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Spring 2021

Airpocalypse Now

Northwestern alumni and faculty break down the deadly consequences of outdoor air pollution. By Amanda Morris

Sunset in Mexico CityImage: Miguel Megevand/Dreamstime

Franz Geiger stood in the middle of Mexico City’s Plaza de la Constitución in June 1990, watching the traffic chug around the city’s epicenter. It was the first day of the rainy season, but cloud cover was sparse. Then suddenly the skies opened and rain deluged the plaza. The abrupt downpour soaked through Geiger’s clothes and turned his crisp, white T-shirt a grimy, streaky black.

“During that time, Mexico City was experiencing the worst air pollution in its history,” says Geiger, a Northwestern chemistry professor. “It was amazing. I remember the rain feeling sticky on my skin. It was a mixture of rain, road dust, combustion engine exhaust and rubber from tires, all stuck together.”

Air pollution is everywhere. According to a recent study led by Harvard University, 8 million people died in 2018 from fossil fuel air pollution. Other recent studies have found air pollution is also linked to lost pregnancies and mental health crises, as well as increased risk for dementia and sight loss. According to the American Lung Association, nearly half of all Americans live in counties that have unhealthy levels of air pollution.

Northwestern researchers and alumni in various fields are working to better understand how air pollution travels from smokestacks and tailpipes into the atmosphere, how it affects our health and why it impacts some populations more than others. Their ultimate goal is to find solutions that could protect our health and the environment.


Air pollution is primarily made up of three substances: ozone, fine particulate matter (or soot, dust and smoke) and nitrogen oxides. All three are byproducts of mining operations, electricity generation and agricultural activities, but vehicles with combustion engines are among the worst contributors. “In the U.S., the transportation sector is the No. 1 source of air pollution and greenhouse gas emissions,” says Daniel Horton, assistant professor of Earth and planetary sciences.

“Car pollution is particularly threatening for population health because, by definition, cars drive and pollute where people live,” adds Hannes Schwandt, assistant professor of human development and social policy. “It’s pollution in the middle of society, including dense population centers.”

Vehicles’ high-temperature engines burn fossil fuels, generating nitrogen oxides, which are harmful to human health. Nitrogen oxides also can mix with other gases in the atmosphere and — when exposed to sunlight — form ozone. Not to be confused with “good” ozone in the stratosphere — the kind that protects us from the sun’s ultraviolet rays — ground-level ozone can damage lungs and constrict airways, making it difficult to breathe. It also worsens respiratory diseases such as asthma.

From a health perspective, ozone is problematic but less life-threatening than other pollutants. “If you’re looking for the cause of early mortality, poor lung development in children or exacerbation of chronic obstructive pulmonary disease, by and large that will be particulate matter air pollution, which comes from burning fossil fuels,” says Scott Budinger ’85, the Ernest S. Bazley Professor of Airway Diseases at the Feinberg School of Medicine.

“In the U.S., the transportation sector is the No. 1 source of air pollution and greenhouse gas emissions.” — Daniel Horton

Clotting Consequences

Perhaps surprisingly, air pollution–related deaths aren’t caused by lung damage or respiratory failure alone. Since the 1930s, researchers have linked air pollution to an increased risk of heart attack and stroke. This was most famously demonstrated during the London smog events in the 1950s. Whenever levels of smoke — generated from coal-burning power plants and coal furnaces in homes — increased, mortalities spiked two days later. Subsequent studies found all air pollution–related deaths after a smog event were from heart attacks and strokes. Similarly, when a poisonous haze blanketed Beijing for nearly a week in 2013, hospital admissions surged by 30% during the city’s so-called “airpocalypse.”

But how does inhaling polluted air affect the heart? Budinger, who studied chemical engineering as an undergrad at Northwestern and is now the chief of pulmonary and critical care at Northwestern Medicine, set out to determine the pathways that particulate matter travels to affect heart health. His team discovered the cascade begins with the alveolar macrophage, or “dust cell,” in the lungs. Responsible for cleaning up particles that enter the lungs, these cells activate when they encounter soot. This induces a cascade of signaling that ultimately releases a group of proteins and small molecules that cause clotting.

“Once those molecules get into circulation, you are more prone to clotting,” Budinger says. “So we think inflammation in the lung increases the risk of blood clots forming in heart arteries, causing heart attacks and strokes.” By understanding this cascade, Budinger and his team were able to pinpoint already-approved medications that could decrease premature deaths.

Pollution in our Backyard

These health issues typically strike disadvantaged populations more severely, as their neighborhoods are often disproportionately overburdened with pollutants. “Housing is much cheaper in areas where the land and air are contaminated,” says Nancy Loeb, who directs the Environmental Advocacy Center at the Pritzker School of Law’s Bluhm Legal Clinic. Because the hazards of air pollution exposure are tangled with the other issues that face low-income populations — including reduced access to affordable health care and higher levels of stress — it becomes difficult to tease apart how air pollution specifically affects the human body.

