Preservatives are used to kill bacteria and fungi and extend the shelf life of products such as foods and cosmetics. Most cosmetic creams should only last two to three months, so preservatives are added in order to increase their shelf life and ensure that they do not become contaminated. Ingredients found in non-natural preservatives used in skin care include formaldehyde, methyl parabon and propylene glycol. Although nothing has been proved, these products have been flagged as potentially dangerous when used over long periods of time. Ascorbic acid (vitaminC) , potassium sorbate and retinyl palmitate (vitaminA) are natural preservatives, as is grapefruit seed extract. Essential oils can be used instead of artificial fragrances.