As shown in Figure 3 (right graph), the percentage of MTHFR C667T CT heterozygous genotype increased substantially in the control group from low AP death rates (Level 2) to high AP death rates (Levels 3 and 4); however, higher percentages of MTHFR C677T CT genotype were noticeable in the AD group than that in the control group with increased air pollution levels. Here, MTHFR is linked to Alzheimer disease.