Battle of the Somme
The Germans began to mix chlorine gas with a different gas, called phosgene. Phosgene gas is colorless, more deadly than chlorine, and smells like mouldy hay. A person does not get sick as soon as they inhale phosgene gas; it does not take effect until 24 hours later. Then the person’s lungs fill with fluid, which can cause them to drown.