On hot, humid summer days many women rely on waterproof mascara to keep their lashes soft and smooth. The main chemical responsible for smoothing the lashes is a type of silicone polymer known as dimethicone copolyol . This chemical adheres firmly to the lashes and repels water. In rare cases waterproof mascara can cause acne, skin irritation and eyelid dermatitis. Dimethicone copolyol is also added to deodorants to make the product spread more smoothly and is included in shampoos to add body and shine to hair.