This study allowed us to characterize airborne concentrations of several chemicals during a common fire training scenario using representative fuel materials. Area gas samples were collected at different instructor locations and heights to provide a better understanding of potential exposures at those locations. The sampling results indicate that airborne contaminants during live-fire training scenarios can exceed applicable short-term occupational exposure limits but vary considerably due to day-to-day differences in environmental conditions (e.g., humidity and wind), instructors’ positioning, as well as the fuel package utilized. These results reinforce the need to maintain airway protection whenever operating in and around the Fire Behavior Lab because even short-term removal of SCBA could potentially result in over-exposures. High concentrations of these compounds, many of which are known, probable and/or possible carcinogens, also present a risk for dermal contamination via penetration around gaps in PPE and/or from cross contamination when handling PPE after firefighting.
In Part A of this series, we characterized the fidelity of the fire dynamics training objectives generated when utilizing each of these fuels in the Fire Behavior Lab training structure along with the thermal exposure risk for firefighting students and fire instructors [
48]. To achieve training objectives for six ventilation cycles in the Fire Behavior Lab, the most consistent fire dynamics were demonstrated with the OSB fuel followed by particle board and plywood, with fiberboard and pallets resulting in less repeatable flashover and rollover demonstration. However, the OSB fuels resulted in the highest heat fluxes with pallets resulting in the lowest. It was found that fuel substitutions may impact thermal risk for students and instructors but can also impact the consistency of the fire dynamics being presented to the firefighting students. A more impactful reduction in thermal risk may be created by controlling firefighters’ elevation within the training structure, regardless of the training fuel used. Increasing the distance from the fire area had the largest impact in reducing thermal risk to instructors.
Several factors can contribute to the variability in sampling time, fire dynamics and ultimately, the magnitude of combustion products. Fuel preparation and fuels sets were carefully controlled for each experiment. All materials used for the five replicates of each fuel were delivered at the same time from the same manufacture and distributor. Fuels were carefully loaded by trained instructors and researchers to be as identical as possible. The training structure was allowed to cool to ambient conditions prior to reloading the fuels. The ignition scenario and ventilation conditions were scripted, controlled and repeatable. However, the ambient conditions, including air temperature, humidity, pressure, wind speed and direction and, in some cases, precipitation, were not possible to control with the outdoor training prop. Each of these factors can impact not only fire development, but also the ventilation of smoke from the training structure. The 0.9 m sampling height, where the head height of a sitting instructor is assumed to be located, will be near the smoke layer that descends to the fire area floor, so small changes in smoke volume and lift can have dramatic impacts on the exposure levels at this height. However, the experimenal design with randomized fuel order should help account for any unintended biases.
4.1 Impact of Fuel Selection in Fire Behavior Lab
PAHs are the most common class of compounds reported in the fire service exposure literature and the range of PAHs measured in the Fire Behavior Lab were consistent with those measured to date. Fent et al. measured total PAH personal gas concentrations (sampled by personal samplers located at chest height) with medians ranging from 2.78 mg/m
3 for fire instructors conducting pallet and straw fuels scenarios in a concrete training structure to 34.0 mg/m
3 for firefighters in an OSB, pallet, and straw fueled scenario in a metal container based structure [
41]. In comparison, the 31 experiments here resulted in total PAH concentrations that ranged from 6.0 mg/m
3 to 33.7 mg/m
3. Median sampling times in our study (Table
2) were similar to the sampling times for instructors (25–30 min) but longer than the sampling times for firefighters (9 to 12 min) [
41]. Personal gas samples collected using similar methods from firefighters responding to controlled residential fires measured a median of 23.8 mg/m
3 total PAHs (range: 7.46 mg/m
3 to 78.2 mg/m
3) and 17.8 mg/m
3 total PAHs (range: 9.77 mg/m
3 to 43.8 mg/m
3) for firefighters assigned to attack and search job assignments, respectively [
16]. Other studies have reported total PAH concentrations of 0.43 mg/m
3 to 2.70 mg/m
3 for particle board-fueled training fires in Australia [
42], 75 mg/m
3 to 180 mg/m
3 for particle board-fueled training fires also in Australia [
47], and 19 mg/m
3 to 41 mg/m
3 for chipboard-fueled training fires in Sweden [
21]; although it should be noted that the latter authors summed 22 PAHs as opposed to 16 here.
