country.
Yearly average temperatures in the U.S. are already more than 1.8 degrees Fahrenheit (1 degree Celsius) warmer than at the beginning of the 1900s. By the end of this century, global temperatures are on pace to be nearly 5 F (2.7 C) warmer. Dangerous extreme heat waves, currently rare, will become more common.
Changing climate is also affecting levels of outdoor fine particulate pollution — for example, through weather changes such as air stagnation events, wind and dust storms, and drier and warmer conditions that contribute to increasingly frequent and intense wildfires.
What to do to stay safe
Further research is needed to better understand these effects, such as the full impact of wildfire smoke exposure. However, enough is known that people should take measures to reduce their risk of harm during periods of extreme heat or air pollution.
That means staying well hydrated and keeping cool. Shopping malls and other air-conditioned public spaces can provide a refuge from heat. Home air conditioning, especially during nighttime, can reduce mortality. A portable air filter in the bedroom can markedly reduce particle pollution levels.
People with symptoms of heat stress, such as headache, nausea, dizziness or confusion, especially the elderly, should seek medical care.
Many county and state health departments already provide alerts about extreme heat and extreme air pollution. Developing a special category of alert during co-occurring extremes may be beneficial to public health.
Governments also need to take steps now to avoid the worst future climate change scenarios. Some best practices for cities include creating cooling shade cover and green space that will also reduce particle pollution.
Erika Garcia receives support from Southern California Environmental Health Sciences Center, National Institute of Environmental Health Sciences grant P30ES007048, and the University of Southern California Office of Research Strategic Development of Research Award.
Rob Scot McConnell receives funding from the National Institute of Environmental Health Sciences
Md Mostafijur Rahman does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
This article is republished from The Conversation under a Creative Commons license.
These cities are better at enduring extreme heat. Here's how they're doing it
Beating the heat
Blistering heat has returned to western Europe, as some countries like France enter into their third wave of the summer with temperatures expected to reach over 100 degrees Fahrenheit (37 degrees Celsius), while more than 80% of the U.S. population will experience temperatures over 90 degrees (32C) within the next week, including in the Mid-Atlantic and Northeast. Around 100 million Americans have been under heat alerts for eight of the past 16 days.
This means hundreds of millions of people living in urban areas are again desperately trying to stay cool.
Air conditioners might keep the indoors cool, but they only add to heat outdoors. And in most cases, they are adding to the climate crisis by increasing planet-warming emissions. Public transportation may be unbearable on a hot day, but driving a car that runs on gas instead just worsens traffic, also adding to heat and emissions. A lack of trees means a lack of shade, and buildings made of dark materials bring hotter interiors, which means more air conditioning.
It's a vicious cycle, but there are other solutions.
Here's how eight cities are taking some of the heat out of their summers.
Medellín, Colombia: Grow trees on the streets, not just in parks
When it gets really hot, people with air conditioning might stay indoors, but not everyone has that luxury and — well, who wants to say in all the time?
For cities that aren't on the coast, parks that offer shade are a good option. Colombia's second-largest city, Medellín, however, has created an entire metropolis of shade with its award-winning Green Corridors project.
The web-like network has transformed 18 roads and 12 waterways into lush green cycling lanes and walkways that connect the city's parks and other frequently visited sites.
Temperatures have come down in these areas and their surrounds by as much as 5.5 degrees Fahrenheit (around 3 degrees Celsius), and officials hope that before 2030, it could shave off up to 9 degrees Fahrenheit (5 Celsius).
"Urban forests are the very best thing for city heat," Kathy Baughman McLeod, director of the Adrienne Arsht-Rockefeller Foundation Resilience Center (Arsht-Rock) at the Atlantic Council, told CNN. "Medellín has dropped the city's average summer temperature, which is remarkable."
By 2019, the city had planted more than 8,000 trees and over 350,000 shrubs. It also uses an area beneath a raised Metro line to collect rainwater that flows down from the bridge, capturing it in a system of pipes to help water the green belts.
