Beyond the Flames: A new study reveals the lasting health risks of wildfire smoke

By Yiying Zhang

“There is a fire on your street.” Alicia Kalvin woke up to see her friend’s urgent text on Jan 7. “The only thing in my mind is to get out of here,” Kalvin recounted later in an interview with CNBC. Outside, as she described last February, the sky glowed red, planes circled overhead, and a neighbor’s house was already engulfed in flames. When Kalvin returned months later, the visible damage had begun to heal. With police escorts repairs, and cleaning, life seemed to be back on track.

However, wildfire smoke’s danger does not disappear even after the sky clears. A new study using millions of U.S. hospitalizations shows a sustained “aftershock” of heart and lung problems that can last for up to three months after the wildfire itself ends, becoming an alert for the public to pay attention to the lingering, invisible health risks that live longer than the flames. “We expected to see health impacts during the fire themselves,” said Yaguang Wei, an assistant professor at the Icahn School of Medicine at Mount Sinai and the main author of the study. “It is surprising that hospitals are still seeing patients months after the smoke has cleared.”

The researchers analyzed more than 21 million hospitalizations across 15 U.S. states between 2006 and 2016. The team linked patients’ ZIP-codes with satellite-based estimates of wildfire smoke PM2.5. Instead of focusing only on short-term exposures, the team extended a traditional case-crossover design to examine health outcomes over a three-month window, comparing hospitalization rates during smoky versus clear periods. The findings were published in Epidemiology on September 5.

A national Medicare cohort study, published in June 2025, also found that long-term exposure to wildfire PM2.5 was more strongly associated with heart failure than exposure to other forms of air pollution, indicating that wildfire smoke carries a uniquely potent cardiovascular risk. Another study, conducted in 2023, of lung cancer patients recovering from surgery showed that wildfire exposure during the first postoperative year was linked to significantly worse survival outcomes.

“Most papers either deal with very short-term exposure, like daily effects, or very long-term exposure, like annual averages,” said Yang Liu, the Chair of the Gangarosa Department of Environment Health at the Rollins School of Public Health of Emory University, who is not involved in the study. “Wei’s study is kind of in between. That’s new in the context of wildfires.”

The reason wildfire smoke leaves such a long shadow lies in PM2.5, tiny particles less than half the width of a red blood cell that penetrate deep into the lungs and enter the bloodstream. Liu noted that wildfire PM2.5 appears more toxic than typical urban sources, likely because fires often burn not only vegetation but also plastics and household chemicals, creating a complex mix scientists still struggle to characterize. “There’s a lot more unknowns than knows at this point,” Liu said. Yet smoke PM2.5 remains excluded from regulatory attainment determinations, since wildfires are treated as natural disasters beyond government control.

The hospital records showed that this microscopic dust storm translated into very real illnesses. Hypertension shows the greatest susceptibility. Increases are seen in arrhythmia, ischemic heart disease, heart failure, asthma, chronic obstructive pulmonary disease (COPD), pneumonia, and acute respiratory infections. The pattern overall was consistent: the more the exposure, the higher the risk.

Kalvin’s story on wildfire is not unique. For many people in parts of the western U.S. and the boreal forest of northern North America and Russia, this is becoming reality more often. NASA satellite data shows that extreme wildfire events are now at least twice as frequent as they were in the 2000s, and the fire season is getting longer, emissions larger. The reach of those fires extends well beyond their point of origin. In June 2023, smoke from massive Canadian wildfires drifted into New York City, turning the sky orange and reminding millions that wildfire smoke can threaten communities far from where the flames burn.

“For the public, the most important thing is to stay aware of local air quality,” Wei advised. “If there is a fire, stay indoors, use air purifiers, and if you go outside, wear a high-quality respirator rather than a simple surgical mask.” Wei also argues that the government should invest in better wildfire monitoring, early suppression systems, and expand prescribed burning strategies. More funding for studies on the health effects of smoke will be critical as fire seasons grow longer and more intense. 

Educations from public health agencies and establishment of early alert systems are equally important. As Liu explained, people tend to protect themselves only when smoke is visible, but not when it has already dispersed and mixed with the air. “It’s a little counterintuitive. When you see smoke, you dodge. When you don’t see smoke, you don’t do anything without knowing the composition of your air pollution has changed,” said Liu. “Because fire smoke is a strictly outdoor source. Changing personal behavior will definitely help.”

But it is always challenging for scientists to link health problems to any environmental event, as isolating the effects of a single cause among all possible factors is rigorous work. To reduce this uncertainty, the researchers adjusted their models for temperature, background air pollution from non-smoke PM2.5, and seasonal patterns in hospitalizations.

But no study is perfect, neither is this one. In Wei’s analysis, smoke exposure had to be estimated at the ZIP-code level using modeled PM2.5, which means individual breathing levels and the sharpest spikes on severe fire days could not be captured. Hospital records only reflect the most serious cases, excluding milder illness treated outside hospitals.

“There are always factors we cannot fully capture,” Wei acknowledged. “For example, if someone quit smoking or had surgery during the study period, that changes their vulnerability in ways our model cannot reflect.”

Liu also noted that the estimates were based on satellite data with a resolution of about 10×10 kilometers. “You don’t get exposed to a regional average, and what matters is the air right outside your home,” he said. Nevertheless, Liu still emphasized that measuring wildfire smoke is inherently challenging, since scientists still lack precise tools to capture its intensity, chemical composition, or fluctuations over time.

Wildfires are often labeled as “natural disasters”, but many are not entirely natural. Human actions can trigger severe fire events, meaning prevention is possible through better land management and stricter fire safety practices.

At the same time, wildfire smoke remains one of the least understood forms of air pollution. There is no consensus on how to best characterize exposure, whether by the number of smoky days, the intensity of peaks, or cumulative averages. Nor is it clear which chemical components are most harmful when firs burn household chemicals. Future work will need higher-resolution exposure data, cleaner biomarkers to trace how the body responds to wildfire smoke, and special attention to vulnerable populations. Recognizing these gaps does not weaken the case for action. Instead, it makes clear that understanding the corresponding shadow of wildlife smoke is now one of the most pressing frontiers in environmental health.