Sunscreen: what SPF and "broad spectrum" mean, and how much you actually need

Most people who say sunscreen does not work for them are using a quarter of the amount it was tested at. Here are the terms and the amount. SPF SPF measures protection against UVB, the rays that cause sunburn. It is tested at an application of 2 mg per square centimetre of skin. Protection rises with SPF but not in a straight line: SPF 15 blocks about 93% of UVB, SPF 30 about 97%, SPF 50 about 98%. Broad spectrum UVA penetrates deeper and contributes to ageing and skin cancer, and SPF does not measure it. In the US, "broad spectrum" on the label means the product passed the FDA's critical-wavelength test (critical wavelength of 370 nm or more), so it protects across UVA as well as UVB. Look for both: SPF 30 or more, and broad spectrum. Other regions use other marks, such as PA+ ratings in Asia and the UVA circle logo in the EU. How much - Face and neck: about half a teaspoon. A common rule is two finger-lengths. - Whole body in a swimsuit: about one ounce, roughly a shot glass. Most people use far less, which in practice gives them a fraction of the labelled SPF. How often Reapply at least every two hours outdoors, and after swimming or sweating. "Water resistant" on a US label means 40 or 80 minutes in water, and the label says which. No sunscreen is waterproof. Patch testing a new product Before putting a new product on your face: 1. Apply a small amount to the inner forearm or behind the ear. 2. Leave it 24 to 48 hours and watch for redness, itching or bumps. 3. For something you will use daily, the stricter method is to apply it twice a day to the same spot for a week. A reaction to one sunscreen is often to a specific filter or a fragrance; note the ingredient list so you can avoid it next time. Sources: https://www.fda.gov/drugs/understanding-over-counter-medicines/sunscreen-how-help-protect-your-skin-sun https://www.aad.org/media/stats-sunscreen