As wildfire smoke plumes travel thousands of miles, sunlight and atmospheric chemistry bleach their color, strip away their campfire aroma and add them to the mix of urban smog.

Smoky skies plagued the East Coast and Midwest again in summer 2026, as wildfires hundreds of miles away forced people to cancel sports practices and run air conditioners around the clock.

Living in the western U.S., I am no stranger to this scene. Wildfires and the thick smoke they produce have become a fixture of summer life here. I also study wildfire smoke as an atmospheric chemist, in particular how the smoke evolves as it moves downwind from the flames to the communities where people breathe it.

To understand what’s in the smoke you might be breathing, you have to look at where it’s been.

When a forest burns, the wildfire emits large amounts of fine particulate matter, or PM2.5, along with nitrogen oxides and volatile organic compounds, or VOCs. The compounds and particles emitted by wildfires can harm human health, including the lungs, heart and organs, and include known carcinogens.

How this potent mix of pollutants transforms over time and distance is a complex puzzle. At the NOAA Chemical Sciences Laboratory, our team tracks this behavior, from studying controlled fuel burns in the lab to flying research aircraft through wildfire smoke plumes. Understanding how wildfire smoke ages is essential for predicting its impacts on human health.

The story of smoke begins with how the wildfire burns. High temperature wildfires behave very differently from cool, smoldering wildfires.

Hot, intense flames produce nitrogen oxides, which can harm a person’s respiratory system and contribute to the formation of secondary pollutants and reactive carbon compounds like aromatic hydrocarbons, which can increase the risk of certain cancers with long-term exposure. They also emit smaller amounts of nitrous acid, hydrogen cyanide and isocyanic acid, all of which can be toxic to humans.

In contrast, lower-temperature smoldering wildfires release a different mix of compounds, but it is still enriched with toxic aromatic oxygenates and ammonia, which can irritate the respiratory system. This isn’t to say that these wildfire emissions are safer for human health, but rather that burning conditions dictate the chemical makeup of the smoke and its subsequent chemical fate.

Wildfires are rarely just one or the other; they are dynamic mixtures that shift throughout the day as air temperatures rise, humidity drops or evening thunderstorms roll in.

Once smoke leaves the flames, three main factors govern its journey: wind speed, atmospheric temperature and sunlight.

Think of a campfire. If you are sitting in the wrong spot, the prevailing wind blows smoke directly in your face. This horizontal movement is called advection. Small fires often don’t generate enough heat to loft their smoke high, meaning you can smell your neighbor’s campfire.

However, massive wildfires generate immense heat and powerful upward winds. This buoyancy acts like an elevator, lifting the smoke plume out of the planetary boundary layer and injecting it into the free troposphere, roughly 1.2 miles (2 kilometers) above the ground. Up there, high-altitude winds take over, transporting the smoke thousands of miles across the continent.

On occasion, wildfire plumes can generate their own weather, making smoke-laden pyrocumulonimbus clouds, which can inject large amounts of particles into the stratosphere.

As smoke travels, it undergoes rapid physical and chemical changes.

First, the plume dilutes as cleaner background air mixes into it. Close to the fire, the smoke is dense and opaque. As it moves downwind, it spreads out and grows more diffuse.

Second, intense sunlight acts as a chemical engine. Solar ultraviolet photons break apart bonds in molecules, creating radicals, which oxidize VOCs. Nitrogen oxides play a key role in driving this chemistry.

Within just hours of being emitted, this mixture reacts to form ground-level ozone, which can irritate the lungs and is a key component of smog.

If smoke is lofted high, into cold free tropospheric air, the chemical aging process can temporarily freeze. The smoke can remain chemically fresh until the air parcel sinks closer to the surface, below about 1.25 miles (2 kilometers), where warmer temperatures can restart the chemical reactions.

When an aging smoke plume passes over a major city, it can mix with urban pollution, such as car exhaust. This interaction can rekindle chemical reactions, creating additional local ozone on top of the fine particulate matter emitted from the fire.

Have you ever noticed that long-distance wildfire smoke doesn’t smell like a campfire? There is a chemical reason for that.

The specific compounds responsible for the classic smoky aroma of a campfire – phenolic compounds like guaiacol and syringol – are highly reactive. Sunlight and the associated chemistry destroy them in a matter of hours. By the time the smoke has traveled across several states, the smoky smell is completely gone.

Sunlight also alters the color of the smoke through chemical bleaching of aerosols. Fresh smoke contains dark brown and black carbon particles that absorb light. Over several days of exposure to sunlight and oxidants, chemical reactions break down these dark compounds.

The result?

Dark, light-absorbing particles turn into light-scattering particles, transforming dark, dense plumes into the milky white haze seen drifting across the eastern skies. These particles efficiently scatter the blue and green light while letting red and orange wavelengths pass through, creating the uncanny feeling of an all-day sunset.

The wildfire smoke inhaled hundreds of miles downwind is chemically distinct from the smoke that left the flames, but it remains a serious health threat and an area of active research. Consequently, smoke transport and its associated atmospheric chemistry represent a major North American air quality issue, one projected to intensify in the coming years.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Michael A. Robinson, University of Colorado Boulder

Read more:
Yes, breathing wildfire smoke can harm your health – here’s what you can do to protect yourself

Wildfire smoke leaves harmful gases in floors and walls − air purifiers aren’t enough, new study shows, but you can clean it up

Wildfires are reversing America’s progress on ozone pollution, the main ingredient in smog

Michael A. Robinson does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.