Assessment of pulmonary toxicity in mice to evaluate human health risks from indoor airborne microorganisms

Indoor air quality significantly impacts human health, with airborne microorganisms being a key pollutant. Although toxicity of indoor chemicals has been well-documented, the effects of inhalable microorganisms are not well understood. This study aims to assess the pulmonary effects of dominant resident airborne microorganisms collected from various locations across the Republic of Korea. Bacteria and fungi were isolated from indoor air samples collected from 240 to 1029 sites, and two strains each were selected for animal testing. C57BL/6 mice were exposed to these microorganisms via intratracheal instillation over four weeks. All exposure groups showed increased inflammatory cells and cytokines in broncho-alveolar lavage fluid, along with histological signs of lung injury, including inflammatory cell infiltration and mucous cell hyperplasia. The benchmark dose10 was then calculated for each adverse outcome as the mouse toxicity point of departure, which was converted to human equivalents to estimate the indoor microorganism exposure threshold capable of inducing toxic responses in humans. This threshold indicates that humans may be exposed to indoor fungi at potentially harmful levels which may lead to disease-associated inflammatory lung injury. Overall, this study provides toxicological evidence that can contribute to the development of improved standards for the management of indoor air microorganisms.