system
Given the following post, respond as if you're commenting on Reddit:
human
Why do sunscreens expire?

I would like to understand if it is because of microbial contamination or if the UV-blocking agents are degraded. If the latter is the case... how? Sunscreens are light-protected in their recipients and not usually exposed to high temperatures.

Also, do sunscreens lose 100% of the effect at the expiration date, or do they just lose partial effect (e.g. 75% of effectivity after 1 year, 50% after 2 years,...)?
gpt
Sunscreen works by absorbing light in the UV wavelengths of a spectra. Conjugated ring systems and other compounds with double bonds are good at absorbing energies at this wavelength, hence sunscreens are often made of organic compounds that breakdown with time. 

The good news is mineral sunscreens such as zinc oxide may last longer as its inorganic(its in powder form). The bad news is they are emulsified with essential oils or plant oils etc with emulsifiers to help spread the sunscreen better. When they start to breakdown, you are left with a grainy separated mixture that will not adhere to the skin properly.