GSTM1 and hydrops fetalis: In subjects with the GSTM1-null deletion, a 10-μg/m3 increase in PM2.5 was associated with a 10.5% decrease in SDNN (95% CI, −18.2 to −2.2; p = 0.015), a 24.2% decrease in HF (95% CI, −39.2 to −5.5; p = 0.014), and a 17.0% decrease in LF (95% CI, −31.0 to −0.2; p = 0.048).