PHAs ( polyhydroxy acids ) is for the ones with sensitive skin to other exfoliating acids. 

PHAs are chemical exfoliants, and are considered 'cousins' of alpha hydroxy acids, as they are, in fact, second-generation AHAs.

They work by exfoliating dead skin cells on the surface, resulting in a more even skin tone and texture, and also help skin-care ingredients penetrate deeper into the layers of your skin, thus boosting their efficacy.

Difference with AHAs & BHA? 

Their difference lies within their molecular structure.

PHA :  The molecules of PHAs are much larger and therefore cannot penetrate as deeply as AHAs and BHAs. Great for sensitive skin, they work exclusively on the surface, without disturbing the deeper delicate layers. They offer less irritation than their acidic counterparts.

AHA : Such as glycolic acid and lactic acid, penetrate the skin more deeply. Preferred for normal to dry skin, and are used due to their affinity to enhance natural moisturizing factors and exfoliate dead cells

BHA : Such ad salicylic acid are better suited to oilier, acne-prone skin. They work their way into blocked pores, dissolving sebum and dead skin cells, and help exfoliate acne that can lead to post-inflammatory pigmentation

Ideal Patient ?

The person who has dry, itchy skin, eczema, or atopic rosacea to try PHAs to help smooth and retexturize skin without irritation. Best try taking things slow with either a PHA-based product,  like  Neostrata's Bionic Face Cream ( coming soon )