A T-shirt is not just a T-shirt. Most clothes are made from cotton and cotton is one of the dirtiest industries in the world. When you buy cotton products, you risk taking part in some really bad stuff!
25 million tons of cotton are produced a year. The same as 25 t-shirts for everyone on Earth. But the consumption of cotton varies a lot. In some Western countries we use an amount of cotten that would corresponds to 100 t-shirts per person.