You have to make sure the filter is HEPA, and calculate whether the device has had the time to do a full air change given the room size and its CADR rating.

Viruses are generally too small for even the best filters, but the droplets that carry them are well within what a HEPA filter can manage.

> Viruses are generally too small for even the best filters...

The average diameter of coronavirus particles is around 0.12 μm. The diameter of the envelope is 0.08 μm and the spikes are 0.02 μm long.

A P100 HEPA filter will remove as much as 99.97% (for P95 it is 95%) of all airborne particulates with an aerodynamic diameter of 0.3 μm, and particles both larger and smaller are removed with an efficiency greater than 99.97%.

Even if viruses are smaller than 0.3 μm, they are an easy target for HEPA filters.

I’m not a doctor, but isn’t this a lost cause?

The masks we all wear are primarily droplet guards, not air filters, they catch splatter that might float. All of the air that blasts out the back and escapes is barely going to be diverted by another mask after it’s already escaped the other mask and has had time to exist, right?

I guess for my threat analysis, if you’ve already invited them inside your house and they’ve stayed long enough that you think their breath particulates are floating in the air, you’ve already lost the war, so to speak.

Probably just a few minutes will be all that matters, I suspect. If you like, go take a walk and wait for the droplets to land?

> I guess for my threat analysis, if you’ve already invited them inside your house and they’ve stayed long enough that you think their breath particulates are floating in the air, you’ve already lost the war, so to speak.

That's why ventilation is important if you are sharing a room with other "external" people.