The stronger the concentration of the peeling agent, the deeper the "potential" to damage the skin. In general, one never uses more than 35% except when treating scars.  Anything stronger than 35%, except for ice pick scars using the cross TCA method or other scars in special circumstances, has a high likelihood of scarring.  One can still get a "superficial" peel with a 35% TCA similar to a 20%. But, one can never achieve a medium depth peel with a 20% TCA on the face as they could with a 35% TCA peel. There are other factors that affect the depth of the peel and one must be experienced with this type of peel.