Exposure to environmental fine particulate matter (PM<sub>2.5</sub>), ultrafine PM (UFPM) and nanoparticles (NPs) are associated with accumulation of amyloid-β<sub>1-42</sub> peptides, phosphorylated-Tau, alpha-synuclein and transactive response DNA binding-protein-43 misfolded aberrant proteins, consistent with the biological definitions of overlapping Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal lobar degeneration (FTLD), and amyotrophic lateral sclerosis (ALS) in 99% of ≤40-year-old Metropolitan Mexico City (MMC) forensic autopsies. The gene discussed is SNCA; the disease is Alzheimer disease.