Fast fashion is very bad for the enviroment this is due to its links to heavy water use, high emissions and massive waste. The production of cheap clothes uses a massive amount of water. THis is becasue the use of cotton in most clothing items and cotton is a crop that requires massive amount of water in order to grow. Furthermore, with trends and styles chaning so fast all of the clothes from the last trend usually goes to waste in order to mkae room for the new style.
According to the UN enviromental programs the fashion industry alone accounts for about 8-10% of total global emissions each year.