Calculation of toxic and mutagenic equivalencies for the PAH concentrations provided another means of comparing the relative health risk from these training fires. It is possible that the PAH composition generated when using different training fuels could be more heavily weighted towards the more carcinogenic compounds (e.g., benzo[a]pyrene), thus resulting in higher TEQ or MEQ estimates. However, the PAH composition was similar across the training fuel types, with approximately 60% in the IARC Group 1, 2A, or 2B categories and with TEQ and MEQ estimates following similar trends as the total PAH concentrations. Nevertheless, it is interesting to note that although naphthalene was the dominant species (accounting for > 50% of total PAHs), benzo[a]pyrene and dibenzo[a,h]anthracene were the most impactful PAHs on a toxicity basis (Figure S1). Kirk and Logan used the same TEQ calculation on personal gas concentrations of PAHs measured during particle board-fueled training fires and found much lower TEQs (0.044 mg/m
3 to 0.063 mg/m
3) than we did at the rear of the structure for the same fuel (0.223 mg/m
3) and different fuels (0.093 mg/m
3 to 0.695 mg/m
3) [
42]. However, this difference was driven primarily by overall lower PAH concentrations in the Kirk and Logan study which may be attributed to differences in structure geometry, fuel and ventilation locations and/or location of gas sample collection.
Area gas concentrations of benzene measured at the 0.9 m working height inside the structure (median range of 19 mg/m
3 to 270 mg/m
3) were generally higher than the personal gas concentrations measured in Fent et al. where the median range was 9.6 mg/m
3 to 29 mg/m
3 for instructors and 10.8 mg/m
3 to 101 mg/m
3 for firefighters during live-fire exercises involving different fuel packages [
41]. Laitinen et al. reported area gas concentrations of benzene ranging from 0.624 mg/m
3 for pure spruce and pine wood-fueled fires to 0.998 mg/m
3 for chipboard-fueled fires (that also included some polyurethane foam and kerosene) to 2.516 mg/m
3 for conifer plywood-fueled fires [
39]. Kirk and Logan also reported comparably lower gas concentrations of benzene during compartment fire behavior training sessions using particle board (4.5 mg/m
3 to > 7.8 mg/m
3) [
43]. However, Fent et al. measured personal gas concentrations of benzene from search and attack firefighters who operated at controlled residential fires (median 121 mg/m
3 and 129 mg/m
3, respectively with peak concentrations near 1000 mg/m
3 for both groups) that were well within the ranges reported here [
16].
All fuels other than particle board produced similar levels of formaldehyde at the rear instructor location at the 0.9 m height (median range of 39 mg/m
3 to 52 mg/m
3 compared to 2 mg/m
3 for particle board). Fent et al. measured comparable levels of formaldehyde between a pallet and straw scenario and one type of OSB (4.6 mg/m
3 vs 4.5 mg/m
3—though a second type of OSB had higher formaldehyde concentrations (35.2 mg/m
3)) [
41]. Laitinen et al. reported mean formaldehyde concentrations ranging from 0.3 mg/m
3 to 1.5 mg/m
3 for training fires involving wood-based fuels in a ‘fire house’, and 11 mg/m
3 for training fires in a ‘gas simulator’ [
39]. Kirk and Logan also reported lower gas concentrations of formaldehyde during compartment fire behavior training sessions (0.53 mg/m
3 to 5.0 mg/m
3) [
43]. The acrolein concentrations measured inside the Fire Behavior Lab structure at the 0.9 m height (median range of 3.4 mg/m
3 to 32 mg/m
3) were similar to the levels measured during live-fire exercises in Fent et al. where the median range was 4.9 mg/m
3 to 60.6 mg/m
3 [
41].
The pallet-fueled scenario resulted in the highest concentrations of hydrogen chloride. Chlorine and other halogens occur in nature and may be absorbed by trees. However, it is unknown why timber used in pallets would contain more chlorine than timber used in the other wood-based products. This result does corroborate the findings in Fent et al. where the pallet and straw fire training scenario produced higher concentrations of hydrogen fluoride and hydrogen chloride than the scenarios that incorporated OSB [
41].
