The Fort McMurray wildfire that started inAlberta, Canada, on May 1, 2016 burned more than 2,300square miles, caused theevacuation of close to 90,000 people, and destroyed nearly 2,000 structures anddamaged another 19,000. Two fatalities were indirectly tied to the fire. Earlyfinancial estimates put the insurance cost at between $2.6 and $4.7 billiondollars. By June 10, the fire was nearly 73percent contained.

By contrast, the2003 Cedar Fire in San Diego County,Calif., while only one-fourth the size of the Fort McMurray fire, displacedmore than 300,000 people, caused 14fatalities, and destroyed 2,400 structuresdue to higher population density in the urban-wild land interface. As theeffects of climate change on seasonal weather become more evident, frequent andsevere wildfires in proximity to residential areas are expected to exposegreater numbers of homes and people to smoke and debris from the fire and itsaftermaths.

As of mid-June, Fort McMurray was under a boil-waterorder. Air quality throughout the region as significantly affected. Arsenic andheavy metals contamination in some undamaged homes made them unsafe toreoccupy. The alkaline ash—one of the main components of wildfires—acts as acorrosive agent, and dust suppress ion compounds sprayed on burned structurescontain crystalline silica. Reconstruction activities can cause thesesubstances to become airborne. Labor authorities urged employers to take allnecessary steps to protect the health and safety of their workers and mitigatehazards once recovery begins.

Wildfires can impact property, the environment, andpublic health from the immediate vicinity of the fire up to several hundredmiles from the source. Insurance claims for property damage are the drivingforce behind most investigations of damage from wildfire smoke. In many cases,these investigations take place several months to a year after the incident.Investigative, sampling, and analytical techniques are primarily intended toconfirm the presence or absence of wildfire residues and determine the degreeof damage to property and assets. The potential human health effects ofwildfire smoke and residues often remain unanswered. Does wildfire residue posea human health hazard? How do we evaluate the potential health hazard posed bywildfire residue? What are the background levels of similar products ofcombustion in homes? And what is the appropriate level of remediation and cleanup?

Modeling Exposure to Wildfire Residues

Smoke inhalation can have acute and chronic effects onthe health of wild land firefighters. Heating and cooking with wood can alsohave recognized adverse health effects on household residents. However, thereis little information on the health risk to residents from exposure to wildfireresidues in homes that are reoccupied after being affected or damaged by smoke.

A conceptual model can serve to explore the potentialadverse health effects of residential exposure to wildfire residues. In thismodel, the wildfire does not reach as tructure. Instead, smoke from thewildfire descends upon a residential area and enters the home through roofpenetrations, make-up air vents, and seams around doors and windows. Gaseousand particulate smoke contaminants entering the building settle on solidsurfaces, including HVAC units and ductwork, and some are adsorbed ontocarpets, floors, walls, and furnishings. When residents return home days orweeks after the fire, the smoke is no longer a direct hazard. Combustion gasesand the more volatile smoke constituents dissipate and become diluted bypassive or active ventilation to reduce odors.

The fate and transport of the residual contaminantsdepends in part on house keeping ractices, the chemicals’ partitioncoefficients, and the type of ventilation. The larger-size fractions of settleddust are removed by regular cleaning, while the respire able-size particulatesmay remain longer in the absence of HEPA filtration. Adsorbed semi-volatileorganic compounds (SVOCs) continue to off-gas following their gas-solid phaseequilibrium kinetics for days, weeks, and even months. At this stage, thepotential health risk from the wildfire residuals is most likely frominhalation, skin contact, and ingestion of particulates—mainly char and ashdeposited by the smoke, as well as the polycyclic aromatic hydrocarbons (PAHs)that have become adsorbed onto the fire particulates and onto surfaces in thehome.

Evaluating Health Hazards

Wildfire smoke is a veritable cocktail of products ofincomplete combustion. Ash and char, the main components of wildfires, maycontain heavy metals, PAHs, and dioxins and furans. Research on firefighterexposure to smoke has identified a number of constituents of potential concern:combustion gases, such as carbon monoxide; volatile organic compounds (VOCs),particularly benzene; aldehydes such as form aldehyde and a crolein; a widevariety of PAHs, including pyrene, phenanthrene, benzo(a)anthracene, benzo (a) pyrene,and benzo (b) fluoranthene resulting from combustion of organic matter; and particulatematter, principally in thePM2.5-size range.

The health effects of wood smoke inhalation range fromacute irritation, inflammatory responses, asthma triggers, and immune systemsuppression to changes in lung function(measured as increased airway resistance);reduced lung function capacity; chronic illnesses, including bronchitis,obstructive pulmonary disease, and cardiac disease; and cancers of the lung,skin, and bladder.

Concentrations of VOCs, PAHs, and particulates measuredat wildfire suppression and prescribed burning operations suggest that theprimary sources of cancer risk in the range of one-in-one-thousand toone-in-one-million derive from exposure to benzene and formaldehyde, and thatnon-cancer adverse health effects above a level of concern (hazard quotient> 1) result from exposure to ac role in and respire able particulate matter.In studies of wild land firefighter risk assessment models, neither carbonmonoxide nor any of the PAHs detected reached levels of concern.

