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Ep 4: WT-SPF Part 2 - How Quickly are you Letting Yourself Age?!

  • Writer: Nat
    Nat
  • Sep 28
  • 5 min read

Updated: Nov 29

The Sun keeps us alive and also damages our skin - it's a love-hate relationship if we've ever seen one. Its core runs on nuclear fusion, and sends out energy across the spectrum. Some of that energy is visible light, but a lot of it is invisible UV that reaches us every day, through clouds and even glass. To understand why sunscreen matters, we’ll break down the two UV types that count: UVB, which burns, and UVA, which mainly contributes to aging. We’ll look at how SPF is tested, what “broad spectrum” actually covers, and the practical steps - application, reapplication, and choosing the right sunscreen for you - that make protection work outside a lab.


The UV Basics


Episode two of the BXB podcast discussed all things sun (go listen to that episode if you haven't heard it already!) but here's a refreshser: the sun emits a spectrum of energy, and the shorter the wavelength, the higher the energy it carries. Ultraviolet (UV) light sits just above visible light in energy, which is why it has such a pronounced effect on skin. UVC is the shortest UV band and is blocked by the atmosphere, so our daily concerns are almost entirely UVA and UVB.


UVA vs. UVB


UVB has shorter wavelengths and mostly affects the epidermis, which is the upper layers of skin. It’s the main driver of sunburn (erythema, not edema) and contributes to skin cancers by directly damaging DNA. UVA has longer wavelengths, penetrates deeper into the dermis, and is the primary driver of oxidative stress and photoaging; it passes through clouds and glass, which is why side-window exposure during driving matters - and yes, you should still wear sunscreen indoors if you're sitting next to a window for this very reason!


Oxidative Stress, kind of


Atoms don’t crave neutrality; they crave full outer electron shells. Oxygen wants two more electrons to feel “complete,” (we're all just looking to feel complete, really) and when UVA exposure helps generate reactive oxygen species (free radicals), those species steal electrons from nearby molecules to stabilize themselves. This electron theft disrupts lipids, proteins, and DNA and is a core mechanism behind wrinkles, loose skin, and uneven tone.


Why You Should Care About UVA


Between 90% and 95% of the UV that reaches ground level is UVA. Most UVB is filtered by the atmosphere, and what remains tends to peak around midday in summer, whereas UVA is present all day, all year. Because UVA penetrates glass, indoor daylight near windows and long drives can quietly add up to significant photoaging exposure. It doesn't cause sunburns, but instead causes spooky effects over time.


How SPF Is Tested (and What It Means)


SPF testing is an in vivo assessment on human volunteers that measures how much product reduces UVB-induced redness. It is expensive and, by design, exposes people to controlled UV doses, which is why the metric historically focused on sunburn. SPF measures protection against UVB only; it doesn’t tell you how well a product handles UVA. There are obviously some ethical concerns with exposing people to skin cancer for money... we'll let you decide what you think about that.


How UVA Protection Is Measured


UVA coverage, on the other hand, is typically evaluated in vitro (AKA without any living organisms involved) by applying sunscreen to textured PMMA plates and measuring how much light passes through 290–400 nm. This captures UVB (280–315 nm), UVA-II (315–340 nm), and UVA-I (340–400 nm) transmission. Many regions summarize UVA performance with the PA rating system (PA+ to PA++++), developed in Japan and widely used in Asia.


What SPF Number to Choose


Most dermatologists recommend SPF 30 as the practical minimum for daily use. The SPF scale is not linear: SPF 15 blocks ~93% of UVB, SPF 30 about ~97%, and SPF 50 about ~98%. Those differences are real but diminishing, so application technique and reapplication matter more than chasing the biggest number on the shelf. However, choosing a higher SPF can give you some breathing room when it comes to application technique, so if you can find an SPF 50 sunscreen you like and will reapply, that's a better bet than an SPF 30.


Reapplication and “Broad Spectrum”


Sunscreen forms a thin film on top of skin that wears down with time, sweat, oil, touching, and clothing. Think of it kind of like a layer of body paint - it can be hard to get an even layer, and over time it can crack and flake off, leading to uneven coverage. Reapply every two hours, and sooner after swimming, sweating, or toweling, to maintain the labeled protection. Look for the “Broad Spectrum” claim so you cover UVB and UVA (both UVA1 and UVA2); without that claim, a high SPF can still leave you underprotected against photoaging.


Mineral vs. “Chemical” Filters


“Chemical” filters are organic (carbon-based) molecules like avobenzone, octocrylene, homosalate, and octisalate that absorb UV and release it as a tiny bit of heat. “Mineral” filters - zinc oxide and titanium dioxide - are inorganic particles that scatter, reflect, and also absorb some UV depending on coatings and size. Neither category is inherently “safer” across the board; effectiveness and tolerance come down to the actual formula, film formation on skin, and whether you apply enough to create an even layer. The best sunscreen is one that you will wear and reapply often; you don't need to stress too much about whether it's chemical or mineral.


Formats, Water Resistance, and Real-World Use


Water resistance helps the sunscreen film stay intact during sweat or swim for the time stated on the label, but it's not perfect armor. Sprays and sticks can work, yet many users under-apply them; if you use them, go slow, use generous amounts, and rub in to ensure an even film (and even then, they are definitely not the best form of protection; a lotion sunscreen is almost always a better way to go). The best protection strategy layers defenses: shade, hats, UPF clothing, and then sunscreen on the exposed parts - because if skin is in the dark, it’s safer from UVA.


Bottom Line


UVA is “spicy daylight” and makes up the majority of UV you encounter, including through windows. Choose a broad spectrum SPF 30+ you like enough to apply generously and reapply every two hours. Treat sunscreen as one part of a system with clothing and shade, and you’ll meaningfully reduce both immediate burn risk and long-term photoaging. Sunscreen is important, WEAR IT!


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