Earlier today an air traffic control tower in Alexandria, Virginia was evacuated due to noxious fumes entering the tower. Thankfully it was determined that a nearby construction site was the culprit. However, this still cause flight delays averaging 2 hours and congestion as other control towers had to take over the flights they were tracking. This is the third busiest air traffic control center in The United States. See the below link to The Washington Post for the full story...
This should be a lesson learned for us all. This facility is crucial to the Federal Aviation Administration (FAA) to track high altitude aircraft. Even a building as secured and isolated as this can be vulnerable to threats. Environmental air units take "fresh" air from outside and push it over coils to which either heats or cools the air coming in and cycles it through the building. These units can be located outside on the ground or the roof and in some cases the supply fan grille will be mounted on an exterior wall high off the ground to protect against vandalism.
Now imagine if you would that instead of a noxious gas that this was a chemical or biological weapon. The air traffic controllers wouldn't have made it out and now several pilots would have lost communication with that tower. While the FAA would transfer those aircraft to other control centers it causes confusion and in the event of multiple attacks sites it could mean that pilots have to land "blind".
This event was out of the hands of The FAA as the incident was controlled by an outside entity perhaps something like this should have been a part of their risk assessment.