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Lemon juice contains citric acid which is a natural bleaching agent. Therefore, applying lemon juice to elbows can help to lighten the skin. To use:  Cut a large lemon in half. Squeeze out some of the juice from both halves so that you have two hollow halves or "cups". Rub each lemon cup over your elbows. You can rub away any excess pulp, but don't wash your elbows for about 3 hours. This gives the lemon juice time to act more deeply. Wash off the lemon juice with some warm water. As lemon juice can be drying, you should moisturize the area with your favorite body moisturizer. Repeat this every day until the dark skin begins to fade. You should see a significant difference within a couple of weeks. A mixture of heavy cream and turmeric works well to lighten the skin on elbows, particularly on people with darker skin tones. Turmeric is a natural bleaching agent which helps to reduce melanin in the skin.   Get some heavy cream (or milk with a high fat content) and boil it until it thickens and becomes clotted. Mix in a half teaspoon of turmeric and blend into a paste. Apply this paste to your elbows (and knees) using a circular motion, let it sit for about 20 minutes, and then rinse it off with water.  Be aware that turmeric can dye the skin a yellowish-orange color that’s especially noticeable on pale skin. The color will fade after a day or two. You can also substitute yogurt for the heavy cream. You won’t need to boil it to achieve the same color-changing effect. This remedy helps to lighten elbows as milk contains lactic acid which reduces skin pigmentation, while the baking soda helps to exfoliate dead skin cells.  Mix enough milk to turn the baking soda into a paste. Apply on your elbows and gently scrub it in a circular motion. Do this for about 3 minutes or until you notice that your skin is lighter. These combinations contains both lactic and acetic acid, which each help lighten your skin color.  To use, combine one teaspoon of yogurt with one teaspoon of white vinegar or lemon juice and mix until they form a consistent paste. Apply to your elbows by rubbing in a circular motion. Leave the mixture on for 20 minutes, and then rinse off and moisturize. Buttermilk can lighten your skin by bleaching and moisturizing it. Oatmeal and almonds can both work as exfoliants and remove dead skin cells.  Grind the oatmeal and almonds and combine them with the buttermilk to create a paste. Rub the paste onto your elbows in a circular motion. Let the paste sit on your elbows for 10-15 minutes and then rinse it off with water.

Summary:
Use lemon juice. Apply heavy cream and turmeric if you have a darker complexion. Make a paste using milk and baking soda. Try mixing yogurt and vinegar or lemon juice. Create a mixture of almonds, buttermilk, and oatmeal.