Another notable finding of this study was the relatively high concentration of methyl isocyanate during the particle board-fueled training fires. Particle board (along with other wood-based products such as OSB and plywood) may contain isocyanate-based glues or polymers, and materials in this family may produce methyl isocyanate (as well as other isocyanate compounds) upon combustion [
56,
57]. Methyl isocyanate is also used in the production of carbamate pesticides. While the straw used in this study was reportedly ‘pesticide free’, the authors ran a follow up experiment using only straw in the ignition barrel and detected similar magnitudes of methyl isocyanate as reported in Table
3 for the training fuels other than particle board. In all five particle board experiments, the ACGIH excursion limits were exceeded in the front and the rear instructor location, while values remained typically an order of magnitude below this limit in experiments with the other four training fuels. The source of the dimethyl phthalate contaminants is unknown. However, phthalates are present in numerous consumer products containing polymers.
4.2 Evaluation of Potential Control Measures
We hypothesized that utilizing a training fuel package that incorporated solid wood products (pallets) along the wall and ceiling would result in a lower concentration of airborne contaminants than fuel packages that utilized panelized wood based products with resins and/or waxes (low density wood fiberboard, OSB, particle board, plywood). We sought to address this question using a common training structure described in NFPA 1402 and 1403 and then altering the fuel packages using materials that are commonly employed in fire service training. For the six-cycle experiments, training fires involving fiberboard, which utilizes wax-based binders, resulted in some of the lowest concentrations of many compounds (including PAHs and VOCs). However, there were no notable differences in chemical concentrations between OSB and fiberboard fuel sets when in the shorter, three-cycle experiments. At the same time, fires involving brand new pallets resulted in the second highest total PAH concentrations at the rear instructor location and the highest BTEXS and acrolein concentrations at the front instructor location. The pallet and fiberboard fuel packages resulted in the longest six-cycle test scenarios. As noted previously, both chemical concentration and exposure duration are important factors that may impact biological uptake (along with PPE protection and other factors). Additionally, as described in Stakes et al., the fire behavior encountered with these two fuel packages were the least consistent in their ability to achieve the underlying training objectives, which can impact the training experience and lessons learned by students [
48].
On the basis of the gas sampling results alone, it may be difficult to select one fuel over another. In comparing the median concentrations across the six-cycle experiments (at the rear instructor location), OSB-fueled fires produced the highest median concentrations of BTEXS and 1,3 butadiene, plywood-fueled fires produced the highest PAH concentrations, particle board-fueled fires produced the highest methyl isocyanate concentrations, and pallet-fueled fires produced the highest hydrogen chloride concentrations. All fuels other than particle board produced similarly high levels of formaldehyde at the rear instructor location. Fiberboard-fueled fires often produced the lowest concentrations of contaminants, likely because the burning rate of the fiberboard decreased to the point where rollover could not be reliably generated after the third ventilation cycle. This explanation is reinforced by the fact that little difference was observed among PAH, BTEXS, and aldehyde concentrations for shorter (three-cycle) experiments involving OSB and fiberboard. Further research is necessary to determine if a substitute fuel can be developed that provides high quality training environments yet results in lower concentrations of airborne contaminants (particularly those that may be carcinogenic). Substitution controls are considered among the most effective under the NIOSH hierarchy of controls [
58]. Finding such a substitute would likely reduce not just potential inhalation exposures (i.e., when respiratory protection is doffed), but also potential dermal exposures from ingress of contaminants through turnout gear or from cross-contamination when handling contaminated PPE. In the meantime, utilizing airway protection whenever in smoke and good hygiene and cleaning practices can be effective control measures for reducing risk.
When mounting training fuels on the ceiling, oxygen concentrations available for combustion will typically be below 10% and often below 5% [
48], resulting in fire effluent with large amounts of products of incomplete combustion. It is possible that different results would be found with other fuel arrangements such as fuels loaded in a ground-level hopper in the middle of the training room. However, mounting fuels on the ceiling is common in many compartment fire behavior training structures (e.g., [
43,
59]), metal container based training structures (e.g., [
41]) and other Class A burn buildings, where fuels are supported at or near the ceiling by chains or steel fuel racks. This mounting arrangment is commonly employed to create smoke opacity and ignition/rollover behavior that firefighters might encounter in ventilation limited fires in real world response. Such conditions can provide important controlled training environments that assist to prepare firefighters for the dynamic fireground.
