1. Spelce D, Rehak TR, Metzler RW, et al. History of U.S. respirator approval. J Int Soc Respir Prot. 2018;35:35.
2. Drexler M, Institute of Medicine (US). How infection works. What you need to know about infectious disease. Washington, DC: National Academies Press; 2010.
3. Guan W, Ni Z, Hu Y, et al. Clinical characteristics of coronavirus disease 2019 in China. N Engl J Med. 2020;382(18):1708.
4. First MW. Hepa filters. J Am Biol Saf Assoc. 1998;3(1):33–42.
5. World Health Organization (WHO). Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations. Geneva: World Health Organization; 2020.