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The following is information from our Environmental Health & Safety team, answering commonly asked questions related to wildfires and air pollution. Have a question? Submit it to campushealth@usc.edu.

What should I know about air pollution?

Air pollution (particulates) such as fine matter released from burning fires are a continuous concern for environmental health, particularly in areas where vehicle exhaust, commercial release of chemical by-products, or other pollutants can impact people and animals by entering the lungs. 

Air pollution exposure can impact health both through exposure (length of time, frequency) and concentration (particulate mass per volume of air, such as micrograms per cubic meter.) 

During exceptional events like wildfires, the concentration of pollutants may change quickly, based on wind direction and acreage currently burning. 

When pollutants are diluted in concentration, this reduces the level of potential harm to health.  

What’s in wildfire smoke? 

It’s a mix of gases like carbon monoxide, carbon dioxide, nitrogen and sulfur dioxides, along with fine particulate matter that are both fine particles and liquid particles, making it a complex mix of pollutants that can impact health when inhaled. The exact composition can vary, but generally include hazardous air pollutants as well as water vapor that can come from burning trees and plants, buildings, cars, and other material that can be harmful to human health. For most people, brief exposure to wildfire smoke may cause mild irritation, but people with respiratory or heart conditions, weakened immune systems, children, and the elderly maybe more sensitive to wildfire smoke. If you need to be outside when the AQI is elevated to 101 or greater, especially if you have a sensitive health condition, it’s recommended to wear an N95 mask. 

What is the red colored chemical the fire department is dropping on the fires? 

The fire retardant used by the fire planes is called Phos-Chek that “slow the rate of fire spread by cooling and coating fuels, depleting the fire of oxygen and slowing the rate of combustion as the retardant’s inorganic salts change how fuels burn.” The main ingredient is a mixture of water, fertilizer (ammonium phosphate), and rust (red color) that fades over time with exposure to sunlight. 

Why does the AQI reading for the campuses sometimes differ from the readings of South Coast AQMD?

USC has our own instruments that collect readings for the campuses; whereas the South Coast AQMD sensors are an estimate of readings covering a wider area.

What does it mean to be under a “red flag warning?”

A red flag warning means warm temperatures, very low humidity, and stronger winds are expected to combine to produce an increased risk of fire danger. Red flag warnings are issued by the National Weather Service. This is not an evacuation warning or order; it means precautions should be taken to reduce the risk of starting a fire under these dry conditions. The following outdoor fire safety tips are particularly relevant during a red flag advisory, and at all times in places with consistent risk of wildfires.

Burning materials: If you are allowed to burn in your area, cover all burn barrels with a weighted metal cover, with holes no larger than 3/4 of an inch.

Lit materials: Do not throw cigarettes or matches out of a moving vehicle. They may ignite dry grass on the side of the road and become a wildfire.

Did you know? The Bureau of Land Management has a comprehensive page on fire restrictions for California, specifically for campfires and hunting with firearms.

I’ve heard that AQI does not measure ash—is that true? How can I protect myself if I am near a burn scar area?

It is correct that AQI does not measure ash. There may be ash in the air when you are near wildfire smoke, or if the wind near a fire zone blows your direction (downwind).

Ash can be variable because it is dependent on your location and the current wind status at that time. If you are in close proximity to a burn scar area, and if you see ash or smoke, it’s best to stay indoors or wear an N95 or respirator when you are outdoors.

Leafblowers should not be used during, after or near a wildfire since it will aerosolize particles and ash into the air.

View information below related to clean-up after a fire:

On Wednesday, January 15, 2025, the LA County Department of Public Health issued a Health Officer Order that prohibits the clean up and removal of fire debris and ash in the burned residential and business areas of the County until a hazardous materials inspection at the property is conducted. This Order is necessary to protect the health and safety of individuals from toxic substances in these areas. There will be additional information forthcoming regarding the fire debris and ash evaluation and removal process as the County moves into the recovery phase of this tragic disaster.

The information in the following link (https://www.mbard.org/wildfire-ash-cleanup-health-resources) is for residents and businesses who are cleaning up ash in structures impacted by smoke, but NOT for those cleaning up burned structures.

Interested in additional safety information related to air pollution, including correct methods of “donning and doffing” (putting on and taking off) an N95 respirator mask? See this air pollution occupational safety sheet from USC Environmental Health & Safety.