Furthermore, NO2− was present in the blood in a detectable range (2 to 3 μg/L of NO2−-N) but did not elevate blood MetHb levels (less than 4.1% of total Hb) to the threshold that is considered to cause subclinical methemoglobinemia (30 to 40% [12]). This evidence concerns the gene GSTM1 and methemoglobinemia.