Yperite (No. 20) is a chemical mixture consisting of 80% bis(2-chloroethyI) sulfide (Yperite) and 20% carbon tetrachloride or monochlorobenzene. Bis(2-chloroethyl) sulfide (Yperite) is a veiy low volatility, highly persistent agent. It is a vesicant gas that first acts as a cell irritant and then a cell poison. This action results in inflammation of the eyes and erythema of the skin. Yperite has a high freezing point (13.9 degrees C, 57 degrees F). Carbon tetrachloride or monochlorobenzene are solvents which dissolve Yperite and lower the freezing point. They are generally added for winter weather operations.