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Why extreme heat is so alarming for the fight against Covid-19

“I’m actually really worried about indoor transmission.”

People walk through the street near downtown on July 1, 2020 in El Paso, Texas.
Cities like El Paso, Texas, are coping with temperatures above 100 degrees Fahrenheit alongside the Covid-19 pandemic.
Cengiz Yar/Getty Images
Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

A heat wave is baking much of the United States this week, with some of the highest temperatures forecasted in Southwestern states battling some of the most troubling coronavirus outbreaks in the country.

Arizona, for instance, is currently suffering from one of the worst outbreaks of Covid-19 with the highest daily reported cases per capita in the country. Meanwhile, Phoenix hit a high of 109 degrees Fahrenheit in recent weeks, and the National Weather Service issued an excessive heat warning for the region through the weekend.

Parts of Texas are also experiencing extreme heat, with temperatures rising into triple digits in cities like El Paso. The Covid-19 outbreak in the state has also worsened in recent weeks, with more than 10,000 new cases reported on Tuesday, beating a previous record.

These states were among those that began relaxing some of their pandemic control restrictions, like closing non-essential businesses, before infection rate had a chance to decline. Now, as the number of new infections and hospitalizations has surged, governors of these states have reimposed some measures and implemented new ones, like mandatory face masks.

However, the combination of extreme heat and a fast-spreading virus in the Sun Belt is now creating a new set of problems that could undermine efforts to control Covid-19. From hampering surge capacity plans for hospitals to increasing people’s likelihood of getting exposed to the virus while sheltering indoors from the heat, heat can make things harder. And temperatures are poised to rise even higher in the southwest in the future due to factors like the urban heat island effect and climate change.

But there are ways to mitigate some of these risks. In particular, tactics like increasing ventilation reduces the likelihood of transmitting the infection indoors. These measures will be key to making schools, offices, and public spaces safe enough to reopen.

The many ways that extreme heat can worsen Covid-19 risks

Early in the pandemic, many hoped summer weather would reduce the transmission of Covid-19. Based on patterns with past coronaviruses, some scientists suggested that factors like ultraviolet light on sunny days, humidity, and heat could potentially reduce the spread of Covid-19 by impairing the virus itself. But the evidence from the United States and other parts of the world shows warmer temperatures have done little to curb the rise in new cases.

“In the context of this escalating pandemic, weather is pretty far down on the list of things that influence spread,” said Katherine Ellingson, an assistant professor of epidemiology at the University of Arizona, in an email.

However, there are several indirect ways that extreme heat can alter how Covid-19 moves through a population and potentially worsens the outbreak.

People spending more time indoors

When temperatures get searingly hot, people spend more time in enclosed spaces, which presents the greatest opportunity for infection if the virus is present. “I’m actually really worried about indoor transmission,” said Davidson Hamer, a professor of global health and medicine at Boston University. “It’s so hot in the Southwest US that people are not outside. They’re being driven inside, so then you have all the issues of aerosol transmission and recycled air, a lot of which honestly we don’t fully understand yet.”

For a state like Arizona, it can be difficult to go outside at all for days at a time. But it’s not clear yet whether this will raise or lower the transmission rate of Covid-19.

“Most of us are indoors, and that in some ways is good because it encourages people to stay home, but it some ways it’s also very challenging because people want to go be around others,” said Saskia Popescu, an infection prevention epidemiologist at George Mason University based in Arizona.

If the heat keeps people isolated, that could slow the pandemic.

However, if parks and open air cafes are uncomfortable in the heat, people are likely to hop from one air-conditioned space to another — from a house, to a car, to a store, to restaurant, and so on.

That’s why infection risk can remain high if people are frequently in groups, even small ones.

“I think people assume, ‘Oh, I’m not in a 50-person spin class inside, I’m just having a few people over, that’s not as risky,’” Popescu said. “When we focus solely on large gatherings, that can be a poor communications strategy.”

Another key concern is that with so many people infected now, there may be a rise in new Covid-19 infections within households as people shut doors and windows and switch on the A/C.

Workplaces for people with essential jobs can also become major sources of transmission as they switch on cooling systems and seal off the outdoors.

So many people can end up in high-risk scenarios for Covid-19 that are nearly impossible to avoid. “And this is where vulnerability to infection comes as a result of socioeconomic and occupational risk factors, not individual choices about where to eat, socialize, or exercise,” Ellingson said.

Increased air pollution

Another issue during heat waves is that air pollution tends to get worse on hot days. Pollutants like ozone form more readily in high temperatures, which in turn can exacerbate breathing problems. That’s particularly troubling for a respiratory infection like Covid-19 that has shown a link between more severe illness and air pollution. In poorly sealed buildings, that outdoor pollution can become indoor air pollution, often leaving the poorest residents of a city little respite.

Building surge capacity for health care becomes more difficult

Extreme heat also poses problems for the response to Covid-19. As hospitals approach their limits of beds, they may struggle to implement their backup plans.

Things like outdoor tents and temporary clinics like those deployed in New York City are much harder to set up in the heat and more expensive to run when they have to be air conditioned. “Those capabilities quickly go out the window,” Popescu said.

