Changes in Indoor Air Quality in Public Facilities before and after the Enactment of Taiwan’s Indoor Air Quality Management Act

South Korea was the first to administer the Indoor Air Quality Control Act in 1996, followed by Taiwan’s implementation in 2012. This study investigated indoor air quality (IAQ) in public facilities before and after the enactment of Taiwan’s Indoor Air Quality Management (IAQM) Act in 2012 to assess the effectiveness of the Act. The study also calculates health risks for employers, and consumers/visitors separately. The mean concentration of carbon dioxide (CO₂) after the IAQM Act’s enactment was higher than before, except for government offices. The lowest attainment rates for CO₂, below 80%, were 73% in hospitals and 78% in libraries. As for formaldehyde, average concentrations were higher after the IAQM Act’s implementation, except for the exhibition room and library. Notably, improvements in particulate matter with a diameter less than 2.5 μm (PM_2.5) levels were evident in hospitals and libraries compared to other environments (attainment rates increased from 85% to 100% and 89% to 94%, respectively). However, in schools, preschools, and public transport spaces, unattainment rates worsened. Regarding cancer risk from formaldehyde exposure in the public, the 95% of upper risk limits ranged from 3.44 × 10⁻⁵ in the public transport system to 8.80 × 10⁻⁴ in preschools. Our findings highlight the necessity of integrating more measurement data after IAQM Act implementation and formulating management strategies based on risk assessments for future investigations.