This study aimed to evaluate the variation in PM10 concentration and identify the factors influencing it in Korean subways during the past decade. The PM10 measured internally by subway companies according to legal requirements was categorized by the subway’s characteristics, which were statistically examined using a mixed effects model to identify the relevant parameters. The average levels monitored near or on the platforms and in the waiting rooms ranged from 53.9 to 92.4 µg m−3, remaining below the Indoor Air Quality Control Act regulatory standard of 150 µg m−3. However, the levels monitored on the platforms far exceeded the average yearly atmospheric environmental standard (50 µg m−3). Based on both univariate and multiple analyses, several subway characteristics, including the presence of a platform screen door (PSD), were found to significantly correlate with the concentration, although slight differences in the significant factors were detected between the cities. Particularly, the absence of transfer lines and the presence of a PSD reduced the platform concentration, except at Busan and during specific years.