Nine miles south of the picturesque French city of Bordeaux, firefighters are at war with a phenomenon straight out of an apocalyptic epic. Scientists call them pyrocumulonimbus, or PyroCbs. NASA calls them the “fire-breathing dragon of clouds”.

The port capital of Nouvelle-Aquitaine, normally bustling with tourists at peak season, is instead drowned in smog, so thick residents are forced to wear the N95 and FFP2 masks many hoped would be a thing of the past as the COVID-19 pandemic subsided.

A NASA satellite passing over the burning pine forests of the south-western Gironde region last Friday showed the PyroCb driving smoke into the city’s skies was visible from orbit. Until this month, these fire systems had been recorded almost exclusively in Australia and North America.

These firestorms occur when fires burn so hot they drive columns of superheated air upward into the atmosphere until it cools at higher altitudes, generating dry storm clouds which power their own winds, and shoot lightning at the blaze below, creating a deadly loop that accelerates the intensity and spread of the inferno.

“This is a phenomenon that has never been observed in France – it is a historic first,” regional fire chief Marc Vermeulen said on Sunday. “It’s like a thunderstorm of fire.”

“This phenomenon will likely continue to generate lightning strikes, but also cause what the literature refers to as ‘ember attacks,’” Vermeulen added, referring to fragments of burning, incandescent wood carried by the wind during megafires, triggering new ignition points far beyond the blaze.

The dense smog has prevented authorities from knowing exactly where the storm is located, or the speed at which it is travelling, forcing mass evacuations now up to 220,000 people in a region of just 1.6 million. If the fire reaches the city, nearly every person living in the region will be forced to flee the flames.

“If the fire reaches the city of Bordeaux, it means that we have to evacuate at least one million people,” Jerome Steffe, mayor of Cestas, said on Tuesday. “We have never, never seen [this] in France. It will be the fire of the century.”

Europe on fire  

The Bordeaux storms of lightning fire are part of a wider crisis setting Europe and the world ablaze, threatening homes, economies, and killing hundreds of thousands through respiratory and cardiovascular disease.

Land burned across the WHO European Region reached 2.2 million hectares last year, up from 1.4 million in 2022, a 57% rise in four years, while Portugal and Spain have each reported more than twice as many fires this year as at the same point in 2025, the UN health agency reported this week. 

“Behind these numbers are real people and communities,” said Hans Kluge, WHO’s regional director for Europe. “Wildfires destroy homes and livelihoods, force evacuations, and place enormous pressure on emergency services and health systems. Tragically, they also claim lives.”

Portugal, Italy, Greece, Spain and Germany are all fighting significant blazes, and the EU has pre-positioned the largest firefighting fleet in its history and revised its guidance for member states on preparing for wildfires.

Americas ablaze 

Nearly a thousand fires are burning across Canada in one of the most intense seasons the country has recorded, pushing smoke far enough south to place more than 120 million Americans under air quality alerts in mid-July alone.

The Canadian fires prompted US President Donald Trump to pitch an idea for a new border-spanning wall, this time built from an AI-generated air filter – dubbed the ‘MERV 16 High Efficiency Particulate Air Filter’ – to stop the “invasion of smoke” crossing his nation’s northern border. 

“The United States is being unnecessarily invaded by filthy, polluted, and unhealthy air. This is Willful Negligence, and becoming a yearly occurrence, costing the United States Billions of Dollars,” Trump said on Truth Social. “Clean Air. Clean Borders.”

In South America, Brazil is bracing for a third consecutive year of drought in the Amazon. Last year’s fire season burned 46 million hectares across the country, the second-largest series of wildfires anywhere in the 21st century. 

Climate crisis death loop 

As wildfires wreak havoc worldwide, dominating global headlines, they are set to accelerate both this year and in the long run as the ongoing El Niño – projected to peak in December and continue through 2027 – and global warming – on pace for 2.8C at current emissions levels – spike temperatures. 

Successive heatwaves, dry soils and drought have created ideal conditions for extreme fire behaviour across Spain and western Europe, the WMO said on Tuesday. The heatwave driving it is the most severe ever recorded in the region and would have been “virtually impossible” without human-caused climate change, according to the World Weather Attribution group

Warming temperatures feed into wildfires. Hotter, drier conditions turn forests into fuel; in turn, burning forests release the carbon that makes conditions hotter and drier. Fires and biomass burning fuel rising temperatures in return, releasing around 1,380 megatonnes of carbon worldwide by the end of last November, more than the United States emitted from fossil fuels across the whole of 2024.

Total area burned by wildfires globally has increased by 50% since 1970. On current emissions trajectories, UNEP projects extreme fires will increase by 14% above current levels by 2030, 30% by 2050, and 50% by the end of the century. 