Avoiding health impacts of heat can create opportunities for transmission

Heat itself is a potent health threat and is the deadliest form of extreme weather, according to the National Weather Service. High temperatures make it harder for the body to shed excess heat, straining breathing and circulation. That can then lead to potentially fatal conditions like hyperthermia and heat stroke.

While most people can avoid the most extreme heat, not everyone has access to air conditioning or can afford the power bills to pay for it. Places like Maricopa County in Arizona do operate public cooling shelters for people who don’t have access to their own cool spaces. But that also creates another opportunity for people to gather in an enclosed space.

The Centers for Disease Control and Prevention has issued Covid-19 guidance for cooling centers that include social distancing, as well as calling on local governments to assist low-income residents with utility bills so they can pay for cooling in their own homes.

Improving indoor air could help curb Covid-19 transmission during heat waves

Right now, scientists are still reckoning with how the virus spreads between people. Initially, the main modes of transmission were thought to be surfaces and also respiratory droplets that usually fall out of the air within 3 to 6 feet of their origin. That’s what informed guidance to wear masks, sanitize surfaces, and stay 6 feet away from others.

But a group of more than 200 scientists this week have called on the World Health Organization to recognize aerosols — tiny particles that can stay aloft longer— as a contributor to Covid-19 infections.

“All the data over the last couple of months point to aerosols as a major mechanism for transmission of this virus,” Hamer said.

That would mean that the virus could spread through the air to a greater degree than some previously thought. This has important implications for strategies to control the spread of the virus, particularly indoors where it can circulate in the air. One preprint study (research that has not yet been peer-reviewed) found that infectious virus particles could be found in an aerosol up to three hours after the aerosol was formed.

The duration can change depending on how much air is circulated in the space, whether the air is filtered, and factors like temperature, humidity, and lighting. But there is potential for Covid-19 to spread through ventilation systems.

“Regarding the spread of the coronavirus via ductwork, the short answer is yes, it is plausible,” said Parham Azimi, a postdoctoral researcher studying environmental health at the Harvard T.H. Chan School of Public Health. “Definitely, in indoor environments, with low ventilation rates or without supplemental control strategies like air purifiers and filtration, we would face a lot higher risk of getting infected.”

In another preprint paper, researchers in the United Kingdom looked back at the Covid-19 outbreak on the Diamond Princess cruise ship where at least 700 passengers out of 3,711 tested positive for the virus. The researchers concluded that inadequate ventilation was a key factor in the spread of the disease on the ship. And another paper looking at a Covid-19 outbreak in a restaurant in Guangzhou, China, found that the direction of airflow from the restaurant’s air conditioner influenced which patrons ended up infected.

That means one of the more effective strategies for limiting the indoor spread of Covid-19 in places like classrooms would be to improve indoor air quality. “The easiest and fastest ways would be increasing ventilation or putting air purifiers inside of each class,” Azimi said. “That would help reduce airborne transmission.”

Similarly, the CDC guidelines for reopening schools recommend that facilities “increase circulation of outdoor air as much as possible, for example by opening windows and doors.”

Increasing ventilation means that less air is stagnant in a given space, so a potentially infectious aerosol has less time to linger. But opening windows during triple-digit temperatures can cause more problems than it solves. And improper ventilation that doesn’t adequately cycle air out of a room could just end up spreading aerosols around.

Air purifiers can also clear out aerosols by using devices like HEPA filters. However, purifiers are limited by how quickly they work since they can only process a small volume of air at a time, becoming ineffective in larger rooms.

There are also more sophisticated ways to improve air quality too, like ultraviolet germicidal irradiation, where ultraviolet light is used to sterilize air. These devices can be integrated into HVAC systems to clean a large volume of air. It’s effective, but expensive, requiring costly hardware and installation. For schools and businesses struggling with their finances amid the pandemic, tactics like UVGI could be out of reach.

The lessons learned from fighting the pandemic during a heat wave could still be applicable in cooler times of year, according to Azimi. Improving ventilation would help reduce infection risks in the winter as people similarly spend more time indoors to avoid the cold.

But one side effect of many of these measures is that they increase energy consumption. During a heat wave, running HVAC systems at a higher capacity or installing air cleaning devices adds to power demands. For consumers, that raises their energy bills, and for utilities, that adds stress to the power grid. Researchers have found that the risk of power outages rises under extreme heat.

The Texas grid operator ERCOT said in May that it’s anticipating record electricity use this summer in the state, driven by heat. While it has more reserve power this year than last, ERCOT says it may still have to issue energy emergency alerts if power demand rises too high.

So improving indoor ventilation during a heat wave isn’t simply turning fans on full blast. It requires planning, balancing costs and benefits, and infrastructure to support it.

And Azimi cautioned that ventilation alone isn’t enough to reduce the risk of indoor transmission of Covid-19; it’s just one piece of a comprehensive strategy that includes measures like physical distancing between people and wearing masks. It is possible to stay cool and stay safe from Covid-19, but it requires anticipating risks and planning for them.