Indoor Air Quality

After the Wildfire Smoke has Cleared
Kristen Shaw Clark Seif Clark, Inc.

By mid-September of 2006, more than 80,000 wildfires had burned nearly 9 million acres in the United States since the first of the year; one million acres more than had burned the previous year, according to the National Inter-agency Fire Center

Typically we think of wildfires occurring in remote areas that affect only the future Christmas tree selection and wildlife. We often don't consider wildfires raging through suburbs; blazing by homes, schools, hospitals, businesses and other buildings where residents will return after the fire.

However, throughout US history, wildfires have plagued developed communities. For example, in October 1871, the Peshtigo Fire burned more than 3.7 million acres in Wisconsin and Michigan. An estimated 1,500 people died in the fire. On September 1, 1894 near Hinckley, Minnesota, small blazes converged to form a firestorm burning 350,000 acres and killing 418 people. The Great Fire of 1910, one of the biggest wildfires ever recorded in North America, burned about 3 million acres in Idaho and Montana, and killed 86 people. In October 2003, The Cedar Fire in San Bernardino consumed 80,000 acres in about 10 hours on the day it started. Three days later the wildfire had torched 206,000 acres. Twelve other fires were burning nearby. The toll by the end of October was 16 people dead, nearly 2,000 homes burned and over 600,000 acres scorched.

Wildfires have long been feared for their potential to destroy lives and homes. The smoke generated by wildfires is also a widely recognized health threat. In fact, some communities are evacuated during large wildfires, not because they are at risk for burning, but because of the smoke.

Smoke is a complex mixture of particles, liquids and gaseous compounds, including polynuclear aromatic hydrocarbons (PAHs), organic acids, particulate matter (PM), semi-volatile and volatile organic compounds (VOCs) and the inorganic fraction of particles. The types of particles, liquids and gaseous compounds released in smoke depend on the fuel type and the amount of fuel, among other factors. The fuel for a house fire or structure fire includes all of the items burned in the building: carpet, carpet pad, paint, electronics, linens, clothing, synthetics, polymers, etc. The fuel for a wildfire is primarily plant material such as wood from trees and shrubs, as well as grasses.

Many of the individual components of wildfire smoke are known to cause both short-term and long-term harmful health consequences, as well as damaging effects to property. For example, several PAHs in wildfire smoke, such as benzo(a)pyrene, benz(a)anthracene/chrysene and benzo(j)oranthenes are carcinogenic. Organic acids such as 2-furaldehyde and vinyl acetate are not only carcinogenic, but can cause property damage by corroding susceptible surfaces.

Another component of smoke, categorized as particulate matter, can be composed of any of the combustion by-products, including PAHs, organic debris and inorganic residues. Numerous air pollution studies have shown that small increases in the concentrations of particulate matter are associated with notable increases in respiratory and cardiovascular disease mortality. The association between increased respirable particulate matter and childhood asthma and other respiratory diseases is also well established.

Particulate matter small enough to be inhaled is segregated by size: particles up to 10 micrometers in diameter (PM10), which the EPA considers "inhalable coarse particles"; and particles smaller than 2.5 in diameter (PM2.5), called "fine particulates." If inhaled, the larger PM10 deposit in the upper respiratory tract, while smaller PM2.5 travel deeper into the lungs and generally are retained within the lungs.

The EPA National Ambient Air Quality Standards (NAAQS) for particulate matter was first issued in 1971, and then revised in 1987 and 1997. In September 2006, the EPA again tightened the PM standards. The revised 2006 standards tighten the 24-hour fine particle standard from 65 micrograms per cubic meter to 35 g/m3, and retained the current annual fine particle standard at 15 g/m3.

Particulate matter is also categorized as "ultra fine particles." Fine particles are less than 2.5 in diameter, while ultra fine particles are only 0.15 to 0.4 in diameter. (By comparison the period at the end of this sentence is about 500 in diameter.) Most ultra fine particles are too small to be removed by HEPA filters, which can remove 99 percent of filtered particles that are larger than 0.3 in diameter. The majority of particulate matter produced in a wildfire is in the ultra fine particle size range.

