One of the chemicals used in WWI was tear gas. Some affects of tear gas were crying, coughing, difficulty breathing, and temporary blindness.
Another chemical that was used in WWI was chlorine. The affects of chlorine were coughing, vomiting, and irritation in the eyes.
Mustard gas was an often used chemical in WWI. This gas damaged the eyes, the respiratory system, and the skin. Mustard gas burns when it comes in contact with skin.
Phosegene and dephosegene were both very harmful gasses in WWI. The affects of them were suffocation, difficulty breathing, coughing, and irritation to the throat and eyes. The affects can last up to 48 hours, possibly causing death.