Vienna: Splish, splash, splosh
Like in much of Europe, many in Vienna don't have air conditioning, so water is a big part of how the Austrian capital keeps cool.
For those who don't have time for a dip in the Danube, the city offers cooling parks with mist-spraying "trees" that people can either "shower" in, or just sit near to enjoy the cooler temperatures they bring their surrounds.
Children, who are generally more vulnerable to extreme heat than adults, are often seen playing in the city's splash pools or running around in pop-up water features — typically hosepipes with holes punched in them — that the city government brings out on the hottest of days, including in areas like Karlsplatz, a popular city square.
Vienna also has a huge number of water fountains for drinking to keep people hydrated — more than 1,100 for its population of 1.9 million — which is important in preventing heat-related illness.
"Air conditioning in homes may sound like a quick and easy solution. But it's not a long-term sustainable solution because of the source of the power and the waste heat that comes off the unit," McLeod said. "So thinking about how to get more airflow, use water features and get windows to open in some of the oldest buildings is key. The nature-based solutions are the best for extreme heat."
Abu Dhabi, United Arab Emirates: Use old cooling techniques and modernize them
Parts of the Middle East are some of the hottest inhabited places on Earth. Temperatures in Abu Dhabi can climb to more than 120 degrees Fahrenheit (over 50 Celsius). Air conditioning is seen as a necessity, and people tend to spend a lot of time indoors.
But people here haven't always had air conditioning, and an ancient Arabic architectural cooling technique has made a comeback — with a modern twist.
Mashrabiya refers to the latticed screens often seen in Islamic architecture, sometimes surrounding a small balcony, that diffuse sunlight and keep buildings cool without completely blocking light. They are designed to encourage a breeze and offer a spot of respite from the heat within a building. The idea is essentially to stop direct sunlight landing on a building's exterior.
That's what inspired the design of Al Bahar Towers, a 25-story building wrapped in more than 1,000 hexagonal shades with built-in sensors that allow them to respond to the sun's movements. When the sun hits the shades, they unfold like an umbrella to ward off the heat. Without these measures, the outside of such a building in Abu Dhabi could reach as high as 200 degrees (around 90 Celsius).
The technique has helped reduce the building's need for air conditioning by 50%. Cool, huh?
Miami: Target heat traps
In a lot of cities, catching the bus can mean a long wait. If it's really hot, the wait can be all the more punishing — unless, of course, that bus stop has been thoughtfully engineered to include natural shade.
Medellín in Colombia may have proved that urban forests, or simply planting more trees, can cool a city a down, but Miami-Dade county has put a lot of thought into exactly which parts of the city need cooling the most.
Neat Streets Miami, a board convened by the county council, recognized that bus stops had become real danger zones during heat waves, so they planted trees around 10 stops. They wrote a guide on which trees work best and where to plant them so that other areas could replicate the project.
And that they have. There are now 71 green bus stops in the country, most of them by communities that applied to the government for resources to green their own bus stops.
To make it more fun, the organizers also held a haiku poetry competition, and selected the best 10 to etch into the sidewalks by the original stops.
The trees have also
missed their bus — look how they wave
their many sad arms
— Ariel Francisco
Athens: Work with what you've got
Not every city has an ancient aqueduct at their disposal, but the Greek capital of Athens does. The Hadrian aqueduct was once used as a main source of water, using a system of pipes that worked with gravity to allow water to flow from its source to the city for human consumption.
The water today isn't drinkable, but the city is looking at ways to salvage the 800,000 cubic meters of water that flows off as waste into the sea each year. One use will be to irrigate new greenbelts to run all along the 20 kilometer structure, which should help take the heat out of the areas around it. The water will also be used for misting, like in Vienna.
Even for cities without infrastructure quite this old, Athens is a good reminder that defunct water systems can sometime be revived.
