After the Fire
Assessing the Potential Health Risks of Wildfire Residues in the Indoor Environment
BY ENRIQUE MEDINA
The Fort McMurray wildfire that started in Alberta, Canada, on May 1, 2016 burned more than 2,300 square miles, caused the evacuation of close to 90,000 people, and destroyed nearly 2,000 structures and damaged another 19,000. Two fatalities were indirectly tied to the fire. Early financial estimates put the insurance cost at between $2.6 and $4.7 billion dollars. By June 10, the fire was nearly 73 percent contained.

By contrast, the 2003 Cedar Fire in San Diego County, Calif., while only one-fourth the size of the Fort McMurray fire, displaced more than 300,000 people, caused 14 fatalities, and destroyed 2,400 structures due to higher population density in the urban-wildland interface. As the effects of climate change on seasonal weather become more evident, frequent and severe wildfires in proximity to residential areas are expected to expose greater numbers of homes and people to smoke and debris from the fire and its aftermaths.
As of mid-June, Fort McMurray was under a
boil-water order
. Air quality throughout the region was significantly affected. Arsenic and heavy metals contamination in some undamaged homes made them unsafe to reoccupy. The alkaline ash—one of the main components of wildfires—acts as a corrosive agent, and dust suppression compounds sprayed on burned structures contain crystalline silica. Reconstruction activities can cause these substances to become airborne. Labor authorities urged employers to take all necessary steps to protect the health and safety of their workers and mitigate hazards once recovery begins.
Wildfires can impact property, the environment, and public health from the immediate vicinity of the fire up to several hundred miles from the source. Insurance claims for property damage are the driving force 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 to confirm the presence or absence of wildfire residues and determine the degree of damage to property and assets. The potential human health effects of wildfire smoke and residues often remain unanswered. Does wildfire residue pose a human health hazard? How do we evaluate the potential health hazard posed by wildfire residue? What are the background levels of similar products of combustion in homes? And what is the appropriate level of remediation and clean up?
MODELING EXPOSURE TO WILDFIRE RESIDUES
Smoke inhalation can have acute and chronic effects on the health of wildland firefighters. Heating and cooking with wood can also have recognized adverse health effects on household residents. However, there is little information on the health risk to residents from exposure to wildfire residues in homes that are reoccupied after being affected or damaged by smoke. 
A conceptual model can serve to explore the potential adverse health effects of residential exposure to wildfire residues. In this model, the wildfire does not reach a structure. Instead, smoke from the wildfire descends upon a residential area and enters the home through roof penetrations, make-up air vents, and seams around doors and windows. Gaseous and particulate smoke contaminants entering the building settle on solid surfaces, including HVAC units and ductwork, and some are adsorbed onto carpets, floors, walls, and furnishings. When residents return home days or weeks after the fire, the smoke is no longer a direct hazard. Combustion gases and the more volatile smoke constituents dissipate and become diluted by passive or active ventilation to reduce odors.
The fate and transport of the residual contaminants depends in part on housekeeping practices, the chemicals’ partition coefficients, and the type of ventilation. The larger-size fractions of settled dust are removed by regular cleaning, while the respirable-size particulates may remain longer in the absence of HEPA filtration. Adsorbed semi-volatile organic compounds (sVOCs) continue to off-gas following their gas-solid phase equilibrium kinetics for days, weeks, and even months. At this stage, the potential health risk from the wildfire residuals is most likely from inhalation, skin contact, and ingestion of particulates—mainly char and ash deposited by the smoke, as well as the polycyclic aromatic hydrocarbons (PAHs) that have become adsorbed onto the fire particulates and onto surfaces in the home.
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