Four of the five worst years for forest fires on record have come since 2020, and 2024 broke the record set the year before, according to World Resources Institute data. 

Scientific attention has followed the smoke. Around 840 studies on wildfires and health have been indexed in PubMed since 1990, roughly three-quarters of them published since 2020.

So what do we actually know about what this smoke does to the human body? How much more dangerous is it than the polluted air billions already breathe? And what, two decades into a worsening global fire era, do we still not know?

“The health impacts extend far beyond the flames,” Kluge said. “Smoke can travel long distances, worsening heart and respiratory conditions and affecting mental health, particularly among older people, children, pregnant women and those with chronic diseases.”

Let’s take a look.

How many people are dying?

Terrifying as the image of fire-breathing lightning clouds conjured out of thin air is, it isn’t direct fire that kills millions – it’s the smoke.

By the last reliable global count in 2018, 221 deaths worldwide were recorded as directly caused by wildfires. Fourteen such deaths occurred in Spain last week, mostly foreign nationals, too late to flee the flames near the capital, Madrid.

Wildfire smoke, meanwhile, killed an estimated 154,000 people in 2024, a 36% increase from 2012, according to the Lancet Climate Countdown, setting an all-time record unlikely to stand for long as temperatures continue to rise. Deaths rose fastest in countries with low Human Development Index scores, up 46% over the same period.

While 117 countries saw increased population exposure to wildfire risks, deaths from wildfire smoke increased in only 85 of those nations, which the Lancet attributes to improved prevention, management, or reduction of available fuel for wildfires due to land-use change and deforestation. 

Around 270,000 children under the age of five die each year from landscape and wildfire smoke, UNICEF research estimates. 

A separate Lancet analysis counting deaths from all landscape fire smoke, including agricultural burning, controlled land-clearing fires and peat fires emitting the same toxic pollutants as wildfires, put annual deaths at 1.53 million per year in the first two decades of this century.

Canada’s record 2023 fires became the first event of their kind to be measured across two continents, quantifying for the first time the threat of what researchers term “long-range PM2.5“.

Over 354 million people across North America and Europe breathed the smoke, causing 5,400 acute deaths in North America and a further 64,300 chronic deaths across both continents. Some 22,400 of those were in Europe, where the smoke arrived thinned but settled over dense population centres.

What’s in the smoke?

Wildfire smoke is a mixture rather than a single pollutant, and what goes into it depends on what the fire burns in its path. While a fire burning through pine forests emits particulate matter, carbon monoxide, nitrogen oxides, and other organic compounds contained in the burning wood, a fire that reaches a village, town or residential area will add noxious chemicals to the mix.

Burnt vehicle batteries, wiring, insulation, paint, roofing, furniture, electronics, plastics, PFAS, heavy metals and countless chemicals blend into the fire smoke. Colorado State University toxicologists have identified at least a thousand distinct compounds in it, including formaldehyde and benzene. Many of those thousand compounds have little to no research on their health effects.

The danger of fire smoke grows as the intensity of its flames grinds its composite elements into ultrafine particles, up to 25 times finer than standard PM2.5. These smaller particles carry hydrocarbons more easily past defences like masks, as well as nose hairs and mucus designed to catch larger particles, allowing them into the bloodstream, heart, kidneys, brain and placenta. 

What are the known health effects?

The respiratory effects are settled well enough that the authors of a review of medical research into wildfire smoke published last month urge researchers to stop studying them. Smoke reliably worsens asthma and chronic obstructive pulmonary disease, filling emergency rooms and leaving lingering health effects long after the smoke has dissipated.

Current research also suggests wildfire smoke appears more dangerous than ordinary urban pollution. A Southern California study found respiratory hospitalisations rose by up to 10% per 10 micrograms of wildfire-specific particulate, against a maximum of 1.3% for the same increase from traffic and industry.

Stanford researchers put the toxicity difference at roughly tenfold, with longer and repeated exposures worsening health risks across the board. Like traditional air pollution, scientists and health experts stress there is no safe level of inhaled particles.

That increased toxicity frequently results in immediate health effects including coughing, headaches, sore throats, and brain fog. While populations such as the elderly, children, and patients with respiratory risks like asthma are the most vulnerable, even healthy adults can see such symptoms last long after the smog clears.

No temporary nuisance

Evidence of the impacts on cardiovascular disease risk is also accumulating rapidly. Yale researchers reported last week that among 65 million Medicare beneficiaries in the US, cumulative exposure was linked to more hospitalisations for ischaemic heart disease, arrhythmia, stroke and heart failure, with heart failure risk more than 20% higher at greater exposure.

“Wildfire smoke should be treated as more than a temporary respiratory nuisance,” said Harlan Krumholz, a leading expert at Yale Medical School and co-author of the study.  

