Original sentence: , a fraction of some ingredients do penetrate through the epidermal barrier to deep layers of skin, to come into contact with living tissue and the bloodstream.Although some sunscreen ingredients do not penetrate the skin surface or the uppermost layers of the stratum corneum.For example, if it stays on the surface or in the stratum corneum, then it is not in contact with living tissue and cannot be photocarcinogenic.If a photomutagenic substance is absorbed into the skin it could potentially act as a photocarcinogen, although many other events must occur in order for a cell to become cancerous (see carcinogenesis).Whether a photomutagenic compound is photocarcinogenic or not depends on several factors
Edited sentence: However, If a photomutagenic substance is absorbed into the skin it will act as a photocarcinogen.However, a fraction of the active ingredients can penetrate through the epidermal barrier to deep layers of skin, to potentially come into contact with living tissue and the bloodstream.If it stays on the surface or in the stratum corneum it is not in contact with living tissue and then it is harmless.The desirable site of action is restricted to the skin surface or within only the uppermost layers of the stratum corneum.Whether a photomutagenic compound is photocarcinogenic or not depends on its location