The subway is the major public transportation system in Seoul with 2.2 million people using it everyday. Indoor air pollution in the subway can be a significant part of population exposure because of the number of people using the subway, time spent in transit and potentially high exposure for certain pollutants. The Korea Ministry of Environment has established the level 2 of recommended standards of <TEX>$PM_{10}$</TEX> and <TEX>$CO_2$</TEX> in subway trains. The aims of this study were to determine the airborne levels of <TEX>$PM_{10}$</TEX>, <TEX>$CO_2$</TEX> and any correlation between pollutant levels and number of passenger in a subway train. The airborne <TEX>$PM_{10}$</TEX> and <TEX>$CO_2$</TEX> were measured on the inside of trains on line #4 for 4 different days from October to November in 2008. Average <TEX>$PM_{10}$</TEX> and <TEX>$CO_2$</TEX> levels were <TEX>$113{\pm}25{\mu}g/m^3$</TEX> and <TEX>$1402{\pm}442$</TEX> ppm, respectively. These levels did not exceed the level 2 of recommended standards of <TEX>$250{\mu}g/m^3$</TEX> for <TEX>$PM_{10}$</TEX> and 3500 ppm for <TEX>$CO_2$</TEX>. <TEX>$PM_{10}$</TEX> level was not correlated with the number of passengers, while <TEX>$CO_2$</TEX> levels were positively correlated with the number of passengers. The findings suggested that <TEX>$PM_{10}$</TEX> in subway trains may have sources other than those directly associated with the number of passengers.