Many of these compounds have also been identified insidehomes, which complicates three valuation of health hazards from wildfire smokeresidues. Background sources of PAHs in urban outdoor air and in homes notaffected by wildfire smoke include smoke from fireplaces and cigarettes, asphaltpavement sealers containing coal tar, and vehicle exhaust. Background PAHlevels in indoor air range from 0.00027 µg/ to 0.05 µg/,approximately twice the background levels found in outdoor air.

PAHs exist in equilibrium between a vapor and a solid phase,and have a strong affinity for organic matter like charcoal. They attach tobuilding materials and furnishings, such as carpet, gypsum wallboard, and evenstainless steel, and slowly off-gas for time periods ranging from hours toweeks or months. As are sult, PAHs are commonly found as a component ofhousehold dust. Typical background levels are in the range of 0.15 to 1.64micrograms per gram (µg/g)of dust. Dust ingestion by children is the secondmost important route of exposure to carcinogenic PAHs, after inhalationexposure. However, household dust needs to contain more than 150 times thetypical PAH background levels to pose a lifetime cancer risk aboveone-in-one-million.

Sampling and Analysis

The residential post-wildfire exposure scenario discussedin this article illustrates some of the uncertainties that must be addressedwhen evaluating human health risk from wildfire residuals. These include thecomposition of the wildfire smoke inside a home; concentration of chemicalconstituents in the smoke; fate and transport of persistent wildfire residualchemicals in the interior environment; partition coefficients of PAHs in char,ash, construction materials, and furnishings; background levels fromnon-wildfire sources; and the effects of ventilation and housekeeping practiceson contaminant deposition and removal rates.

The wildfire impact investigator needs to have a clearunderstanding of the sampling technique and analytical methods to ensure thatthey are compatible with each other and meet the desired objectives. Varioustechniques for particulates, metals, VOCs, SVOCs, or gases are applicable towildfire residue investigations.

Depending on the objective of the investigation,particulate analysis of soot, char, and ash can be conducted through opticalmicroscopy using reflected and transmitted light, transmission or scanningelectron microscopy (TEM or SEM) supported by X-ray diffraction techniques formaterial composition, or a combination of methods. Sample collection techniquesneed to consider the desired analysis to maintain sample integrity. Forinstance, wipe sampling with water or solvents can dissolve ash particles andrender them invisible to microscopic examination. By the same token, some labpreparation techniques can destroy the sampling media, or the sample itself.

Some of the most common sampling techniques in wildfireproperty damage claims are tape lift sand micro-vacuum sampling for particulatemicroscopy analysis, often used as part of Visual Area Estimation (VAE)analysis. This method identifies various stypes of particles in the sample,such as soot, ash, char, mold, cellulose, soil minerals, and other opaqueparticles, and reports them as percentages or numbers of each componentparticle in the observed field. The laboratory interprets the percentage orcount in a range from normal background to uncommon, and makes recommendationsas to the source of the particles or the level of cleaning needed. The resultsand interpretation of this part iclecharacterization method can vary betweenlaboratories. Because VAE an alysisdoes not determine actual surface or massconcentrations of the identified particle types in the home, it is notappropriate for human health risk assessments.

Air sampling methods for gases, VOCs, and SVOCs used intraditional industrial hygiene orindoor air quality investigations can be usedin wildfire residue as sessments. The objective of the investigation willdictate the sampling and analytical technique. Direct-reading instruments forcombustion gases and VOCs can be used for initial screening. Bulk samples offurnishings or building materials can beanalyzed in environmental chambers foroff-gassing chemicals to identif ypersistent odors. Due to the long time lagbetween the fire and the smoke damage investigation, sampling methods that candetect residual concentrations in the parts per billion range (ppb)—such as EPATO-15 and TO-17 for VOCs, and XAD-coated samplers for SVOCs—followed by gaschromatography and mass spectrometry are often necessary. These methods candetect hundreds of chemicals at ppb and sub-ppb concentrations.

The experienced laboratory can help sort through thebackground “noise” to identify the signature chemical compound combinationsthat indicate the presence of apart icular wildfire residue. Choosingexperienced and qualified wildfire residue laboratories and working closelywith the analysts to develop the sampling strategy is essential to produceusable and defensible data to support the interpretation of results. This isespecially important in wildfire residue investigations due to existing gaps inpublished standard methods in this emerging area of practice.

Sampling and analytical methods specifically designed toquantify concentration and distribution of the wildfire residuals in thebuilding and allow calculation of dose by the major routes of exposure arenecessary to evaluate the human health risk from wildfire smoke. Industrialhygienists involved in wildfire residual impact investigations from the healthrisk perspective need to utilize sampling and analytical methods that canevaluate potential health hazards. This data can also inform the development ofclean-up levels and post-remediation verification protocols.

ENRIQUE MEDINA, MS, CIH, CSP, FAIHA, is president ofAlliance Consulting International in SanDiego, Calif., and a member of the AIHAEnvironmental Issues Committee. He can be reached at (619) 297-1469 oremedina@pulse-point.com.