Effects of wetting events on mass timber surface microbial communities and VOC emissions: Implications for building operation and occupant well-being

Gwynne Mhuireach ^1* Susan Collins ² Leslie Dietz ³ Patrick Finn Finn Horve ¹ Aurelie Laguerre ⁴ Dale Northcutt ¹ Jason Stenson ¹ Kevin Van Den Wymelenberg ⁵ Elliott Gall ⁶ Mark Fretz ¹

• ¹ University of Oregon, Eugene, United States
• ² Ginkgo BioWorks (United States), Boston, Massachusetts, United States
• ³ Oregon State University, Corvallis, Oregon, United States
• ⁴ Salk Institute for Biological Studies, La Jolla, Illinois, United States
• ⁵ University of Nebraska System, Lincoln, Nebraska, United States
• ⁶ Portland State University, Portland, Oregon, United States

Humans have used wood as a construction material throughout history. Currently, mass timber products, such as cross-laminated timber (CLT), are becoming more popular as a structural material, since they are renewable and have a lower carbon footprint than concrete or steel. Nonetheless, some building types, such as healthcare, veterinary, and food manufacturing, avoid using structural mass timber due to concerns about microbial growth in the event of wetting. One solution is to use protective coatings on mass timber products to increase moisture resistance, although the coatings themselves may generate concerns about volatile organic compound (VOC) emissions. Natural uncoated wood also produces VOCs, some of which may have intrinsic antimicrobial effects. Methods: In this study, we inoculated coated and uncoated cross-laminated timber (CLT) blocks with a mock microbial community and isolated each block within individual sealed microcosms.We characterized VOCs and surface microbial communities from the CLT blocks before, during, and after wetting periods of varying durations. VOC concentration and emission rate were analyzed with chromatography-mass spectrometry (GC-MS), while microbial community abundance, diversity, and composition were analyzed through qPCR and shotgun metagenomics. Results: VOC emissions were elevated immediately after inoculation, then decreased through the remainder of the experiment, except for a plateau during the wetting period. VOCs from uncoated CLT blocks were primarily terpenes, while coated blocks emitted VOCs associated with coatings, plastics, and industrial solvents, as well as terpenes. One VOC—acetoin (3-hydroxy, 2-butanone)—was present at high levels across all samples immediately after microbial inoculation. Bacteria comprised 99.54 % of the identified microbial sequences. The plastic control microcosm (not containing a CLT block) had higher abundance of viable bacteria for the majority of the study, and uncoated blocks tended to have higher abundance than coated blocks, possibly due to biocidal ingredients in the coating. Prior to wetting periods, microbial composition was driven primarily by sampling day, whereas surface type played a larger role during and after wetting periods.