system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: We all know the UV rays from the sun is dangerous and we can use sunscreen for protection. If it is so dangerous, why are weaker SPFs produced?
gpt
SPF 15 is the lowest to be considered sunscreen. It blocks 93% of UV light. The skin is capable of naturally stopping a little UV light. So that is sufficient to get the job done, maybe give you a little tan.

SPF 30 stops 97%

SPF 50 stops 98%

SPF 100 stops 99%

Edit: Different strengths are developed because different skin tones require different levels of protection. The skin produces melanin that is basically our own natural sunscreen. The more melanin you produce the darker your skin. When UV light is detected it triggers melanin production which gives you a tan. If you want that then you use a lower strength sunscreen that let's in enough UV to trigger melanin production. If you dont want that then you use a stronger sunscreen. High SPF sunscreen is for those with very fair skin that dont produce much melanin leaving them highly susceptible to sun burns.