Chemical sunscreens, like octylmethylcinnamate, octinoxate, PABA derivatives like padimate A and padimate O and salicylates like octisalate, absorb UVB rays and release them as heat to keep them from penetrating the skin. Benzophenones, like oxybenzone, avobenzone (Parsol 1789) and sulisobenzeone, help with shorter wave UVA protection. Physical blockers like titanium dioxide and zinc oxide reflect and disperse the sun’s UVA. (These are the blockers that go on our skin white and chalky.) A combination of these – for instance, octylmethylcinnamate, oxybenzone and titanium dioxide – would be considered broad spectrum. 
 
SPF is about coverage and time. SPF 15 gives you around 93% UVB ray protection, while SPF 30 offers around 97% and SPF 50 around 98%. You’ll never get 100% coverage, so many experts say it’s pointless to go above SPF 30 or 50, and for most of us, that’s enough. For people like my husband, who has vitiligo, or those who have a history of skin cancer, sun burn or light sensitivity issues, seemingly tiny increases in protection can make a massive difference between enjoying the outdoors or hiding inside from May to October. 
 
How long you can remain in the sun is also determined by the SPF of your chosen product. If you can normally sunbathe for 10 minutes without burning, using SPF 15 means you can lay in the sun for up to 150 minutes without worry. If you slather on some SPF 30, it means you can play for 300 minutes or five hours happily sunburn free. But you shouldn’t…
 
Why? Because we need to re-apply a liberal amount of sunscreen every two hours if we expect it to work for us. Make sure you apply it 30 minutes before going outdoors, and use a lot more than you think you need. You’ll want to apply at least 30 ml or 1 fluid ounce – a shot-glass full – every two hours of outdoor fun. (Yep, this means if you’re spending the day at the beach, you could use up to half or more of that 8 ounce bottle per person!)
 
If you’re sweating a lot playing or exercising outdoors, choosing one that’s “water resistant” – one that will remain on the skin after immersion in water for 40 minutes – or “very water resistant” – remains after 80 minutes - means less stinging and burning as the sunscreen drips into your eyes. (And if you still have the product on your hands, rubbing your eyes only makes matters worse!) 
 
Is there a reason to use sunscreen every day? Yes! Even though we may not be outside for very long, windows of buildings only screen out UVB, not UVA rays, the ones that penetrate deeper and cause premature aging. Better to choose a facial product with full spectrum protection today than spend your money on every expensive serum that comes along ten years from now. 
 
Given how expensive sunscreen can be, why shouldn't we make it at home? Because it isn’t a cosmetic, it's a drug, which undergoes rigourous testing to ensure it works as advertised. Making an effective and safe product isn’t as easy as adding x% titanium dioxide or zinc oxide to a lotion bar or using an oil, like raspberry seed, in a whipped butter. The reality is that you can’t test the efficacy of such a product at home. There are so many variables to consider - your skin type, brightness of the sun, degree of shade, activities in which you participate in the water, how much you sweat, and even then, all you can really say is that it worked that one time you tried it. What you can't test in your home workshop are the chemical and physical interactions between sunscreens and ingredients we use in lotions, like oils, emulsifiers and preservatives that can only be tested in a well set-up lab. (Some non-ionic emulsifiers, like emulsifying wax NF, could de-activate physical blockers, for instance.) 
 
If you've read this column and you're still thinking about making sunscreen at home, I beg you to reconsider. If I were to give you a shampoo or body wash recipe you don’t like very much, the worst outcome is a bad hair day or a bit of dry skin that can be moisturized away. If I give you a sunscreen that doesn’t work, I’m not only hurting you now if you suffer from a horrible sunburn, I’m setting you up for accelerated photo-aging and possible skin cancer. And isn't the point of making products at home sharing the joys of your creativity and making people happy? 

References:

1. https://www.skincancer.org/prevention/sun-protection/sunscreen/sunscreens-ex…
2. https://www.badgerbalm.com/s-30-what-is-spf-sunscreen-sun-protection-factor.…
3. https://www.webmd.com/beauty/sun/high-spf-sunscreens-are-they-better
4. https://www.cancercouncil.com.au/skin-cancer/
5. http://swiftcraftymonkey.blogspot.ca/2014/06/blast-from-past-suncreens-what…
6. https://www.melanomafoundation.org/prevention/facts.htm
7. https://www.fda.gov/drugs/resourcesforyou/consumers/buyingusingmedicinesafel…
8. http://healthycanadians.gc.ca/healthy-living-vie-saine/environment-environn…