In a recent paper by Borro et al. (2020), the variability of the infection rate, the mortality rate, and the case fatality rate as a function of particle concentration was estimated for PM2.5 particles only, recognizing a primary role of these smaller particles in inducing an over-expression of the angiotensin conversion enzyme 2 (ACE-2) in the human respiratory system (see papers by Gemmati et al. 2020; Devaux et al. 2020; Bunyavanich et al. 2020; Leung et al. 2020), and consequently in enhancing COVID-19 epidemiologic impact. The gene discussed is ACE2; the disease is infection.