Each data point represents a 3.5-hour average concentration from each day of sampling. The linear regression analysis reveals a moderately strong relationship between light-rail line personal exposure concentrations with ambient levels and a weaker relationship between subway line personal exposure concentrations with ambient levels. This suggests that the light-rail line is more influenced by ambient PM levels than the subway line. Also, given the relatively small y-intercept of the regression lines, local emissions are presumably the main source of pollutants for the light-rail line. On the other hand, the subway line is less influenced by urban ambient air emissions. In addition, the linear regression analysis also implies that on a day with high episodic ambient PM concentrations, light-rail commuters may be subjected to comparable or higher personal exposure concentrations than subway commuters.