Given the CDC is taking a nap right now, I wanted to share what I learned at a building science conference a couple weeks ago.
For mitigating your personal risk, think of it in order of filtration, ventilation, UV penetration.
Upgrade your home or office HVAC filters to MERV 13 if you have forced-air and the system can handle it. If not, or in addition, build a few Corsi-Rosenthal boxes and have them running whenever people are sharing an indoor space. Aim for a clean air delivery rate (CADR) of 1 air change per hour or higher.
Ventilate either through mechanical ventilation or opening doors and windows. A low-cost CO2 monitor (we got the Aranet) will give you a good sense of ventilation levels (but not filtration) so you can determine if you feel safe in a space. Some friends will leave a space if it gets above 1400, bc to them, that means they’re breathing in too many other people’s exhalations.
The UV stuff is a little more technical so I won’t go into it unless people are interested.
Long story short, with masking, filtration and ventilation, your chances of contracting this thing, even in an enclosed space, are immensely lowered. Stay safe. 
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