90 Million Hospital Stays Show Wildfire Smoke Hits the Heart and Lungs Harder

When wildfire smoke rolls in, most of us think of it as a short-term problem. Stay inside for a few days, and it passes.

But a new study suggests its health effects are more persistent than that.

It looked at around 90 million US hospital stays, and at what happens after two years of living with smoky air.

It compared wildfire smoke with ordinary air pollution from traffic and industry. Gram for gram, the smoke looked roughly twice as harmful.

What are the health effects of wildfire smoke?

Wildfire smoke is a mix of air pollutants. The biggest threat is fine particles, called PM2.5, small enough to reach deep into the lungs.

According to the World Health Organization, these particles are linked to early deaths. They can also cause or worsen disease in the lungs, heart, brain and other organs.

Most advice focuses on the smoky days themselves. The WHO also says more research is needed on long-term effects, especially in children, older people, pregnant people and the chronically ill.

That long-term question is exactly what this new study tackles.

The study: 20 states, 14 years

What the researchers did

The team was led by researchers at the Icahn School of Medicine at Mount Sinai in New York. They studied hospital records from 20 US states between 2006 and 2019.

That included 57 million hospital stays for heart and blood vessel problems and 33 million for lung problems.

The states ranged from fire-prone California, Oregon and Washington to New York and Georgia.

Splitting smoke from other pollution

The pollution they studied is PM2.5. These are tiny particles, small enough to reach deep into the lungs and even the bloodstream.

Using satellite data, ground monitors and machine learning, the team estimated how much PM2.5 in each ZIP code came from wildfires. The rest came from other sources, such as traffic and industry.

Comparing each case with itself

Instead of comparing different people, the study compared each hospital stay with the years around it.

It asked: was smoke higher in the two years before this hospital stay than in nearby years? This design automatically cancels out things that don’t change much, such as genes or smoking history.

Why two years?

The team chose a two-year window before looking at the results. It covers the year of the hospital stay and the year before.

Using only one calendar year could misjudge exposure for someone admitted in January. Leaving out the admission year could miss the period that matters most.

How much smoke are we talking about?

Wildfire smoke is still a small part of the air most people breathe, on average.

California had the highest levels of both types.

But averages hide the bad years. In extreme smoke years, wildfire smoke has made up over half of the yearly PM2.5 in many parts of the US.

Smoke also travels. Its fine particles can drift hundreds of miles, reaching people far from any flames.

It has even reversed some of the gains in air quality that decades of clean air rules had achieved.

The results: smoke hit harder, gram for gram

Heart and lung hospital stays

For every 1 µg/m³ increase in two-year average PM2.5, the risk of hospital stays went up for every condition studied.

In every case, the rise linked to wildfire smoke was bigger.

Longer exposure, bigger effect

When the team looked at three- or four-year averages, the link with wildfire smoke grew stronger. The link with ordinary pollution stayed about the same.

That suggests smoke damage may build up over time.

Who was hit hardest

The risks were higher in some groups:

• Hispanic people and people from other racial or ethnic minority groups.
• People living in big metropolitan areas.
• Communities with less education and more deprivation.
• People who also had obesity or diabetes.

The authors suggest several reasons. City air carries other pollutants, such as nitrogen dioxide and ozone, that may add to the damage.

People with obesity often have weaker heart and lung function and more background inflammation. Deprived communities tend to have poorer healthcare access and fewer ways to avoid smoke.

The link with heart hospital stays was also generally stronger in people under 65. That may partly be because the most vulnerable older people had already become ill earlier.

The everyday pollution caveat

There’s an important twist. Because ordinary pollution levels are so much higher, it still drove more of the overall risk in this period.

Per unit, smoke was worse. But in total, everyday pollution still mattered more.

That balance may shift. Under future climate scenarios, yearly wildfire PM2.5 could reach 10 µg/m³ in some regions.

A gap in the rules

The authors point out a policy blind spot. In the US, wildfire smoke is largely exempt when judging whether an area meets national air quality standards.

