In subjects carrying the GSTM1-null deletion and at least one allele with ≥ 25 HMOX-1 microsatellite (GT)n repeats, a 10-μg/m3 increase in PM2.5 in the 48 hr before the HRV measurement was associated with a −12.7% decrease in SDNN (95% CI, −20.6 to −3.9; p = 0.0059), a 27.8% decrease in HF (95% CI, −43.0 to −8.5; p = 0.0073), and a 20.1% decrease in LF (95% CI, −34.5 to −2.7; p = 0.0261). This evidence concerns the gene GSTM1 and hydrops fetalis.