PUFA stands for Poly-Unsaturated Fatty Acid, which means that the fatty acid has more than one (poly) double bond in the carbon chain.
They’re unsaturated because they’re missing out on what saturated fatty acid has - hydrogen atoms.
That makes the bonds incomplete.
So picture a chain of links that is missing a joint or two on every single link - it wouldn’t be very strong or stable, and because of this instability PUFAs are prone to oxidation, which basically means their chain gets all messed up and broken and causes problems with how your body reacts to it.
When exposed to heat, light or moisture they react with oxygen and produce toxic, free-radical particles that are harmful for our bodies both internally and externally.
Whilst they’re present in mainstream products, they are widespread in natural ones too and include oils commonly known for ‘beautifying’ like almond, argan, grapeseed, rosehip, sesame and sunflower etc.
Often we think that by choosing natural we’re doing our skin a favour, when in actual fact we could be damaging it.
Continued use of these products is understandably bad news for skin.
In studies the oxidisation of PUFAs has been shown to lead to cell destruction - and the result of this is unstable, damaged skin prone to breakouts, chronic dryness, 'age' or 'liver' spots, sagging and wrinkles.
Here's an informative video on the dangers of linseed (flax) oil. Although it's not about skincare or health, it does demonstrate how quickly this super-polyunsaturated oil (commonly touted as a health food) oxidises.