The buildings will also be surrounded, so there will be a second protective layer. Now, I don’t know if these layers will be enough to stop the chemical element, because chemicals are all molecules; they will definitely pass through any and every net. So, no matter how much protection is done, there will be some amount of gases and particles in the air. Since you have such massive buildings being demolished, even a small amount will actually be a lot of material, which will be released into the surroundings and dissipated into the periphery depending on the air velocity. If the air is moving at high speeds, it will dissipate faster. However, if the air is moving slower, it will tend to stagnate and dissipate slower. But you should expect it to go several kilometres, from what I have heard. Something similar happened at Kochi a couple of years ago, and there was a lot of debris.