They talk about dichloramine and trichloramine which are the products of chlorine reaction with ammonia, but also any amino acid - which means it can come as we

They talk about dichloramine and trichloramine which are the products of chlorine reaction with ammonia, but also any amino acid - which means it can come as well from reactions with any part of our body it gets in contact with, like almost 2 sq. meters of our skin. How did they rule out all those other sources?