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SPF stands for Sun Protection Factor. But it only measures protection from UVB rays. Since UVB rays cause redness and sunburn, this was used to measure SPF. Simply put, the time taken to burn with sunscreen divided by the time taken to burn without sunscreen for a specified dose of UVB is SPF. To simplify, it is literally a rating of sunscreen vs. no sunscreen.
SPF was developed when we did not fully understand UV-induced sun damage. UVB causes sunburn, which is an immediately visible form of sun damage. UVB is also absorbed directly by skin DNA resulting in sun damage. In a lab setting, the dose of UVB required to cause sun damage is far less than the dose of UVA required. So all sun science was geared towards UVB protection. In summary, do sunscreens really work? Yes! As long as it provides both UVA and UVB protection.
We know now that UVA poses a far greater danger to our skin. First, it forms a much higher proportion of solar radiation. Solar UV is 95% UVA and only 5% UVB. UVA radiation is relatively constant and present even during cloudy days. UVA radiation easily penetrates glass windows and hence is a threat even indoors. Due to longer wavelengths, UVA radiation also penetrates much deeper into the skin.
UVB rays account for less than 5% of solar UV. They are strongest around noon and during the summer. They are only able to penetrate the upper layers of the skin and cannot get through glass.
Choose a sunscreen of moderately high UVB protection like SPF30 and ensure it has a high UVA rating. In case you are likely to be exposed to bright midday sunlight, say an afternoon on the beach, you could look for an SPF 50 but it is vitally important not to compromise on UVA protection.
Want to know how much SPF you actually need? Read our blog on the topic!