This study also provided the opportunity to characterize the impact of administrative control measures which would relocate fire instructors or firefighters that work in the structure. The gas sampling results indicate that instructors at the rear location will be working in an area with higher airborne concentrations of the compounds studied than the front instructor location (contrary to the relatively lower air temperatures and heat fluxes at the rear location [
48]). This phenomenon likely results from heated smoke traveling along the ceiling, impacting the relatively cool vertical obstruction at the back of the structure, and then descending onto the instructor. This differs from the front location where combustion byproducts remain at elevated heights and temperature. While the front instructor is typically charged with teaching the class and running the scenario in the structure, the rear instructor is most often responsible for operating the vents. However, these vents can be operated from outside the structure—where airborne exposures may be two to three orders of magnitude lower. Also, by remaining outside the structure, the rear instructors would not experience the spikes in pressure when the vents were closed [
48]. More research is needed to understand if such pressure spikes could provide a mechanism for driving airborne contaminants underneath the firefighters’ PPE through gaps in protection. Even though airborne concentrations of all compounds were lower outside of the structure, potential exposures remain, particularly for formaldehyde, which had median concentrations above the NIOSH STEL for all fuel types. Thus, students and instructors should continue to use PPE—and particularly SCBA—even when operating immediately outside of the training structure.
The follow up three-cycle experiments allowed us to evaluate the impact of height on potential exposures. While most data were collected 0.9 m above the floor to simulate approximate head height for a kneeling/crouching fire instructor, it is conceivable that instructors could get even lower inside the structure, especially with structural modifications such as using a taller container in the observation area or by having instructors and students lie on the ground in the observation area (anecdotally, this practice was employed when these training structures were first being utilized). Overall, gas concentrations of BTEXS, aldehydes and many of the higher molecular weight PAHs were reduced when sampling at 0.3 m and 0.6 m from the floor. The one outlier in this trend was for the lower molecular weight PAHs in the fiberboard experiments where higher median concentrations were measured at 0.3 m vs. 0.6 m height. Further study is needed to understand the potential cause for this unexpected finding. TEQ and MEQ, which are better estimates of potential toxicity than individual PAH concentrations, were approximately two to four times lower at 0.6 m and 0.3 m than 0.9 m height. Thus, orienting instructors and firefighters in a manner that keeps them lower in the observation chamber will likely reduce their exposure to many hazardous emissions. Some training academies have installed benches in their Fire Behavior Lab to make it more comfortable to observe the scenario, but this should be reconsidered if it increases head height (above 0.9 m) due to potential increased thermal and chemical exposure risk. It is possible that the design of the structure could be modified to lower the observation area using a taller container. As with any change in training environments, such modifications should be investigated holistically, considering impact on instructional objectives and other possible risks, such as larger potential fall distances from the fire area to the observation area.
4.3 Limitations and Future Work
While this study provides the most complete characterization of compounds measure during live fire training involving different types of fuels to date, there are important limitations to this work. Changes in weather and ambient conditions throughout the study likely contributed to variability in results. This might explain why variability in air concentrations for some compounds were greater within a fuel package than between fuel packages. Conducting experiments in a large indoor lab environment would allow control over these ambient conditions and may allow tests of statistical significance to be conducted. However, the training structure is most commonly used in an outdoor environment, so the variability reported here is representative of the typical use case. The fuels selected in this work are among the most common in the fire service today, but there are other materials that can be used for training fires.
Future research should expand upon this work to study how different fuels and other control interventions during live-fire training impact the biological uptake of chemicals by firefighters and instructors. Such research should consider the holistic impact of these control measures on chemical and thermal exposure as well as learning objectives. A similar study should be conducted using alternative fuel orientations particularly with fuels lower in the compartment and with more ventilation (such as a traditional hopper in the middle of a concrete structure). It is likely that differences in individual fuel components may have more impact on evolution of products of combustion when utilized in more well ventilated burning conditions. Studies should also be conducted using combinations of these fuels, such as using fiberboard and pallets on the structure ceiling, which has been found to improve fire dynamics repeatability compared to these fuels used individually. Future research should evaluate the elemental composition and yields of training fuels in order to characterize exposure hazard using bench scale methods that can then be coupled with these large-scale results. Finally, a tradeoff analysis should be conducted to evaluate the fire dynamics training benefit compared to the exposure risk presented by firefighters’ and instructors’ immersion in the vent limited fire conditions.