Impact of energy retrofits on toxicity and inflammatory potential of indoor dust and associations to IEQ and occupant health

Understanding how energy retrofits affect indoor environments is essential for ensuring occupant health and revealing underlying mechanisms. This intervention study examined whether energy retrofits influence the in vitro toxicity and inflammatory potential of indoor dust, and whether these outcomes, reflecting overall exposure rather than individual pollutants, could explain observed improvements in self-reported health post retrofits. Additionally, associations with measured indoor environmental quality (IEQ) indicators were explored to identify factors driving indoor toxicity. Settled dust was collected before and after energy retrofits from 75 apartments in Finland and Lithuania. Toxicity was assessed via effects on cell metabolic activity and viability, and inflammatory potential was measured through interleukin (IL)6 and IL8 production in co-cultures of lung epithelial (A549) and monocyte (THP1) cells. Their associations with retrofit status, IEQ variables and self-reported health outcomes were analysed using pairwise pre-post testing and generalised linear mixed modelling (GLMM). Results suggest a context-dependent effect of energy retrofits on dust toxicity, indicating that impacts may be shaped by factors such as climate, building type, and the extent or type of retrofit measures. The inflammatory potential correlated strongly with dust microbial load and diversity in house dust, regardless of retrofit status. Dust toxicity and inflammatory potential were associated with self-reported health outcomes but did not explain the observed reduction in infection-related absence from work or school after the retrofits. Finally, the observed associations with IEQ were country-specific and findings emphasise that indoor dust toxicity is influenced by numerous factors, many of them unrelated to the building itself.