Why air quality can still be bad even if the sky doesn’t look smoky

By Kasha Patel, CNN

(CNN) — Wildfires are expected to bring unhealthy air to more than 120 million people this weekend, but the smoke and its toxic particles may not always be clearly visible. Some skylines are shrouded in obvious abnormal orange tinges with faint burnt odors. Other horizons may carry no visible indication that something is afoul in the atmosphere — but that doesn’t necessarily mean the air is safe.

“Just because it does look a little bit less brown and orange today doesn’t necessarily mean that the smoke’s not there,” said Dan Westervelt, an atmospheric scientist at Columbia University.

The visibility of smoke can be affected by several factors including the chemicals and particles it carries, how long the plume has been in the air, the weather and angle of the sun.

While the human eye can’t always perceive the danger, air quality monitors can accurately track the number of pollutants in our atmosphere. That’s why it’s important to follow local safety guidance — staying indoors, running air filters and wearing an N95 mask — even if everything looks clear.

There’s more to wildfire smoke than meets the eye, scientists say, and what you can’t see can hurt you.

What makes the skies hazy during a wildfire?

When a tree burns, the heat breaks down plant material and releases flammable gases. Some of those gases react with oxygen and become carbon dioxide and water vapor in the air. But if the fire doesn’t have enough oxygen to fully burn the gases, it can produce more thick smoke filled with countless tiny particles and chemicals.

The signature yellow, orange and brown-ish color of wildfire smoke comes from a group of pollutants aptly called “brown carbon,” an umbrella term that covers thousands of compounds, said Westervelt. These pollutants tint the sky because they absorb sunlight at shorter wavelengths — responsible for blues and ultraviolet — and allow the longer wavelengths behind reds, yellows and oranges to pass through the atmosphere and reach our eyes.

Brown carbon falls under a class of pollutants known as particulate matter 2.5 (PM 2.5), named because they span about 2.5 micrometers in diameter — about 30 times smaller than the width of a human hair. These fine particles can evade our body’s natural defenses and travel deep inside our lungs, causing breathing or heart issues.

PM 2.5 particles are one of the most serious threats to people, prompting all the safety warnings you see to stay indoors with good air filters. They can travel hundreds of miles and remain in the air for days to weeks.

Wildfires also emit hydrocarbon gases like benzene, toluene, xylenes and ethylbenzene, which are “hazardous air pollutants themselves,” Westervelt said. But he said they can undergo additional chemical processing in the atmosphere and become brown carbon and PM 2.5 particles.

The smoke also contains a larger class of pollutants known as particulate matter 10 (PM 10), about seven times smaller than the width of a human hair. Still microscopic, these particles also pass through our nose and throat to cause breathing and heart issues, but they can fall out of the air faster thanks to gravity or rain. They remain airborne for hours to days — concentrating and obscuring skies closer to the source of the fire.

What are the invisible pollutants in a wildfire?

Not all wildfire pollutants announce themselves with color. As smoke travels, it can lose its hue without losing its chemistry. Sometimes it even becomes more toxic over time.

Take brown carbon, for example. Photons of sunlight can break up the chemical bonds in the particles and transform its chemical identity, a process known as oxidation, Westervelt said. The plume may appear clearer because it no longer absorbs light in the same way.

This oxidation process “can play a big role in how brown the brown carbon is, so to speak, but it also changes their toxicity as well,” Westervelt said.

This clearer smoke can trigger the formation of other colorless and odorless harmful pollutants. Ozone is a toxic gas that acts like sunburn for our lungs. It doesn’t come directly from burning vegetation, but concentrations tend to increase during wildfire activity. That’s because wildfire smoke contains gases like nitrogen oxides and volatile organic compounds, which react in the sun and heat to create ground-level ozone.

This highly reactive chemical can inflame our lungs, causing muscles to constrict and making breathing painful, according to the Environmental Protection Agency. (When ozone is high in the sky, however, it can act as a shield to protect us from harmful ultraviolet radiation.)

Formaldehyde is another invisible gas that can form after wildfires in the same way as ground-level ozone. Formaldehyde can irritate our eyes, throats, noses and lungs, but can also cause cancer — suggesting that exposure to persistent wildfire smoke could become more toxic as time goes on.

Does an orange sky always mean bad air quality?

On the flip side, the sky can sometimes appear brown or orange without affecting our breathing air. It all depends on the location of the smoke plume.

Smoke high in the sky can act like a filter. When it stays several hundred meters above us, Westervelt said it may not have as much of an immediate effect on our air quality.

“When it’s above a couple 100 meters in the atmosphere … you won’t see as many high (air quality index numbers),” Westervelt said. “The closer it is, the more it’s going to affect us.”

All of this explains why the network of air quality monitors that measure particles at ground level — including PM 2.5 and ozone — are so important. The best way to know if you are breathing harmful air is to check the air quality reports — available at AirNow — that the government makes based on these sensors.

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