Proposed Toxin-Based Indoor Air Quality Limits for Fungi and Microbes

Indoor air pollution often contains microbial contaminants such as bacteria and fungi, accounting for over a third of indoor air pollutants. Despite existing regulations for chemical pollutants, most countries lack specific, legally binding safety limits for microbial exposure, especially for fungi, leaving a significant gap in indoor air quality policies.

Addressing this issue, researchers led by Professor Wonsuck Yoon from Korea University have developed the first species-specific health risk estimates for indoor airborne microbes utilizing animal toxicity data. Published online on June 9, 2025, in the Journal of Hazardous Materials, this study marks a pioneering effort to assess human health risks from indoor microbes through benchmark doses derived from laboratory animal studies.

The research team collected microbial samples from over 500 busy indoor locations across South Korea, including restaurants, retail stores, and transit hubs, to identify the most prevalent fungi and bacteria. Four representative species were selected for controlled inhalation studies in mice, which involved exposing the animals to inactivated microbes via their airways over a period of four weeks. The study observed that exposure led to toxic effects such as increased inflammatory cytokines, immune cell infiltration, and lung tissue damage, particularly from fungal species.

Based on these findings, the researchers calculated species-specific benchmark doses and translated these into estimated safe exposure limits for humans. Notably, the study suggests that even fungal levels below current South Korean and WHO guidelines could pose health risks, highlighting the need for more nuanced microbial regulations. Conversely, the acceptable levels for bacteria were consistent with existing standards.

Professor Yoon emphasized that this research underscores the importance of moving beyond simple microbial counts to more precise, species-level assessments for indoor air quality. The approach, however, currently relies on inactivated microbes and culturable strains, indicating a need for future studies on live organisms and more comprehensive microbiome evaluations to better reflect real-world exposure.

These insights could influence various applications, including regulatory policies, building health certifications, air purifier performance standards, and occupational safety protocols, especially in densely populated environments. Ultimately, this study sets a foundational step toward smarter, health-optimized indoor environments by establishing species-specific microbial exposure limits.

Source: https://medicalxpress.com/news/2025-07-toxicity-based-exposure-limits-indoor.html