Effects of energy retrofits on the indoor microbiota in Northern European apartments

While the urgent need for energy saving is triggering energy retrofit measures in buildings, there are still large gaps in our understanding of how these measures may impact indoor environmental quality, including particle, chemical, and microbial exposure, and ultimately human health. Here, we explore the effects of energy retrofits on the indoor microbiota characteristics of 193 apartments in 40 apartment buildings in two Northern European countries, Finland and Lithuania. Amplicon sequencing was used to characterise fungal and bacterial microbiota compositions in airborne, settled dust samples collected from apartments before and after energy retrofits, mostly targeting upgrades in insulation, heating and/or ventilation systems, and windows. Pairwise pre versus post testing of the most abundant fungal and bacterial taxa and diversity metrics, (generalized) linear mixed modelling ((G)LMM), and Analysis of Composition of Microbiomes (ANCOM) differential abundance testing were used to explore the effects of retrofits on indoor microbiota while accounting for confounding factors. The results indicated significant, country-dependent changes in the relative abundances of individual bacterial and fungal taxa, and a smaller proportion of human-sourced bacteria in Finnish buildings after compared to before retrofits. As in previous studies, factors such as outdoor temperature and type of ventilation were found to influence both bacterial and fungal composition indoors. To the best of our knowledge, this is the first dedicated, multi-country study on impacts of energy retrofits on indoor microbiota. Our novel findings offer a foundation for future research into this topic, including studies on the potential health relevance of energy retrofit associated indoor microbiome changes.