Why was the ww1 trenches in a zig zag?

Because if an enemy did get across the no-mans-land and into their trenches, they couldn't just shoot from one spot to kill everyone, they would have to run in zig zags to try to get to everyone

Air pollution: “combustion results in gases such as nitrogen oxides, nitrogen dioxide and particulates. the main causes of air pollution include transport, domestic combustion and industrial processes. pollutants from these sources can cause problems in the immediate vicinity and a distance away.”