“Lower-income communities do experience more health issues and higher instances of certain illnesses,” says Loeb, a clinical professor of law. “It’s hard to say there’s a single contributing factor, but there’s a good indication that air pollution contributes.”

Human behavior provides another tricky variable. “If you live next to a highway, you might try to keep your windows closed or avoid sitting outside,” Schwandt says. “It’s difficult to measure how pollution affects people because we might underestimate how much people shield themselves from pollution sources.” 

To help unravel these complex relationships, researchers look for “natural experiments.” For Schwandt, Volkswagen’s emissions scandal presented a perfect opportunity.

In 2015 Volkswagen sold cars that could detect when their emissions were being tested. When sensing a test scenario, the car automatically switched on a set of devices to lower its emissions. Once the car was on the road again, the devices switched off, enabling the vehicle to emit nitrogen oxide pollution 40 times higher than the U.S. Environmental Protection Agency’s limit.

“Suddenly, pollution increased among economically advantaged population groups in areas with many cheating cars,” Schwandt says. “We had this once-in-a-lifetime setting to study the effects of air pollution on more advantaged populations that didn’t live near highways or power plants.”

In areas with more of these particular Volkswagen models, Schwandt’s team found lower infant birth weights. Wealthier and healthier groups, such as married white mothers with college degrees, particularly felt these effects because “clean diesel” Volkswagen cars were marketed to those groups.

“Just informing people about these risks could have a really big impact,” he says. “It’s the consumers themselves who produce this pollution on a daily basis. And we care about the impacts we have on our and our neighbors’ health.” 

air pollution separator

Tricky Trade-offs

In another natural experiment, Franz Geiger and former Kellogg School of Management assistant professor Alberto Salvo, now an associate professor of economics at the National University of Singapore, spotted an opportunity to compare the effects of ethanol versus petroleum-based gasoline on air quality. Vehicles in Brazil can switch between the two fuel types.

“São Paulo is a natural laboratory because it has the largest flex-fuel vehicle fleet,” Geiger says. “We looked at the choices consumers made at the pump and how that affected air quality.”

After gathering and reviewing air quality data, the duo was shocked to find ethanol, which is often considered a “green fuel,” increased ground-level ozone.

The team dug into air chemistry to explain the process. They found nitrogen oxides actually curb ozone concentrations. When vehicles use ethanol, nitrogen oxide concentrations decrease, leaving ozone unchecked.

“The benefit is reduced nitrogen oxide and ultrafine particulate matter, but that comes at the price of increased ozone,” Geiger says. “Ethanol also carries heavy metals because it comes from corn and sugar that are grown using low-quality water. Because the crops aren’t going to be fed to humans or animals, wastewater is used and emitted when the ethanol is burned. We think ethanol is sustainable, but there’s a complex story behind it.”


Although the problems associated with air pollution feel overwhelming, solutions already exist. According to research from Daniel Horton’s group, switching to electric vehicles could massively benefit society, saving billions of dollars and thousands of lives. Horton’s group creates high-resolution air-quality simulations based on atmospheric chemistry, emissions data and meteorology.

“If you have an electric vehicle, you’re no longer emitting pollutants from the tailpipe,” he says. “However, emissions can be generated from the electricity source used to charge the battery.”

That said, electric vehicles have a net benefit in the U.S., where electricity is generated from diverse sources, including wind, solar and geothermal. The story is more complicated in China, where electricity is predominantly generated from burning coal.

When looking at the U.S., Horton and former student Daniel Peters ’19 combined their climate model with public health data. They found that if electric vehicles replaced 25% of combustion engine vehicles currently on the road, the U.S. would save approximately $17 billion annually by avoiding damage from climate change and air pollution. In a much more aggressive scenario — replacing 75% of cars with electric vehicles — savings could reach as high as $70 billion a year.

“Vehicle electrification could prevent hundreds to thousands of premature deaths annually,” says Peters, who now works for the Environmental Defense Fund. “It would not only curb greenhouse gas emissions but also reduce the health burden of harmful air pollution.”

While electric vehicles might be unaffordable for many Americans, Horton believes that recent commitments by automakers to manufacture more electric models — in conjunction with a renewed push for federal and state governments to offer tax breaks, subsidies and incentives — should make widespread adoption possible. Cities also could encourage adoption by installing more vehicle charging stations and electrifying their public transport systems.

“Because they save on gas and maintenance, modern electric vehicles are simply a better value than internal combustion engine vehicles,” Horton says.

Electric vehicles also are much simpler to build, Geiger adds. “They don’t need a transmission or complicated valves. They require significantly fewer components on the assembly line.” 

Air Quality in the COVID Era

The novel coronavirus pandemic has demonstrated how electric vehicles could positively affect air quality. As people drove less, U.S. greenhouse gas emissions decreased by 10% in 2020.

