MTHFR and Alzheimer disease: With a change in AP deaths from low (Level 2) to high (Level 3 and 4), there was a substantial increase in the percentages of MTHFR C677T TT homozygous genotype in both case and control groups (Figure 3, left graph); however higher percentages of MTHFR C677T TT genotype were noted in the AD group than that in the control group with increased air pollution levels.