In the summer of 2002, the smoke in the air caught Loretta Mickley’s attention. She was on vacation in Western Massachusetts when the sunlight turned strange.

“There was something kind of sparkly about the air,” she recalled. “I’d never seen anything like it, and I said to my husband, ‘What’s going on?’ I’m an atmospheric chemist – I should know, right?”

The shimmering particles turned out to be soot, carried by wind from a wildfire in Quebec. That was over two decades ago. Since then, smoke has gone from a curiosity to a central focus of Mickley’s work.

She’s now a senior research fellow at the John A. Paulson School of Engineering and Applied Sciences at Harvard University. Her research is part of a growing wave of attention on wildfire smoke – where it comes from, what it carries, and what it’s doing to our health.

Wildfire smoke wasn’t always on the radar

Wildfires have burned for as long as the planet has existed. But for many years, their consequences were overlooked.

“For a long time, smoke was not considered very important,” Mickley said. “People thought of climate change as temperature and sea level rise, and that was it.”

“They didn’t realize there could be this other consequence of climate change that could be terribly scary, terribly bad for human health – not just close to the fire, but many miles downwind.”

That perception is changing fast. Today, scientists are working across disciplines to understand smoke in detail.

Chemists break down its chemical components. Engineers build tools to sample the air in and around fire zones. Medical researchers track how it enters the human body. Environmental scientists model future fire risks as the planet heats up.

Smoke travels far – and so does the risk

Mickley has co-authored dozens of studies on wildfire smoke since 2015. Her work spans continents, even when she doesn’t travel to the fire sites herself.

“I look at agricultural fires in India, and their smoke, which affect millions of people,” she said. “I don’t go to these places, but we’ve done work in the Amazon, work in Australia … working with local people as much as possible.”

What researchers are finding is that wildfire smoke is not just dirty air – it’s toxic. When vegetation, buildings, or vehicles burn, they release not only soot, but also heavy metals and organic compounds that can damage the body.

According to Mickley, many of these chemicals can “attach to and react with human cells.”

In places like Southern California, where wildfires often spread into developed areas, the smoke can contain harmful synthetic compounds like PFAS – chemicals linked to cancer and hormonal disruption. These compounds don’t break down easily and can linger in the environment for decades.

Tiny particles, big health risks

The real threat lies in the size of the particles. Most of the health damage comes from PM2.5 – particles that are 2.5 microns in diameter, or smaller. That’s about 20 times smaller than the width of a human hair.

Pollen grains are roughly four times larger than PM2.5 particles, and they get stuck in the nose. By contrast, PM2.5 specks are small enough to enter deep into the lungs and also into the bloodstream.

“It causes a cascade of inflammation, and it can wreak havoc on health,” stated Nicholas Nassikas, assistant professor of medicine at Harvard Medical School. He’s also a pulmonologist at Beth Israel Deaconess Medical Center.

Human cost of wildfire smoke exposure

In a study of wildfires in Australia, Mickley and her colleagues found that pregnant women exposed to prolonged smoke had higher risks of premature birth, miscarriage, or needing intensive care for their newborns.

And It’s not just lung or heart disease that are involved. Smoke may worsen COVID-19 symptoms. Early research hints at a link between smoke exposure and dementia.

Furthermore, hospitals report a rise in emergency visits for multiple conditions after days of smoky air – especially among older adults.

At the Harvard’s T.H. Chan School of Public Health, researchers are digging deeper into these effects. Kari Nadeau and her lab are part of the LA Fire HEALTH Study, which looks at how smoke affects people who live and work along the fire lines.

Smoke’s impact on human cells

In a June study, researchers examined blood samples from 31 Californians – both firefighters and civilians – who had been exposed to wildfire smoke.

The experts compared these samples to a control group with no exposure. The results were striking.

“Normally, immune cells are highly regulated, but after smoke exposure, some of those cells begin to behave abnormally – to be hyperactivated,” said study co-author Abhinav Kaushik. Some immune cells, burdened with cadmium and mercury, simply died.

Heavy-metal exposure was most severe among long-term firefighters. Kaushik noted that this may explain the chronic diseases that firefighters experience disproportionately, from cancer to asthma. “Smoke has surfaced as a public-health emergency – no doubt about that,” he added.

The team’s next step is to expand the study and look for a biomarker – something in the blood that would signal smoke exposure. That could help doctors catch issues early or measure the long-term impact.

Big fires, bigger numbers

Even now, the data tells a troubling story. Mickley and co-authors at Harvard and the University of British Columbia estimated that wildfire smoke caused nearly 37,000 excess deaths in 2020 alone.

The range is wide – between 25,000 and 47,000 – but the implication is clear. “In my mind, the exact number is not so important, just that we know it’s a big number,” Mickley said.

“Our goal in giving these numbers is to highlight the sense of urgency and the possible human hazards that follow wildfire smoke which, in turn, is caused by climate change.”

A new climate regime

Wildfires feed on dry air, lightning, and heat – all of which are becoming more common as the planet warms. In 2014, Mickley and then-postdoctoral researcher Xu Yue projected that California’s burn area would double by mid-century under one climate scenario.

What used to be considered an “extreme fire year” in Southern California – about 25,000 acres burned – could become routine. That threshold has already been surpassed in 2025.

At public talks and alumni events, Mickley shares practical steps: wear N95 masks on smoky days, use HEPA filters indoors, and rethink how we manage fire.

Rather than suppressing every blaze, she advocates for controlled burns to prevent massive, fuel-fed wildfires. However, Mickley stresses that individual actions and local policies can only do so much.

“Wildfire is natural. We’re entering a new climate regime that is not – that is at least in large part, due to the greenhouse gases we’re pumping into the atmosphere. And this is a recipe for disaster.”

The full study was published in the journal Environmental Science & Technology.

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