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.
Oxygen is abundant in the human body - which is heated to roughly 37 °C - and so your body is a perfect environment for the oxidation of polyunsaturated fatty acids, which then block every part of the thyroid hormone pathway, causing and promoting hypothyroidism.
These are the most commonly used polyunsaturated oils, starting with the worst:
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.