The juice from the citrus fruit lemon, often touted in the world of skin care as being a natural option for brightening skin. The truth is that lemon juice is highly acidic, with a very low pH that's exceedingly sensitizing to skin. Lemon juice has a pH of 2, and comparing an ingredient with that pH to just one number up (that being pH 3) is like the difference between a minor tremor in the ground and a catastrophic earthquake.
Lemon juice applied to skin can cause photosensitivity, which is due to a volatile fragrance chemical known as limonene, which is abundant in lemon juice.
Bottom line: There's no research showing that lemon juice has a positive effect on brightening an uneven skin tone, yet it's clear this ingredient is a potent skin sensitizer. Given its antioxidant content, lemon juice has its benefits, but far better to consume it orally than apply it topically.