“The COVID emission changes are similar to replacing a large fraction of combustion vehicles with electric vehicles powered by renewable energy,” says Jordan Schnell, a former postdoctoral fellow in Horton’s laboratory.

Now a research associate at the Cooperative Institute for Research in Environmental Sciences (CIRES), Schnell works to discover how pandemic lockdowns have affected air quality, which could provide a glimpse into the future.

“It’s like a real-world experiment showing how places will respond to emissions reductions that will likely occur as we move toward cleaner technologies and introduce more regulations,” he says.

To approach the COVID emission question, the CIRES team compared two models: an emissions dataset that reflects reduced traffic during COVID lockdowns and an identical simulation that shows what 2020’s emissions would have been without lockdowns. However, the results are not straightforward. With fewer cars on the road, nitrogen oxide decreased across the board. Ground-level ozone also dropped in most parts of the country. But without nitrogen oxide to “eat up” the ozone, ground-level ozone actually increased in some urban areas. Particulate matter also mostly decreased, depending on location and time of year.


While driving an electric vehicle can decrease one person’s greenhouse gas emissions, Geiger, Horton, Schwandt and Loeb all agree that air pollution is an enormous problem that requires local, national and global regulations. 

“Government can also subsidize electric cars, public transportation and car sharing,” Schwandt says. “There are many ways to respond to this problem.”

Loeb maintains that governments should rethink local zoning laws, which often push industrial businesses into lower-income communities that “are unfairly overburdened with pollution.”

“We need systemic change,” says Horton, who felt optimistic after President Joe Biden’s inauguration. Within his first days in office, Biden signed multiple executive actions focused on climate change and air quality. Among these initiatives, Biden committed the federal government to buying only zero-emission vehicles.

“These commitments are no-brainers,” Horton says. “Study upon study has demonstrated that the switch to electric vehicles will reduce greenhouse emissions. My group’s research also has shown that the switch to electric vehicles will reduce harmful air pollutants, lowering the public health burden attributable to transportation emissions. By embracing electric vehicle technology, the Biden administration gives us hope.”

Amanda Morris ’14 MA is senior editor of science and engineering in the Office of Global Marketing and Communications.

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Reader Responses

  • Mr. Lucchesi’s comment about electric cars suggests some larger problems. Our response to the challenge of climate change consists of a list of initiatives that rate high on a “feel good” scale. But these ideas haven’t necessarily been thoroughly assessed versus other options.

    One example is from the other day, when President Biden announced an initiative to encourage the construction of wind farms along our coastlines. This is almost certainly a good idea. But from a cost and effectiveness perspective, how does it compare to other possible investments in reducing carbon emissions from power plants?

    Speaking of electric cars, I think we have been too quick to write off hybrid cars. What if we invested in improving this technology, and a hybrid could get 100 MPG? It would produce a quarter of the emissions a typical conventional car does, and we wouldn’t need to worry about what sort of power plant the electricity was coming from.

    But keep going. Has our U.S. government developed a comprehensive plan for the combination of initiatives we will advance over the coming years and decades, and have we “costed these out?”

    In what is one of the greatest challenges the human race has ever faced, shouldn’t we lock a crew of analysts in an office for a few months to develop and cost out our overall program? There will be some “ifs” — the best solutions must still be invented — but we can at least develop a comprehensive, well thought-out, flexible plan for what we need to do.

    Jerry Schulz ’74 MS, Milwaukee, via Northwestern Magazine

  • Nello, you're right electric vehicles aren't instantaneously better than internal combustion engine vehicles. If you're interested in lifetime carbon emissions, have a look at this EV v. ICE carbon emission/cost calculator with 2021 data from MIT:
    My lab's work tackles the "other" emissions question, i.e., air pollutants. EVs win out there too.

    Daniel Horton Evanston

  • “Airpocalypse Now,” about the deadly consequences of air pollution, makes the case for Northwestern to be a leader in moving away from the fossil fuels which cause this pollution.
    The campus could gain prominence by becoming a functioning laboratory and demonstration for solar panels, geothermal energy and maybe wind turbines, as well as for energy efficiency, electric vehicles and storage batteries.

    Richard Barsanti ’61 Western Springs, Ill., via Northwestern Magazine

  • As the recent Wall Street Journal article “ Are Electric Cars Really Better for the Environment?” explains: “The environmental cost of a car includes both building it and fueling it. That means factoring in emissions associated with oil drilling and power plant smokestacks, as well as from mining metals such as nickel and cobalt that are needed for electric-car batteries.”

    The full environmental cost to produce an electric vehicle is much higher than to produce an internal combustion vehicle.

    It’s only after many years of use that the cumulative environmental cost of an electric vehicle is less than an internal combustion vehicle.

    Nello Lucchesi ’77 Glencoe, Ill., via Northwestern Magazine

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