The juice from the citrus fruit lemon, often touted in the world of skin care as being a natural option for lightening brown spots or "bleaching" skin. The truth is that lemon juice is highly acidic, with a very low pH that's exceedingly irritating 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 what's known as a phytophotodermatitis (PPD) upon exposure to sunlight. Results from this can range from a red rash to brown discolorations, which is ironic given that many people turn to lemon juice to lighten brown spots (Source: Photodermatology, Photoimmunology, and Photomedicine, December 2005, pages 318-321). This 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 lightening dark spots, yet it's clear this ingredient is a potent skin irritant. Given its antioxidant content, lemon juice has its benefits, but far better to consume it orally than apply it topically. For lightening discolorations, look for products with proven ingredients such as vitamin C, niacinamide, and hydroquinone--and leave the lemon juice for jazzing up a salad!