I've been taking astaxanthin for about 6 weeks and it works amazing for preventing sunburns.
Astaxanthin is a super strong antioxidant you take orally that protects the skin. I've been taking 12 mg day (2 pills) and I've stayed outside in the sun for literally 5-6 hours with no sunscreen and come away with only a little redness and no pain and within a day I just turn dark.
It's hard to put an SPF rating on it, but it's got to be a 4-8. It's definitely not like wearing SPF 30 but it is a HUGE difference.