How did poison gas effect WW1

The use of poison gas in World War I was a major military innovation. The gases ranged from disabling chemicals, such as tear gas and the severe mustard gas, to lethal agents like phosgene and chlorine. This chemical warfare was a major component of the first global war and first total war of the 20th century. The killing capacity of gas was limited — only 4% of combat deaths were due to gas — however, the proportion of non-fatal casualties was high, and gas remained one of the soldiers' greatest fears 
Poison gas was indiscriminate and could be used on the trenches even when no attack was going on. Whereas the machine gun killed more soldiers overall during the war, death was frequently instant or not drawn out and soldiers could find some shelter in bomb/shell craters from gunfire. A poison gas attack meant soldiers having to put on crude gas masks and if these were unsuccessful, an attack could leave a victim in agony for days and weeks before he finally succumbed to his injuries.   
By the time the war ended, the main user of poison gas was Germany, followed by France and then Britain. Though poison gas was a terrifying weapon, its actual impact, rather like the tank, is open to debate. The number of fatalities was relatively few - even if the terror impact did not diminish for the duration of the war. 
PHEW!!

They were super useful because if they were thrown in the trenches there were not much places to go. They just had to be careful for where the wind was blowing.