Los Angeles: Paint the town white
This one is a little more controversial.
Some cities have experimented with painting rooftops white to reflect sunlight and keep buildings cool, but Los Angeles went a step further and is painting entire roads white. Dark things like asphalt absorbs sunlight and emits that energy back into the air as heat. Painting the asphalt white would theoretically nip that process in the bud, and lead to cooler air temperatures.
The idea has some merit. Researchers Ariane Middel and V. Kelly Turner found that technique did cool the streets themselves by around 10 degrees. But there was a major knock-on effect. The same researchers also said it was likely the extra heat reflecting off the roads was being absorbed by ... people.
That means if you're a few blocks away, the white streets may help you feel cooler, but if you're on the street, you could actually feel hotter.
Nonetheless, LA is continuing with this program to see what works and what doesn't. It currently uses a grayish-white substance called CoolSeal, once used to help hide grounded aircraft from satellites, but it's possible that another type of paint could yield different results.
Painting rooftops has had greater success.
Results vary depending on the level of heat and materials a roof is made of, but in places like Ahmedabad in India, which gets seriously hot, cool roofs have shaved 3-8 degrees Fahrenheit of the heat in homes. According to Berkeley Lab's Heat Island Group, a black roof could be as much at 54 degrees (around 30 Celsius) hotter than a white roof.
Another option is the green roof. Cities all around the world have created "gardens in the sky" to cool down buildings.
Paris: Get really organized
The French capital gets seriously hot.
Temperatures there have surpassed 104 degrees Fahrenheit (40 Celsius) this summer, but the combination of high-rise buildings, limestone monuments and busy asphalt roads means it can feel even hotter.
The city has a strong urban heat island effect, where it is often 18 degrees hotter in the city center on a summer's day than it is in the Parisian hinterlands.
But Paris Mayor Anne Hidalgo has been credited with putting in place some of the world's most innovative measures to combat heat, and the city's heat plan is truly comprehensive.
The main result is a city full of "cool islands." Parisians can use an app called EXTREMA to guide them to more than 800 cool spots — parks, water fountains and air-conditioned museums, for example — and get there via a naturally cooled walkway. The idea is that a cool island is always a maximum seven minutes' walk away for everyone.
Like Vienna, Paris uses mist machines on hot days. It also has dozens of new "splash fountains," in addition to its many traditional fountains, which are very shallow pools with fountain-like effects.
Paris' heat plan involves a register that identifies the most vulnerable, so officials can check up on them by phone and offer advice on staying cool. Kindergartens get temporary air conditioners in their classrooms, and public parks and pools stay open for longer hours into the night. And like LA, Paris is trying to take the heat out of its roads and sidewalks by "demineralizing them," using more porous materials. Now that sounds like a plan.
Seville, Spain: Name your heat waves
The world has been naming hurricanes, cyclones and typhoons for decades for a reason: A named storm makes you sit up and pay attention. The southern Spanish city of Seville is taking that approach with heat waves, becoming world's first to do so.
The July heat wave there was named Zoe.
"Naming heat waves is a positive thing because it means we're recognizing how lethal they are, and that they're here to stay. It's not a fluke heat wave," the Arsht-Rock's McLeod said. "This is this is something we're going to be living with for a long time, no matter what we do with our emissions."
But there's more to what Seville is doing than naming. Arsht-Rock is working with Seville on a new categorization system for heat waves based on projected negative health outcomes. The idea is to avoid scientific jargon that most people don't understand and link alert levels to what a heat wave is likely to do to people.
A 2018 Brown University study of 20 heat warning systems in the United States found that only Philadelphia's heat warning system was effective in saving lives, partly because it uses health-based metrics.
"Besides physical interventions for heat, naming and categorizing heat waves is the best, most immediate thing you can do," McLeod said. "Because that's the key — heat is killing people, and that's because people are not aware of the magnitude of the problem."