The majority of wildfire smoke particulates are in the fine particulate category. These wildfire smoke respirable particulates can contain organic materials that may have significant long-term health effects, such as PAHs, aldehydes, VOCs and organic acids. The toxicity of particulates retained in the lungs varies with chemical composition. Chemical changes of smoke particulates may occur in the form of chemical reactions with other aerosols. Particles may stick together or break apart, changing the size distribution over time.

Research has confirmed that fine particles outdoors will infiltrate indoors, even with all of the windows and doors closed. Some studies have found that as much as 70 to 100 percent of the fine particles outdoors will infiltrate indoors. Many commercial buildings and schools mechanically draw outdoor air into the buildings. Usually, the outdoor air is filtered before it is supplied to the occupants. However, standard HVAC air filters will not remove most of the ultra fine wildfire smoke particles. Also, many schools that rely on portable buildings for classrooms bring in outdoor air by installing continuous exhaust fans. Unfiltered outdoor air is brought indoors by keeping the classrooms under negative pressure.

When heavy concentrations of tiny wildfire smoke particles enter a home, school or other building, the particles can eventually settle out of the air to deposit on horizontal surfaces, or plate out on vertical surfaces, penetrate upholstery, drapes, and insulation; or electrostatically adhere to electronic components or other charged surfaces, as well as impact on surfaces in the path of air currents. Settled respirable particulate matter can be re-entrained into the air by even small disturbances.

Research shows that large wildfires produce in excess of 36 tons of particulate matter per minute, which is 2,160 tons of particulate matter per hour. Under some conditions, wildfires can produce 30 times that amount of particulate matter.

Following the Rodeo-Chediski Fire in July 2002, Arizona's largest wildfire, studies by the Arizona Department of Environmental Quality reported in the draft document "Air Pollution Monitoring at the Rodeo-Chediski Fire: June-July 2002" that on June 25, 2002 alone there were 148,500 tons of PM10 produced by the fire and dispersed in the smoke; on average that is 61,875 tons of particulate matter per hour. Of the hundreds of thousands of tons of respirable particulate matter dispersed in wildfire smoke, most particulates are less than 1 ?m in diameter. In fact, more than 80 percent of the particles are less than 0.3 ?m in diameter.

Of the hundreds of thousands of tons of respirable particulate matter produced in a large wildfire and dispersed in the smoke, only 2 to 15 percent is graphitic carbon, or what is commonly called "soot".

Smoke contamination in a building is most commonly determined by looking only for the presence of "soot," or carbon black, which can produce "ghosting" that is visible on light colored surfaces. However, because soot is only a tiny fraction, as little as 2 percent, of the complex components of smoke, it is an unreliable marker for smoke contamination. By looking only for soot, as much as 98 percent of the smoke particulates that have contaminated homes, schools and other buildings are disregarded.

Additionally, most of the chemicals, compounds and particles in smoke are pale in color or even colorless, therefore en masse would have a light gray or white appearance similar to household dust. Much of the indoor wildfire smoke contamination does not have a visual appearance common to structure fire smoke contamination. If surfaces in buildings near a large wildfire sustain levels of soot deposits sufficient to cause the discoloration commonly associated with structural fires, then the amount of contamination from the other constituents of wildfire smoke, will be substantial.

Laboratory analyses for soot from surface samples are often performed using light microscopy, or sometimes by transmission electron microscopy (TEM). However, the smallest size of particles that can be resolved with the highest quality light microscopes is 0.2 ?m. Ultra fine soot particles cannot be identified with light microscopy. TEM can identify carbon-based ultra fine particles. However, TEM soot analysis also has limitations. Different wipe sample media can collect various amounts of settled particulates from a surface. Other organic dust and debris collected on the surface sample, including the non-soot smoke particles, can interfere with analysis by overwhelming the sample.

