From flames to haze, wildfire smoke transforms as it travels, and it can harm your health in different ways

Wildfire smoke from Canadian blazes has periodically tainted New York City’s skyline with an orange haze. As smoke plumes span thousands of miles, sunlight and atmospheric chemistry bleach their hue, strip away their smoky aroma, and blend them into urban smog mixes. Hazy skies troubled the East Coast and Midwest once more in summer 2026, as distant wildfires compelled people to postpone sporting events and crank up air conditioners continuously.

Those living in the Western U.S. are accustomed to this spectacle. Wildfires and their thick smoke have become a recurring summer phenomenon, and as an atmospheric chemist, I study how smoke evolves as it traverses downwind from the flames to the communities it affects. To comprehend what’s in the smoke you may inhale, one must consider its journey.

Wildfire smoke is often above the atmospheric layer where it can be perceived, but when atmospheric conditions push it closer to the ground, people breathe in unhealthy particles. At the NOAA Chemical Sciences Laboratory, researchers track smoke’s evolution, from controlled lab burns to flying research aircraft through smoke plumes. Grasping how wildfire smoke transforms over time and distance is crucial for predicting its health impacts.

Smoke's origin lies in the wildfire's burn intensity. Scorching flames differ from smoldering fires. Intense heat yields nitrogen oxides, which can damage the respiratory system and contribute to forming secondary pollutants and reactive carbon compounds like aromatic hydrocarbons. These compounds increase cancer risks with prolonged exposure. Nitrous acid, hydrogen cyanide, and isocyanic acid are also emitted, all of which can be toxic to humans.

Smoke rises high into the air. As it travels, its chemical makeup evolves. Lower-temperature smoldering fires release a distinct set of compounds, yet they still contain toxic aromatic oxygenates and ammonia, which can irritate the respiratory system. Wildfires are not solely one or the other; they are dynamic mixtures that fluctuate throughout the day as temperatures rise, humidity drops, or evening thunderstorms arrive.

Three factors dictate smoke's journey after leaving the flames: wind speed, atmospheric temperature, and sunlight. Envision a campfire; if you’re in the wrong spot, the prevailing wind can direct smoke straight at you. This lateral movement is called advection. Small fires typically don’t generate enough heat to elevate their smoke, allowing you to detect a neighbor's campfire.

However, massive wildfires release immense heat and robust upward winds. This buoyancy functions like an elevator, lifting the smoke plume above the planetary boundary layer and propelling it into the free troposphere, around 2,000 feet (600 meters) above the ground. Here, high-altitude winds carry the smoke thousands of miles across the continent.