The idea of acid can strike terror into the hearts of many - bringing back (well-repressed) memories of litmus-testing liquids in a lab. However, it's worth remembering that pH 7 = neutral, and that anything less is considered acidic - including skin (5.5), milk (~6.5), lemon juice (2) and yoghurt (~4). 

In skin care, acids have an amazing effect on the vitality of your complexion. When applied in a controlled way, they act like a trip to the gym for your cells - kick-starting the creation of essential elastin and collagen (the building blocks that keep skin firm and supple), accelerating turnover, restoring radiance and battling spot-causing bacteria. 

The beauty benefits are both rapid and cumulative. But just as you'd mix up your workouts to hone your whole body, it's important to mix up your acid treatments to help keep your cells 'guessing' and make sure they don't get complacent. 

Try to incorporate 2-3 acid-based products into your skin care ritual (not consecutively!!) - a gentle toner, a super-charged peel and a night-time treatment (for example), which you can alternate throughout the week. 

Here's a brief guide to which acids do what...

Lactic acid is one of the gentler acids. Derived from milk (it's why Cleopatra's milk bath was so skin beneficial), this is ideal for regular resurfacing - dissolving the 'glue' that binds dulling dead cells to skin's surface, revealing the healthy new skin cells beneath.

You'll find lactic acid in The Ordinary's game-changing {Lactic Acid 10% + HA}, as well as January Labs' {Retexturising Night Cream}

Glycolic acid (most-commonly derived from sugar cane), like lactic acid, is a member of the alpha-hydroxy (AHA) group of 'skin-friendly' acids. Powerfully penetrative, most at-home treatments have concentrations of between 5 and 15% - to stimulate turnover and encourage cells to create collagen.

Because of its high skin affinity, this acid has been known to irritate initially (some people experience redness and 'tingling' until their skin becomes acclimatised). Build up your tolerance gradually... the anti-ageing benefits are worth the perseverance.

Glycolic acid is a key ingredient in Pixi's bestselling {Glow Tonic} - currently in {Supersize}!! - as well as Alpha-H's cult {Liquid Gold} overnight treatment.

Malic acid is another member of the AHA family, with effects very similar to glycolic (retexturising and 'rebuilding'). It's one of a cocktail of acids in Dr. Dennis Gross Skincare's phenomenal {Alpha Beta Peel} - a daily skin-brightening 'peel' that supports healthy turnover, fortifies skin and refines the appearance of fine lines and pores.

Salicylic acid is a BHA (beta-hydroxy acid), which has a slightly different function to its 'alpha' cousins. Great for congested or acne-prone skin types, this penetrates deeply to purify pores and help combat spot-causing bacteria. Look for salicylic or 'white willow bark extract' - which works similarly but is better suited to easily-upset complexions.

Find it in skyn ICELAND's {Blemish Dots}, as well as Sunday Riley's {U.F.O. Ultra-Clarifying Face Oil}

PHAs (or poly-hydroxy acids) both refine your skin's texture and bind with moisture to create an invisible 'film' between face and the elements. Best for those with dehydrated, dry or more mature complexions, you'll find 'poly-hydroxys' in Zelens' {PHA+ Bio-Peel Resurfacing Facial Pads}, which help to soften lines, bolster defences and leave skin feeling beautifully 'plumped' and dewy.

AND finally... a word of caution. Because acids basically dissolve the 'stratum corneum' - the uppermost, super-thin layer of 'scurf' that can lead to skin looking lacklustre, this also causes freshly exposed cells to be more vulnerable to sun and elemental stress. Invest in a good quality sunscreen, and slather it on during the daytime to keep your skin smooth, glowing and, most importantly, safe!

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