“Apply 20 minutes before sun exposure.” You’ve probably seen this label on your sunscreen before, and if you’re like most people, you’ve probably ignored it most of the time. Is early application necessary for both chemical (organic) and physical (inorganic) sunscreens, and why? Here’s what’s going on.
All sunscreen ingredients absorb UV before application
Applying sunscreen before going into the sun is recommended for both organic (chemical) and inorganic (physical) sunscreen filters.
There’s a misconception that sunscreens containing organic filters need to be applied earlier to allow the sunscreen to “activate” by binding to the skin. This isn’t true. Both organic and inorganic sunscreens work even when they’re in the bottle!
Organic filters need delocalised pi electrons to absorb UV and turn it into heat. In simpler terms, this means that alternating double and single bonds in the structure of an organic sunscreen filter absorb UV light. These don’t change when they interact with skin, so their UV absorbance properties don’t change once applied.
In fact, chemical sunscreen filters will absorb UV when applied to glass, which is how cosmetic chemists estimate SPF before sending the sunscreen off for expensive SPF testing in human volunteers.
What happens after sunscreen application?
If they don’t need to activate, then why do sunscreens need to be applied in advance?
Sunscreen ingredients in sunscreen are a lot like pigment in paint. While the pigment is coloured, to get even coverage it needs to be distributed evenly in the paint. Sunscreen is a lot like this – for it to protect you evenly, it needs to form a uniform film that stays on your skin, before you get sun exposure.
Sunscreens are emulsions that need to dry to be even
Sunscreens are usually emulsion formulas. This means that they contain both oil and water, which don’t mix together. They’re forced to hang out together using surfactants. Emulsions usually look uniform, but if you zoom in using a microscope, you’ll see that they contain droplets of one component scattered in the other component. In sunscreens, this is usually droplets of water suspended in sunscreen. Different ingredients will be in the water and oil parts.
When you apply the sunscreen on your skin, some of it will evaporate or absorb to leave a thin UV-protective layer on top of your skin in a process called de-emulsification. That’s why SPF testing is measured after waiting for 15 minutes for the sunscreen to dry down. But until the sunscreen settles down to form the film, you aren’t necessarily getting the labelled protection. It’s likely that you’ll have a microscopically spotty layer on your skin until it dries.
Sunscreens need to bind to skin
Sunscreens dry and physically bind to skin to form a film, much like paint drying on a piece of paper (as opposed to binding chemically). If you put on clothes or apply makeup or sweat or move too much while the sunscreen is drying, it’ll transfer away like wet paint and you’ll end up with uneven, inadequate coverage.
One study found that 8 minutes was required for the sunscreen to dry down enough that brushing against clothing didn’t affect it significantly. However, different sunscreens will have different required times based on their formulas.
It takes time to apply sunscreen
A final reason for applying sunscreen before sun exposure is that applying sunscreen takes time. So while you’re applying sunscreen to your arms and legs, another part of your skin may already be burning! This is especially important for parents who could be getting sunburned while they’re applying sunscreen to their children before they apply it to themselves.
Note that these reasons matter for both organic and inorganic sunscreens!
Let sunscreen dry before you go outside for better protection, regardless of whether it’s organic (chemical) or inorganic (physical).
BP Binks et al., Evaporation of sunscreen films: how the UV protection properties change (open access), ACS Appl Mater Interfaces, 2016, 8, 13270-81.
JP Hewitt, Aqueous dispersions of hydrophobic TiO2: a novel and effective way to use physical sunscreens (open access), Chimica Oggi – Chemistry Today.
JK Robinson & AW Rademaker, Sun protection by families at the beach (open access), Arch Pediatr Adolesc Med 1998, 152, 466-70.
The Beauty Brains, Why do I have to wait for sunscreen to work? Episode 19, 2014.
ZD Draelos, What is the appropriate dry-down time for sunscreen? Dermatology Times, 2012.