Page 17 - CleanScience_Fall22
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About the Authors
Joe C. Spurgeon,
Joe C. Spurgeon, PhD
Franco Seif
Eugenia Mirica, PhD
Franco Seif,
Eugenia Mirica
SYNOPSIS
Wildfire smoke residues were evaluated in 343 northern California houses that were potentially impacted by one of 22 wildfires. A total of 1,715 wet-wipe samples were collected from five hard-surface sampling areas in each house, including exterior surfaces, attics, interior window sills, interior hard surfaces, and return air plenums.
The samples were analyzed for char, ash, and soot. Char was detected in 363 of the samples, ash was detected in 37 of the samples, and soot was detected in four of the samples. Char was the primary wildfire smoke residue based on the frequency of detection and was the most useful for evaluating the impact of wildfire smoke residues on structures.
Char concentrations on interior surfaces were primarily detected in four concentration ranges: <1%, 1%–2%, 3%–10%, and >10%. More than half (55%) of char concentrations on interior surfaces in impacted structures were 1%–2%, with an additional 28% exceeding 10%.
Char concentrations were less than 1% in all five sampling areas in 147 (43%) of the 343 houses and were 1% or more in at least one sampling area in 196 (57%). Defining sampled sur- faces with a char concentration of 1% or more as having been impacted by wildfire smoke residues was a practical criterion, it was consistent with the laboratory LOQ, and it was a useful guideline for evaluating impact.
Houses closest to the wildfire were impacted by char to a greater extent than those farther from the wildfire. About 74% of exterior samples and 65% of interior samples with char concentrations of 1% or more were collected within one mile of the wildfire. Although peak concentrations decreased with distance from the
wildfire in the range of 1–30 miles, the average concentrations did not vary substantially in the range of 6–50 miles.
The average char concentrations on exterior surfaces, interior window sills, and interior hard surfaces declined at small but relatively constant rates during the first 10 months. Therefore, the elapsed time between the wildfire and the inspection may need to be considered when evaluating initial conditions.
The char concentration measured for one sampling area was not a good indicator of the char concentrations measured at other sampling areas. There was at least a 3% difference in the aver- age char concentration between the interior window sills and hard surfaces in 44% of the 143 houses that had a char concentration of 1% or more in both locations. These results suggested that the inspection and sampling strategies for evaluating the impact of smoke residues should include the concepts of “similar impact areas” and “similar restoration areas.”
The wet-wipe sampling method was effective for sampling wildfire smoke residues, especially the dominant residue (char). The method could be applied to smooth and intricate hard surfaces, as well as heavily loaded surfaces. The sample preparation step increased the homogeneity of subsamples for analysis, which reduced analytical variability and dispersed obstructing debris particles.
The wet wipe method allowed composite samples to be collected, with each composite sample representing the result for three to five individual surfaces. Composite samples increased the probability of detecting wildfire smoke residues, resulted in a better characterization of the space that was sampled, and reduced sampling cost.
FALL 2022
THE JOURNAL OF CLEANING SCIENCE | 17
