Everyday Exposures: Understanding How Controllable Home Environment and Air Purifier Operation Influence Indoor Microbial Burdens and ERMI in Real‐World Residences

Indoor microbial exposures, including pathogenic, opportunistic, and non‐pathogenic fungi and bacteria, are increasingly recognized as important determinants of respiratory and immune health, yet most indoor air quality (IAQ) research emphasizes chemical pollutants and particulates over biological contaminants. To address this gap, a two‐phase investigation examined how home environmental conditions (relative humidity, ventilation type, and UV exposure) and consumer‐grade air purifier operation influence microbial burdens and standardized fungal risk metrics across both settled dust and respirable aerosols in real‐world residences. Concentrations of total fungi, bacteria, and nontuberculous mycobacteria were quantified using droplet digital PCR, and ERMI scores were externally processed. Overall, ventilation emerged as the dominant driver of residential microbial burdens, with natural ventilation producing the highest fungal concentrations in both dust and respirable aerosols, whereas humidity and UV exposure had no measurable effects. ERMI values were highly sensitive to ventilation, explaining over 55% of observed variation, and consistently classified the study site as a high‐mold environment. Purifier operation significantly reduced fungal and bacterial concentrations in settled dust but did not affect ERMI scores and was associated with a slight, non‐significant increase in respirable microbial aerosols, likely due to particle resuspension. These findings underscore the importance of considering realistic and dynamic home conditions when interpreting ERMI values, highlight limitations of consumer‐grade purifiers for improving airborne microbial exposures, and advance understanding of microbial contributions to residential IAQ.