What was the most common gas used in world war 1?

Nerve gasses were used in the trenches because they were denser than air, and when released would sink and fill the trenches where the gasses blew. The gasses would, essentially, kill you entirely if you breathed them in, so soldiers had to wear chemical gear including gas masks and the like. Often soldiers would not be able to put on the gear fast enough and fall victim to the gasses, and often the side who released the gas would be surprised by a change in wind direction and be killed by their own gasses.