Uh, toxins are any element or chemical compound that cause damage to part of your body. In order to do any significant damage, they have to be present in certain amounts, but they do exist, and they can build up. That's how anaphylactic allergies can suddenly appear or worsen at any age. You can suddenly become allergic to peanuts, penicillin, latex, or just about anything else, because the toxins associated with them (proteins that mimic dangerous pathogens and trigger an overreactive autoimmune response) aren't quickly flushed by the body. You get rid of toxins by no longer putting them in your body. And yes, you do sweat them out. Flushing toxins via sweat is basically ineffective compared to flushing them down the toilet, but your pores do release some very basic toxins.