Just played the game again. When you play the level, Quaraitine Zone, the walls (from the outside) are chipping away. But anyway, the firefiles left and the citizens are getting sick as Tess mentioned. I think infection will bring down Boston QZ.
Concrete weathers pretty fast, since even slightly acidic rain (common in most developed countries due to pollution) will wear on concrete and stone structures over time. It may not be much year after year, but after 10-20 years, the walls would likely be a bit weaker. Assuming they haven't been maintained, of course.