The first World War was infamous for two things; the number of deaths and poison gas.
Poison gas was known about long before the First World War. Military officers often refused to use it because they considered it to be a uncivilized weapon. The French army was the first to use gas when they fired tear gas at the German troops. After the Germans were gassed attacked they realized that they had to retaliate in full force. In October of 1914 the German army began firing shrapnel shells in which the steel balls have been treated with a chemical agent. The Germans first used this attack at the battle of Ypres against the French. The gas that was in the balls destroyed the victims respiratory organs of its victims. .
All of the gas compounds that came out of the war were originally one of the three types. A Lachrymator (tearing agent) this gas caused temporary blindness and greatly inflamed the nose and throat of the victim. An Asphyxiant these were the poisons that killed everyone. These gases destroyed the respiratory system so the victim would usually die within 48 hrs of inhalation. A Blistering Agent was the most dreaded by all soldiers. Unlike other gases the Blistering Agent doesn't attack your respiratory system but this gas acts on any exposed moist skin. The Blistering Agent would produce large burn-like blisters when it comes in contact with the skin. .
LIST OF MAIN GASES USED IN WWI AND WHAT SIDE HAD THEM.
BENZYL BROMIDE (GERMAN, TEARING AGENT).
CHLORINE (BOTH SIDES, ASPHYXIANT).
CYANIDE (ALLIES/AUSTRALIA, ASPHYXIANT).
MUSTARD GAS (GERMAN, BLISTERING AGENT).
DIPHENYLCHLOROARSINE (GERMAN, ASPHYXIANT).
ETHYLDICHLOROARSINE (GERMAN, ASPHYXIANT).
ETHYL IODOACETATE (BRITISH, TEARING).
TRICHLOROMETHYLCHLOROFORMATE (GERMAN, ASPHYXIANT FIRST WITH DELAYED REACTIONS) .
XYLYL BROMIDE (GERMAN, FIRST TEARING AGENT USED IN THE WAR) .
As you can see the Germans were far ahead in the gas game than the allies. Germans had been working on gas warfare 20 years before WWI even happened.