The CDC's masks page is full of useful information about what makes a mask good at blocking germs. I'm afraid I have bad news about that unbranded box of "coffee filter" masks you got online.
Counterfeit (fake) KN95 respirators. BE AWARE: About 60% KN95 respirators in the United States are counterfeit (fake) and DO NOT meet NIOSH requirements.
That the CDC has a page strenuously warning us against thinking "any mask will do" while its own director is saying "any mask will do" is truly dystopian, but there you go.
I've switched entirely to 3M N95 masks (see above) bought in person at the hardware store. They're ugly and plain, but you know what? They're real NIOSH N95 masks.
