Vulnerable Populations and Indoor Air Quality: A Comparative Review across High-Risk Populations in Front-Line Occupational, Child Day Care, and Elderly Long-Term Residential Settings

Abstract Indoor air quality (IAQ) is an often overlooked factor in public health, especially for groups that spend long periods indoors, including firefighters, children in day care, and elderly in long-term care facilities (LTCFs). These populations face heightened exposure to airborne pathogens, chemical irritants, and fine particulate matter. This review synthesizes research from 1990 to 2025, including peer-reviewed studies, technical reports, and government guidelines, to assess IAQ challenges across fire stations, day care centers, and LTCFs. It highlights shared vulnerabilities, setting-specific risks, and opportunities for cross-sector improvement. Across all three environments, inadequate ventilation, the absence of enforceable IAQ standards, and inconsistent use of control technologies contribute to poor air quality. Conventional approaches, such as passive ventilation and high-efficiency particulate air (HEPA) filtration, provide only partial protection, often failing to remove pathogens, volatile organic compounds (VOCs), and ultrafine particles. Mechanical ventilation systems offer more control but are frequently outdated, energy intensive, or limited by air recirculation. Even when buildings meet recommended air changes per hour (ACH), real-world studies show uneven pollutant removal due to airflow patterns, structural obstructions, and occupant movement. These shortcomings disproportionately affect biologically vulnerable groups, underscoring the limits of ventilation alone. Emerging technologies, particularly compressive heating air sterilization technology (CHAST), offer promising alternatives. CHAST systems rapidly inactivate airborne microorganisms, degrade VOCs, and remove particulate matter without generating harmful byproducts. Laboratory and field evidence suggests they can significantly reduce bioburdens across diverse indoor settings. Improving IAQ in high-risk environments requires strategies combining ventilation optimization, continuous monitoring, and active air sterilization technologies. We recommend enforceable IAQ standards for vulnerable population settings, routine IAQ surveillance, and incorporation of advanced pathogen-mitigation technologies into building design and public health preparedness. Future research should develop standardized IAQ performance metrics and integrate IAQ monitoring with infectious disease surveillance. Elevating IAQ to a national and global public health priority is essential for strengthening resilience against future outbreaks.