1. Water carried by steam. Boiler make-up water contains impurities. After entering the boiler, the feed water is heated to steam and the impurity is mostly transferred to boiler water, so many times cycle like this, the concentration of impurity in boiler water is higher and higher. The boiler water with high concentration impurities is carried by saturated steam is called water carried by steam. Water carried by steam refers to mechanical carrying, which is the first reason for steam pollution. 

2. Salt dissolved in Steam. Under higher working pressure of boiler, the steam can dissolve some salt. Salt dissolved in steam is called selective carry, which is the second reason of steam pollution.