Controlled burns and fire management have also focused mainly on protecting property. The authors argue that reducing people’s exposure to smoke deserves the same attention.

Why might smoke be worse?

Wildfire smoke isn’t the same as exhaust fumes.

It contains more very small particles, more carbon-rich material and more irritating organic chemicals. Smaller particles can travel deeper into the body.

Lab studies in mice have found that wildfire particles cause more lung inflammation than the same dose of ordinary pollution.

In mice exposed to smoke from smouldering peat fires, researchers have also seen reduced heart function. The particles may also carry more chemicals that trigger oxidative stress and inflammation.

What earlier research found

Southern California hospital data

A 2021 study in Nature Communications looked at respiratory hospital stays in Southern California.

A 10 µg/m³ rise in wildfire PM2.5 was linked to 1.3% to 10% more respiratory hospital stays. The same rise in other PM2.5 was linked to just 0.67% to 1.3% more.

What burns matters

A 2025 meta-analysis pooled studies of wildfires and heart health.

Fire exposure was linked to a 4% higher risk of heart and blood vessel problems overall. Fires that burned buildings or peat showed stronger links than fires in natural vegetation.

How it fits together

Short-term studies have already linked smoke days to more breathing problems. The new study adds that living with more smoke over years is tied to heart and lung hospital stays too.

Its effect sizes are larger than earlier short-term estimates. The authors think that’s because damage builds up over longer exposure.

A multi-city study across eight countries and territories has pointed the same way. Per unit, wildfire smoke keeps looking worse than other fine particles.

How much should you trust this?

Promising. It’s enormous and well designed, but it relies on modelled pollution, not personal measurements.

What makes it convincing

• It covers around 90 million hospital stays across 20 states.
• The self-controlled design cancels out many hidden differences between people.
• Results held when the team changed the model, the exposure window and the control years.
• Stronger links with longer exposure fit the idea of cumulative harm.
• The authors declared no competing interests.

What makes me cautious

• Smoke levels were estimated for each ZIP code, not measured on individuals.
• The wildfire smoke model was only moderately accurate.
• The same person could appear more than once, which may make results look more precise than they are.
• The data end in 2019, and cover only 20 states.
• It can’t fully account for time spent indoors, air purifiers or other pollutants.

What this means for you

If you live somewhere with regular wildfire smoke, it’s worth taking seriously, even in years when it seems brief.

• Check air quality on smoky days. Local air quality alerts tell you when to limit time outdoors.
• Create a cleaner room at home. Close windows and use a portable air purifier with a HEPA filter if you can.
• Use a well-fitting mask outdoors. Respirators such as N95 or FFP2 filter fine particles far better than cloth masks.
• Take extra care if you’re at higher risk. That includes people with heart or lung disease, diabetes, obesity, older adults and children.
• Don’t ignore symptoms. Chest pain, severe breathlessness or a worsening cough need medical attention.

The World Health Organization explains the health effects of wildfires and of outdoor air pollution.

In this PBS video, experts explain how wildfire smoke makes people sick:

What we still don’t know

1. Does what burns change the harm? The study couldn’t separate forest fires from fires that burn homes.
2. Do masks and purifiers cut long-term risk? They reduce exposure, but long-term benefits haven’t been measured.
3. What about recent fire seasons? Fires since 2019 have been severe in many places.
4. How does smoke affect other organs? This study looked only at heart and lung hospital stays.
5. Why are city dwellers more affected? Other pollutants or stress may play a role.
6. Are some ages more vulnerable? This study found few differences by age, but it wasn’t designed to focus on children.

My take: smoke is not a short-term problem

What I like about this study is its long view. Most smoke research looks at a few days; this looks at two years.

I’m cautious about the exact percentages. Pollution was estimated by ZIP code, and nobody measured what each person actually breathed.

But the direction matches lab studies and earlier hospital data. Gram for gram, wildfire smoke looks worse than the pollution we’re used to.

As fire seasons lengthen, clean indoor air on smoky days looks like one of the more useful habits you can build.