The most commonly used gas in WWI was ‘mustard gas’]. In pure liquid form this is colorless, but in WWI impure forms were used, which had a mustard color with an odor reminiscent of garlic or horseradish. An irritant and a strong vesicant (blister-forming agent), it causes chemical burns on contact, with blisters oozing yellow fluid. Initial exposure is symptomless, and by the time skin irritation begins, it is too late to take preventative measures.