I've been using sunscreen for years but never really understood how it works. My dermatologist recommended one with chemical filters and another with physical filters, but I don't know what the difference is or which one I should choose based on my skin type.
Chemical filters absorb UV radiation and transform it into heat that your skin gets rid of, while physical filters (like zinc oxide or titanium dioxide) act as a barrier that reflects the rays without letting them through. The chemical ones are usually easier to apply and don't leave white residue, but some can irritate sensitive skin; the physical ones are more stable and less irritating but leave that whitish finish you see. Do you have sensitive or reactive skin, or more normal skin? That would help you decide: if it irritates easily, try physical filters first; if you prefer convenience and your skin tolerates it well, chemical ones work just as well.
The difference is in how fast they work: chemical ones need about 15 minutes to activate because they have to be absorbed into your skin, while physical ones work right away like a shield. I have sensitive skin and chemical sunscreens would burn me terribly, so I switched to physical ones and don't regret it, even though they leave a white cast if you don't blend them well.
Basically, what differentiates the two is the mechanism and also how your skin tolerates them. Chemical ones do need that time to absorb and activate (though sometimes less than 15 minutes depending on the formula), but the advantage is they leave less white residue and feel lighter, so if you have oily or sensitive skin that doesn't like heavy products, they're more comfortable. Physical ones act instantly as a barrier, but a lot of people find them really pasty or white-looking on the face, which means they end up not using them because it's annoying - and sunscreen you don't put on doesn't protect you at all. My recommendation is that you try both on small areas first and see which one causes you less irritation and which one you actually end up applying every day without complaining.
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