Particulate matter (PM) samples were collected from inside ten childcare centers, and from their adjacent outdoor environments in Seoul, Korea during the summer, autumn and winter seasons. The concentrations and distribution of microbial size of the airborne bacteria and fungi in bio-aerosols were also investigated. The average indoor concentrations of fine particles less than 2.5 μm (PM2.5) in the residential areas ranged from 37.1 μg/m3 (±5.8 μg/m3) to 45.2 μg/m3 (±5.3 μg/m3), while indoor PM2.5 concentrations in centers with roadways nearby ranged from 48.9 μg/m3 (±9.5 μg/m3) to 52.9 μg/m3 (±7.7 μg/m3), and up to 51.1 μg/m3 (±6.4 μg/m3) in residential areas located near construction sites. The concentrations of particulate matter indoor in childcare centers were correlated with the corresponding outdoor locations, in residential areas (R2 of 0.64 for PM10 and 0.66 for PM2.5), near roadways (R2 of 0.72 for PM10 and 0.76 for PM2.5), and near construction areas (R2 of 0.45 for PM10 and 0.62 for PM2.5). The distribution of bio-aerosols showed that 69.4% to 78.1% of the airborne bacteria in the outdoor environments existed in stages 1-3 (over 3.3 μm), while from 59.2% to 78.6% existed in stages 2-4 (2.1-7.0 μm) inside the childcare centers. When the efficiency of air purifiers was compared with the location and characteristics of the indoor of child care centers, the removal efficiency of particulate matter with new data that may characterize indoor air quality.