There are no standards for analysis so that different qualified labs may have various methodologies for analysis, resulting in different findings for the same samples. Soot, or carbon black particle assessment is based on a visual assessment of morphology. Carbon black particles resulting from combustion are irregularly shaped. Identification of combustion by-product soot particles in a field of hundreds or thousands of irregularly shaped particles requires extensive experience in particle identification. Unburned wood or biomass particulates distributed in smoke have none of the characteristics of carbon black and are not identified in TEM analysis. Particles of metal oxides may appear the same as carbon black. Particles considered to be carbon black can be additionally assessed by energy-dispersive x-ray spectrometry (EDX). However, confirmation by EDX requires nearly pure carbon residue, so that fragments of hydrocarbons and unburned portions of biomass that retain hydrogen and oxygen atoms may not be confirmed as carbon black or by-products of combustion.

Other types of laboratory analysis for combustion by-products are available, such as Gas Chromatography Flame Ionization Detector (GC FID), for identification of fuel products, specifically diesel fuels. Data to identify combustion by-products in settled wildfire smoke particulates is not available in commercial analytical laboratories. Research laboratories that study wildfire smoke have the technology and the methodology for quantifying samples that are properly collected. However, most research laboratories study airborne smoke, not settled smoke particles. The cost for the analysis is usually prohibitively expensive. So, as indoor air quality professionals, what can we recommend to homeowners, property managers and school administrators in communities impacted by wildfire smoke?

Careful cleaning is the most prudent course of action. Depending on the type of building or facility and the level of smoke contamination, remediation professionals with personal protection equipment may be required to return a home, school or other facility to a safe indoor air quality condition.

The Federal Emergency Management Agency (FEMA) provides wildfire smoke remediation guidelines in a pamphlet titled "Tips From State And FEMA On Smoke Removal And Fire Cleanup" (available on-line The FEMA document outlines cleaning and remediation actions homeowners should undertake following a wildfire to reduce smoke and ash contamination of their properties. The course of actions specified by FEMA includes:

  • Pressure washing the exterior of the home, walks and automobiles
  • Washing all interior walls and hard surfaces with mild soap or other appropriate cleaning solutions or products and rinse thoroughly; including inside cabinets, drawers and closets.
  • Launder or dry clean all clothing.
  • Cleaning all household items.
  • Cleaning all carpets, window coverings, upholstered furniture and mattresses with steam or other appropriate equipment to clean, disinfect and deodorize.
  • A precaution not provided in the FEMA pamphlet is that cleaning actions should be performed in a way to minimize the re-entrainment of particles. Cleaning methods that should be avoided include vacuuming, dry dusting, sweeping, and vigorous wiping that will aerosolize smoke particulates from surfaces. In addition, cleaning of the interior of electronic components, such as computers, stereos and televisions; as well as refrigerator condenser coils and fan or other appliances that would attract particulates should also be performed.

    Smoke contamination of a building can be reduced by keeping windows and doors closed and closing any outdoor air intake HVAC during the fire. Turning off air conditioning is most favorable if the building is evacuated, since the cooler air indoors will likely promote outdoor air leaking indoors.

    In summary, wildfire smoke can degrade indoor air quality, even after the outdoor smoke has cleared. Fine and ultra fine smoke particulates can penetrate homes and businesses, even when the buildings are closed and the HVAC is turned off. Soot is only a minor component of wildfire smoke. When determining the level of smoke contamination and the appropriate remediation for a building, all types of smoke particulates should be considered. When residents return to their homes, schools and other buildings, precautions should be taken to minimize exposure to smoke particulates retained indoors. Personal protection equipment, such as respirators and gloves, should be utilized during a thorough cleaning of the building, HVAC and contents to help restore indoor air quality. In some cases, specialty cleaning by remediation professionals may be required.