ER worker masks are rated against body fluids - hardware store ones are not. People in ERs tend to, er, exude all manner of body fluid and in times of heavy ER traffic, staff will often wear those anyway. Far more and far worse than COVID-19 can be transmitted by body fluids.
https://multimedia.3m.com/mws/media/1794572O/surgical-n95-vs-standard-n95-which-to-consider.pdf
Many “hardware store masks” have “N95” stamped right on them.