“Cumulative exposure is linked to a higher risk of cardiovascular hospitalisation in older adults. Clinicians should help high-risk patients prepare for smoke events, and public health leaders should expand timely alerts, clean-air spaces, and access to effective filtration, especially for communities with fewer resources.”

A study of 25 million Medicare beneficiaries published in January found each microgram of smoke particulate in a three-year average raised stroke risk by 1.3%, nearly twice the effect of pollution from other sources, amounting to 17,226 additional strokes a year among older Americans.

 “In other words, the cardiovascular toll of wildfire smoke reaches well beyond the fire lines and well beyond the day the sky turns orange,” Kai Chen, Yale associate professor of epidemiology and co-author of the study added.

Attention deficits and pregnancy complications

Research on wildfire smoke’s effect on brain health and cognition is more in its infancy compared to respiratory and cardiovascular health, but our understanding is growing.

A study of 10,228 American adults playing a brain-training game found heavy smoke the previous day was associated with a 117-point drop in mental performance scores, with the sharpest effect in the first three hours and some impairment persisting for two days.

Mental health emergencies also rise in tandem with the smoke plumes. Analysis of 86,609 emergency department visits during California’s 2020 season found 8% more mental health presentations per 10 micrograms of wildfire smoke, 15% more for depression and 29% more for mood disorders. Children’s risk rose 35%, and Black patients were more than twice as likely to present with a mood disorder, the authors found.

Exposure to wildfire smoke during pregnancy has also been linked to adverse outcomes. One study of mid-pregnancy placentas found raised levels of a fetal immune cell associated with inflammation and impaired placental function in women who had breathed smoke, suggesting particles reaching the placenta may trigger an inflammatory response in the fetus.

Other studies have linked smoke exposure to premature births, miscarriages, and cellular damage to placentas.. Another paper released this year suggests a possible link with third trimester exposure to autism diagnoses, though the authors acknowledge the study is the first to examine such a premise.

Another especially vulnerable population to the smog is people with pre-existing kidney problems. Among 52,995 dialysis patients across 22 US states during the 2023 Canadian smoke episode, the mere presence of a plume overhead was associated with an 18% increase in same-day mortality.

A young science

A new research avenue is wildfire’s potential effects on cancer. A novel study tracking over 90,000 US adults presented in April found increased wildfire smoke exposure raised risks for multiple cancers, including blood, lung, breast, bladder and colorectal. No associations were found with other cancer types, such as ovarian or melanoma, the authors said. Its findings have not yet been peer-reviewed.

While new avenues of research into wildfire smoke are intriguing, caution is warranted with how young the evidence base is on many fronts.

In June 2024, Health Policy Watch reported on research presented to the Alzheimer’s Association International Conference linking wildfire smoke to an 18% increase in dementia risk, posing significantly greater risks to brain health than other forms of air pollution. 

That figure continues to be widely cited by media outlets to this day, despite being retracted and replaced by its own authors the following month, after a newly discovered coding error revised their estimate down to a confidence interval that cancelled out the statistical significance of their data.

Many other angles of wildfire smoke’s health effects continue to rely on just a handful of papers. Blood pressure has been examined twice worldwide, once in Montana and once among twenty hypertensive adults in Athens. Kidney disease has two studies, eczema flare-ups essentially one research group, and the cancer findings are the first of their kind. 

Further research will be required to validate the links.

A generation growing up in smoke

UNICEF, which published a technical note on wildfire smoke and children in 2024, estimates wildfire smoke particulate is up to ten times more harmful to children’s respiratory health than particulate from other sources, and most harmful to those under five.

Infants breathe more rapidly than adults and take in more air relative to their body weight. They also deposit fewer particles in the nose, so a higher proportion reaches deep into lungs that are still forming.

Particulate exposure has been linked to reduced lung function, stunted growth, and neurological and metabolic dysfunction in children. Early-life exposure to wildfire smoke specifically has been associated with immune dysregulation and lung function deficits still measurable in adolescence.

Global studies of wildfire smoke and pregnancy outcomes remain sparse, and the associations found in children and adolescents are, UNICEF notes, at a nascent stage.

But what is known already is reason for alarm. Each additional microgram of fire-derived particulate raises the risk of child mortality by 2.3%, UNICEF estimates, while each microgram of wildfire smoke across a trimester raises the odds of preterm birth by 13.2%.

What the smoke does to a child over the course of a lifetime is the question the research, picking up only two decades ago, has not yet addressed. The children growing up today as wildfires become routine rather than rare, for billions around the globe will be the involuntary subjects of a vast, unplanned experiment testing this very question.

Image Credits: UNEP, Cal Fire, Shazadul Alam.