P

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Parabens

Commonly used preservatives in food and cosmetics that prevent bacteria and fungus from forming.

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Peptides 

Proteins used in some skin care products for anti-ageing benefits. There are numerous types of peptides.

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pH

A measure of the acidity or alkalinity of a substance. Water has a neutral pH of 7. A healthy skin barrier has an acidic pH of 4.5 to 5.5. And pH-balanced skin-care products generally fall on the slightly acidic side of neutral.

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Phthalates

These common plasticizers, used some nail polishes to increase flexibility and in some shampoos and cleansers to carry fragrance, are controversial because of a possible link to disruption of the human endocrine system. Their role in a possible increase in breast cancers in women is currently being studied.

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Platelet Rich Plasma (PRP)

A portion of one's own blood that's rich in growth factors. After drawing blood from a patient's arm, and spinning it down in a centrifuge to isolate the PRP, doctors customarily inject it into the scalp to encourage new hair growth, or inject or apply it topically to skin to jumpstart healing for enhanced cell turnover and collagen renewal. (See also: vampire facial.)