Citrus fruit that’s a potent skin sensitizer and irritant. Although it can have antibacterial properties, the irritation can hurt skin’s immune response. 
The juice from lemon is often touted as being a natural option for lightening brown spots or “bleaching” skin. The truth is that lemon juice is highly acidic (has a very low pH of 2) and exceedingly irritating to skin. Lemon juice applied to skin can cause what’s known as a phytophotodermatitis (PPD) on exposure to sunlight. The symptoms can range from a red rash to brown discolorations, which is ironic given that many people, mistakenly, turn to lemon juice to lighten brown spots. The PPD is due to a volatile fragrance chemical known as limonene, which is abundant in lemon juice. Lemon, whether in juice or oil form, is a must to avoid in cosmetics. 
- Matura M, Goossens A, Bordalo O, Garcia-Bravo B, Magnusson K, Wrangsjö K, Karlberg A. Oxidized citrus oil (R-limonene): a frequent skin sensitizer in Europe. J Am Acad Dermatol. 2002;47(5):709-14.