Wildfires are expected to bring unhealthy air to more than 120 million people this weekend, but the smoke and its toxic particles aren’t always clearly visible. Some horizons are covered with a decidedly unusual orange hue, accompanied by a faint burnt smell. At other horizons, there may be no visible signs that something is wrong in the atmosphere, but that doesn’t necessarily mean the air is safe.
“Just because it looks a little less brown and orange today doesn’t necessarily mean there’s no smoke present,” said Dan Westervelt, an atmospheric scientist at Columbia University.
Smoke visibility can be affected by several factors, including the chemicals and particles the smoke carries, how long the smoke is in the air, the weather, and the angle of the sun.
Although the human eye cannot always detect danger, air quality monitors can accurately track the number of pollutants in the air. So even if everything seems fine, it’s important to follow local safety guidance, including staying indoors, turning on your air filter, and wearing an N95 mask.
Scientists say there’s more to wildfire smoke than meets the eye, and what you can’t see can hurt you.
When wood burns, the heat decomposes the plant material and releases flammable gases. Some of these gases react with oxygen to form carbon dioxide and water vapor in the air. But if the fire doesn’t have enough oxygen to completely burn the gas, it can produce thick smoke packed with millions of tiny particles and chemicals.
Westervelt says the characteristic yellow, orange and brownish colors of wildfire smoke come from a group of pollutants aptly called “brown carbon,” a collective term that covers thousands of compounds. These pollutants color the sky because they absorb shorter wavelengths of sunlight that cause blues and ultraviolet rays, allowing longer wavelengths behind reds, yellows, and oranges to pass through the atmosphere and reach our eyes.
Brown carbon falls into a class of pollutants known as particulate matter 2.5 (PM 2.5). Particulate matter 2.5 has a diameter of about 2.5 micrometers, which is about one-thirtieth the width of a human hair. These particles can bypass our body’s natural defenses and penetrate deep into our lungs, causing breathing and heart problems.
PM 2.5 particles are one of the most serious threats to people, with all the safety warnings telling us to use proper air filters and stay indoors. They can travel hundreds of miles and remain airborne for days or weeks.
Wildfires also release hydrocarbon gases such as benzene, toluene, xylene and ethylbenzene, which are “hazardous air pollutants in their own right,” Westervelt said. But they can undergo additional chemical processing in the atmosphere, turning them into brown carbon or PM 2.5 particles, he said.
Smoke also contains a larger type of pollutant known as particulate matter 10 (PM 10), which is about one-seventh the width of a human hair. Although these particles are still microscopic, they can pass through our noses and throats and cause breathing and heart problems, but they can also fall out of the air faster thanks to gravity and rain. They remain in the air for hours to days, concentrating and obscuring the sky close to the source of the fire.
Not all wildfire pollutants announce themselves by color. As the smoke moves, it can lose its hue without losing its chemistry. In some cases, they can become more toxic over time.
Let’s take brown carbon as an example. Photons from sunlight can break chemical bonds within particles and change their chemical properties, 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 “may play a big role in the brown color of brown carbon, so to speak, but it also changes its toxicity as well,” Westervelt said.
This clearer smoke can cause the production of other colorless and odorless harmful pollutants. Ozone is a toxic gas that acts like a sunburn on your lungs. Although it does not result directly from burning vegetation, concentrations tend to increase during wildfire activity. That’s because wildfire smoke contains gases such as nitrogen oxides and volatile organic compounds, which react with sunlight and heat to create ground-level ozone.
According to the Environmental Protection Agency, this highly reactive chemical can irritate the lungs and cause muscles to contract, making it difficult to breathe. (However, when ozone is high in the sky, it can act as a shield to protect us from harmful UV rays.)
Formaldehyde is also an invisible gas released after wildfires, similar to ground-level ozone. Not only can formaldehyde irritate our eyes, throat, nose, and lungs, but it can also cause cancer. This suggests that exposure to persistent wildfire smoke can become more toxic over time.
Conversely, the sky can appear brown or orange without affecting the air we breathe. It all depends on the location of the smoke column.
Smoke in the sky acts like a filter. Westervelt said if it stays a few hundred meters above us, it may not have a big impact on air quality right away.
“Once you get above 100 meters in the atmosphere, you don’t see as many high (air quality index) numbers,” Westervelt said. “The closer it gets, the more impact it will have on us.”
All of this explains why a network of air quality monitors that measure particles such as PM 2.5 and ozone on the ground is so important. The best way to know if you’re breathing harmful air is to check the air quality reports that the government creates based on these sensors (available on AirNow).
