What’s in wildfire smoke?

Billowing clouds of Canadian wildfire smoke have transformed blue skies into apocalyptic orange scenes across the eastern United States.

But the fuzzy skylines are more than just a bleak sight. The haze is a noxious mixture of particles and gases that can spread across long distances, potentially harming anything with a heartbeat.

How bad is the wildfire smoke in your city? Use our tool to find out.

Many of the microscopic particles sneak into our lungs and impair breathing. Others irritate our skin, throat and eyes. Some chemicals dissipate quickly, while others can linger for months in our atmosphere. Under severe circumstances, the best defense is to stay inside or wear a mask when outside — and even then, experts say, you could be breathing contaminated air.

"Most wildfires and wildland fires put out hundreds of chemicals," said Rebecca Hornbrook, an atmospheric scientist at the National Center for Atmospheric Research. "It's a very complex mixture of chemicals that are emitted."

The exact chemical compositions and concentrations vary among wildfires, but all smoke clouds share common elements and hazards to people and the environment.

Here's a look at the ingredients of wildfire smoke:

It's one of the main components of wildfire smoke. The particulate matter — solid particles and liquid droplets — released when wildfires burn through trees, buildings and other materials, can be made up of a variety of different things, including soot and organic compounds.

These particles can vary in size, but when it comes to human health, experts are especially concerned about fine particulate matter, known as PM2.5.

By the end of the 21st century, scientists say wildfire smoke will probably be the dominant source of PM 2.5 particles in the United States. Research showed they contributed to a large portion of asthma visits to emergency departments and deaths, transported from wildfires located thousands of miles away.

"That's probably the most dangerous component of the smoke," Hornbrook said.

These minuscule particles — so named because they are typically 2.5 micrometers or smaller, meaning roughly four of them could fit across a single piece of dust — can bypass the defense mechanisms of the upper airway, such as the mucous membrane that lines the nasal cavity, and cause damage deep within the lungs. Some of the smallest particles may even be able to pass into the bloodstream and travel to other organs.

PM2.5 is "the thing that's really going to kill people or send them to the hospital," said Anthony Wexler, director of the Air Quality Research Center at the University of California at Davis.

The effects of inhaling particulate matter can vary from symptoms more akin to those of allergies — such as stinging eyes, a scratchy throat and a runny nose — to coughing, wheezing and shortness of breath, according to the Centers for Disease Control and Prevention.

In more severe cases, PM2.5 exposure has been linked to heart attacks and stroke, as well as to lung cancer and damage to cognitive functions.

Known as HAPs, these are another concerning wildfire emission. These pollutants have also been linked to cancer and birth defects, among other health problems, according to the Environmental Protection Agency. Examples include benzene, formaldehyde, lead and mercury compounds.

In wildfires, HAPs are formed when a material is only partially burned, known as incomplete combustion. Take for example burning sugar, Hornbrook said. Sugar is composed of carbon, hydrogen and oxygen. If you add a flame and have enough oxygen, then all of the carbon in the sugar will react to form carbon dioxide and water. Incomplete combustion occurs when not all of the carbon is converted into carbon dioxide. Other more complex compounds form instead.

"Some of it ends up as other things like benzene or formaldehyde," Hornbrook said. "If those were to continue being oxidized or combusted, they would eventually form carbon dioxide, but they escape into the gas phase and they are transported away before they’re able to be completely combusted."

Wildfires also emit a host of different gases, such as large amounts of carbon dioxide, Wexler said. "Anything that you burn releases carbon dioxide, adding to the growing load of carbon dioxide in the atmosphere."

Hornbrook said carbon monoxide, formed through incomplete combustion, is also present in the smoke but that levels are usually not high enough to be poisonous to humans.

Additionally, the blazes can release nitrogen oxides, including nitrogen dioxide, or NO2 — an air pollutant that primarily comes from emissions from cars and power plants.

Breathing air with a high NO2 concentration can irritate people's airways, with short-term exposure leading to worse cases of asthma or triggering other respiratory symptoms such as coughing, wheezing or difficulty breathing.

Nitrogen oxides can also react with other chemicals in the air to form particulate matter and ozone, both of which can be harmful to the human respiratory system when inhaled, according to the EPA.

Fires can also be a source of sulfur dioxide, a pollutant that typically comes from industrial activity, said Erica Smithwick, a wildfire expert at Pennsylvania State University. This gas can transform into secondary pollutants and affect air quality, human health and climate.

Wildfire smoke can also carry bacteria and fungi from forest floors, Wexler said. Some fires create a large amount of updraft, which can then send microbes into the air, where they could come into contact with people and potentially cause infections, he said.

The majority of smoke consists of water vapor, which Hornbrook said is not really dangerous to people. But it can affect weather patterns, including rainfall and the formation of clouds. It can also exacerbate climate change. Water vapor can absorb Earth's heat and prevent it from escaping to space, amplifying global warming. Increased temperatures can then prime conditions for more fires, which can emit more water vapor. Then the cycle continues.

"We’re experiencing effects of climate change right now," Penn State's Smithwick said. "You have these existing climate-induced air pollution effects in inner cities already. On top of that, you add the wildfire smoke from distant areas. It's a continuing landscape and we better be prepared."

Latest news: Smoke from Canadian wildfires is spreading south over much of the Midwest, Ohio Valley, Northeast and Mid-Atlantic, bringing dangerous air pollution to the United States. Expect smoky conditions through the end of the week. Follow live updates on the wildfires and the impact on air quality.

Where the wildfire smoke is the worst: Satellite images show smoke covering the Northeast and extending into the Carolinas. Air quality in Philadelphia, D.C. and New York City was particularly poor, and officials urged at-risk residents to wear high-quality masks outdoors. See how bad the wildfire smoke and air quality are in your area.

Air quality and your health: Breathing in wildfire smoke is bad for your health. The EPA uses a color-coded system to measure air quality. Here's an explanation of what Code Red, Code Purple and more mean. Learn how to protect yourself including which air filters and air purifiers to choose for your home.