Dan Zhao

Despite considerable efforts to characterize the microbial ecology of the built environment, the metabolic mechanisms underpinning microbial colonization and successional dynamics remain unclear, particularly at high moisture conditions. Here, we applied bacterial/viral particle counting, qPCR, amplicon sequencing of the genes encoding 16S and ITS rRNA, and metabolomics to longitudinally characterize the ecological dynamics of four common building materials maintained at high humidity. We varied the natural inoculum provided to each material and wet half of the samples to simulate a potable water leak. Wetted materials had higher growth rates and lower alpha diversity compared to non-wetted materials, and wetting described the majority of the variance in bacterial, fungal, and metabolite structure. Inoculation location was weakly associated with bacterial and fungal beta diversity. Material type influenced bacterial and viral particle abundance and bacterial and metabolic (but not fungal) diversity. Metabolites indicative of microbial activity were identified, and they too differed by material.

Despite considerable efforts to characterize the ecology of bacteria and fungi in the built environment (BE), the metabolic mechanisms underpinning their colonization and successional dynamics remain unclear. Here, we applied bacterial/viral particle counting, qPCR, 16S and ITS rRNA amplicon sequencing, and metabolomics to longitudinally characterize the ecological dynamics of four commonly used building materials maintained at high humidity conditions (~94% RH). We varied the natural inoculum provided to each material by placing them in different occupied spaces, and we wet the surface of half of the samples of each material to simulate a flooding event. As expected, different materials showed different bacterial and viral particle abundance, with wet materials having higher growth rates and lower alpha diversity compared to non-wetted materials. Wetting described the majority of the variance in bacterial, fungal and metabolite structure, and material type only influenced bacterial and metabolic diversity, while location of inoculation was only weakly associated with bacterial and fungal beta diversity. Metabolites indicative of microbial activity were identified, as were those that were native to the surface material. Glucose-phosphate was abundant on all materials (except mold-free gypsum) and was correlated with Enterobacteriaceae , which could indicate a potential bacterial nutrient source. A compound consistent with scopoletin, a plant metabolite with antimicrobial activity, was significantly negatively correlated with Bacillus and positively correlated with Pseudomonas and enriched in medium density fiberboard (MDF) materials. In wet samples, the alkaloids nigragillin and fumigaclavine C, both with antimicrobial properties, were significantly positively correlated with the fungal phylum Ascomycota. Nigragillin, was also negatively correlated with Bacillus and Pseudomonas abundance. Thiabendazole and azoxystrobin (anti-fungal compounds) were highly abundant on mold-resistant gypsum wallboard and likely directly influenced the decreased fungal growth observed on this material. The mold-resistant gypsum material also showed a significant increase in bacterial alpha diversity, and bacterial and viral particle abundance, as well as a decrease in metabolite diversity, likely a result of reduced fungal growth. Penicillium taxa were positively correlated with thiabendazole, which suggested the persistence of resistant strains. Also, specific to the wet samples, Bacillus abundance was positively correlated with the azoxystrobin, suggesting bi-directional competitive adaptation, and positively correlated with metabolites known to interfere with Pseudomonas biofilm formation, which could explain the anti-correlation between these taxa. As expected, high moisture conditions enabled faster growth of inoculating microorganisms, whose composition, chemistry, and competition was shaped by surface material, suggesting that both fungal and bacterial growth need to be considered when determining the impact of dampness in built environments.

Much of human exposure to airborne particles of outdoor origin, including fine particles smaller than 2.5 μm (PM_2.5) and ultrafine particles smaller than 0.1 μm (UFPs), occurs in residences. High-efficiency central HVAC filters are increasingly being used in residences, but questions remain about their effectiveness in reducing indoor PM_2.5 and UFPs of outdoor origin in homes operating under realistic conditions (e.g., with HVAC systems operating only to meet heating or cooling demands). Here dynamic building energy and indoor air mass balance modeling are combined to estimate the impacts of 11 HVAC filters (minimum efficiency reporting value [MERV] 5 through high-efficiency particulate air [HEPA]) on indoor concentrations of PM_2.5 and UFPs of outdoor origin in multiple vintages of prototypical single-family residences relying on either infiltration or mechanical ventilation systems in 22 U.S. cities. Results demonstrate that higher-efficiency HVAC filters can meaningfully reduce indoor proportions of outdoor PM_2.5 and UFPs inside residences, but home vintage, climate zone, and ventilation strategy strongly influence the outcomes due to widely varying air exchange rates, HVAC system runtimes, and sources of ventilation air. Higher efficiency filters had a greater impact in older, leakier homes relying on infiltration alone and in new homes relying on supply-only mechanical ventilation systems designed to meet ASHRAE Standard 62.2.

Most desktop 3D printers designed for the consumer market utilize a plastic filament extrusion and deposition process to fabricate solid objects. Previous research has shown that the operation of extrusion-based desktop 3D printers can emit large numbers of ultrafine particles (UFPs: particles less than 100 nm) and some hazardous volatile organic compounds (VOCs), although very few filament and printer combinations have been tested to date. Here we quantify emissions of UFPs and speciated VOCs from five commercially available desktop 3D printers utilizing up to nine different filaments using controlled experiments in a test chamber. Median estimates of time-varying UFP emission rates ranged from ~108 to ~1011 #/min across all tested combinations, varying primarily by filament material and, to a lesser extent, bed temperature. The individual VOCs emitted in the largest quantities included caprolactam from nylon-based and imitation wood and brick filaments (ranging from ~2 to ~180 μg/min), styrene from acrylonitrile butadiene styrene (ABS) and high-impact polystyrene (HIPS) filaments (~10 to ~110 μg/min), and lactide from polylactic acid (PLA) filaments (~4 to ~5 μg/min). Results from a screening analysis of the potential exposures to these products in a typical small office environment suggest caution should be used when operating many of the printer and filament combinations in enclosed or poorly ventilated spaces or without the aid of a combined gas and particle filtration system.

Much of human exposure to fine particulate matter (PM_2.5) of outdoor origin occurs in residences. High-efficiency particle air filtration in central heating, ventilating, and air-conditioning (HVAC) systems is increasingly being used to reduce concentrations of particulate matter inside homes. However, questions remain about the effectiveness of filtration for reducing exposures to PM_2.5 of outdoor origin and adverse health outcomes. Here we integrate epidemiology functions and mass balance modeling to estimate the long-term health and economic impacts of HVAC filtration for reducing premature mortality associated with indoor PM_2.5 of outdoor origin in residences. We evaluate 11 classifications of filters (MERV 5 through HEPA) using six case studies of single-family home vintages and ventilation system combinations located in 22 U.S. cities. We estimate that widespread use of higher efficiency filters would reduce premature mortality by 0.002–2.5% and increase life expectancy by 0.02–1.6 months, yielding annual monetary benefits ranging from $1 to $1348 per person in the homes and locations modeled herein. Large differences in the magnitude of health and economic impacts are driven largely by differences in rated filter efficiency and building and ventilation system characteristics that govern particle infiltration and persistence, with smaller influences attributable to geographic location.