Human skin is naturally slightly acidic, with a pH between 4.5 and 5.5. Lemon juice is a much stronger acid, with a pH of about 2. Because of lemon juice's low pH, it can cause irritation, redness and contact dermatitis. Lemon juice and other citrus fruits can make your skin more sensitive to the sun. So, if you apply it to your skin and head outdoors, you could get a painful chemical burn.