Some kind of protection is better than none. Each of those handy, relatively inexpensive, disposable masks serves a slightly different purpose and risk profile.
If you are serious about protecting yourself (and working in D.C., you certainly should be serious about it), and you want to count on something like these, I'd suggest you also carry snug-fitting goggles to shield your eyes, as without them your eyes would be very vulnerable to biological or chemical contaminants.
I'm not personally well-versed in either of these masks, but this first one is the one that looks to me like the most worthwhile option (in fact I am going to look into this one further for my own purposes) ...
Air Aid Emergency Mask:
The second mask also I'm sure has some benefits, but being N100, I suspect it is actually quite difficult to breath through when it is properly seated around your mouth and nose ...
MSA Affinity Ultra N100:
Bottom line--preparedness is about peace of mind. If you are comfortable and confident in your preps, then you are halfway to your goal. Do your research and when you make your decision, feel good about it and move forward to the next challenge in your journey.