Zach and the others had told the citizens that they would be coming back shortly to help them with the underworld energy and their poisoning, so it would be best if they didn't leave the city. But they could still leave the city just to go beyond the area with underworld energy and catch some fresh air.
It wasn't enough to even start working the underworld energy out of their systems. Far from it, in fact. But just getting away from the puddles of puke and pile that littered the city was a great relief for the citizens.
Lying down in the grass instead of on the stone streets didn't ease their suffering, technically. But it was better to suffer on something more comfortable.
The Underworld Exterminators had mixed reactions. Some had been against the plan to quarantine everyone since it didn't seem humane. Others had seen the Underworld's tricks and were all for it.