UVA: The invisible danger
Even behind a window. Even at the office. Even in a car. Your skin remains exposed to ultraviolet rays. Not to all of them, but to the most silent ones: UVA rays, which don't burn the skin but cause it to age.
It's a persistent misconception: no sunburn = no damage. The photobiological reality is more nuanced. And understanding it durably changes your approach to SPF on a daily basis, including indoors.
Does glass block UV rays?
The ultraviolet spectrum is divided into three bands. UVC rays (100–280 nm) are absorbed by the atmosphere before they even reach the Earth's surface. UVB rays (280–315 nm) are very largely blocked by ordinary glass. Standard window glass filters out more than 97% of UVB rays. These are the ones that cause sunburn, which is why you don't burn behind a window.
UVA rays (315–400 nm), on the other hand, pass through glass without difficulty. A study published in the British Journal of Dermatology (Moehrle et al., 2003) measured UVA transmission through standard glazing: between 50 and 75% of UVA rays penetrate, depending on the thickness and type of glass.
It is also important to understand that UVA rays represent approximately 95% of the UV rays that reach the Earth's surface. They are the ones that penetrate deep into the dermis, where collagen and elastin are formed.
Photoaging, an invisible mechanism
UVA rays act differently than UVB rays. Where UVB rays burn and trigger an immediate and visible response, UVA rays work silently, over the long term. You don't feel them. You don't turn red. But the damage accumulates, and one day the skin pays the price.
The mechanism of photoaging
UVA rays generate reactive oxygen species (ROS), free radicals that damage the DNA of skin cells, degrade collagen and elastin fibers, and alter melanocytes. The result, which is progressive and cumulative, is wrinkles, sagging, and pigment spots. This is called photoaging, which is responsible for up to 80% of the visible signs of skin aging.
An iconic case study was published in the New England Journal of Medicine in 2012 (Gordon, New England Journal of Medicine, 2012): an American truck driver, exposed for 28 years to UVA rays on the left side of his face (the window side), showed very marked asymmetrical skin aging—deep wrinkles, sagging skin, and hyperpigmentation—only on that side. His right side was comparatively spared. This clinical observation has become a reference illustration of the cumulative effect of UVA rays through glass.
What dose are you really exposed to behind a window?
UVA exposure indoors depends on several variables: window orientation, time of day, season, and type of glazing. Measurements taken in a professional environment show that workers sitting near a south or west-facing window can receive, in a single workday, a dose of UVA equivalent to several minutes of direct exposure outdoors in summer.
This is not negligible on the scale of a career or a sedentary life. A person spending eight hours a day sitting less than two meters from a window accumulates, over ten years, thousands of hours of low-dose UVA exposure: this is precisely the exposure profile that promotes chronic photoaging.
Driving a car also deserves special attention. Modern windshields generally filter UVA better than side windows, which are often made of untreated glass. According to an American study analyzing the laterality of skin cancers in patients who drive, lesions appear significantly more often on the left side (the driver's window side) in North American drivers. A result consistent with chronic lateral UVA exposure (Butler & Fosko, Journal of the American Academy of Dermatology, 2010).
Sun protection at the office: a habit to build
If UVA rays pass through glass, the logical conclusion is clear: daily sun protection remains relevant even when you spend your day indoors.
This does not mean you must systematically apply SPF50 cream before checking your emails. But for anyone working near a window or driving regularly, incorporating photoprotection into your morning routine is a matter of consistency, not excessive caution. This is, in fact, what the American Academy of Dermatology and the Société Française de Dermatologie recommend: sun protection every morning, applied like any other skincare product.
It is precisely this realization that is at the heart of the philosophy of Substance of Light®. Our formulas have been designed for protection from morning to night, in summer and winter, indoors and outdoors. Protection for daily life.
Two criteria are worth keeping in mind to effectively protect yourself from UVA rays at the office:
The protection spectrum, first: an SPF index alone does not guarantee UVA protection; it only measures resistance to UVB rays. To cover UVA, you must check for the PA rating (PA+++, PA++++) or broad-spectrum protection. This is the approach taken for all SOL® formulas: balanced UVA/UVB coverage, with rigorously selected next-generation filters.
The texture, second, because protection that doesn't fit into your daily life simply won't be worn. This is where the Brume Protectrice Urbaine SPF25 PA+++ finds its meaning: its ultra-fine mist can be sprayed throughout the day without touching your face or disturbing your makeup. A ten-second gesture, invisible in your routine.
What this means in practice for your routine
The goal is not to live in fear of the sun. It is to understand that photoaging does not distinguish between a day at the office and a day at the beach; it occurs wherever UVA rays circulate, which is to say, everywhere.
In January as in July, on overcast days as under bright sunshine, UVA radiation is present. It is on this principle that Substance of Light® has structured its two photoprotection products: the Fluide Hydratant SPF50 PA++++ to anchor protection from the morning, and the Brume Protectrice Urbaine SPF25 PA+++ to maintain this coverage throughout the day, including the exposure you don't see coming.
FAQ — Frequently Asked Questions
Does office window glass block UVA rays? No, not entirely. Standard glass lets between 50 and 75% of UVA rays through. Only glass with special UV treatment offers significant UVA filtration.
Should I apply sunscreen indoors? If you work regularly within two meters of a window, yes. Chronic low-dose UVA exposure is one of the main factors of photoaging.
Does the SPF of a cream also protect against UVA? The SPF rating exclusively measures protection against UVB rays. For UVA, you must check for the PA+++/PA++++ rating or broad-spectrum protection. An SPF50 cream without a UVA indication does not protect you from photoaging.
Bibliography
- Moehrle M, et al. “UV exposure in cars”, British Journal of Dermatology, 2003 https://pubmed.ncbi.nlm.nih.gov/12925188/
- Gordon J. “Unilateral Dermatoheliosis”, New England Journal of Medicine, 2012 https://www.nejm.org/doi/full/10.1056/NEJMicm1104059
- Butler ST, Fosko SW. “Increased prevalence of left-sided skin cancers”, Journal of the American Academy of Dermatology, 2010 https://pubmed.ncbi.nlm.nih.gov/20226568/
- Liebel F, et al. “Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes”, Journal of Investigative Dermatology, 2012 https://www.researchgate.net/publication/221817338_Irradiation_of_Skin_with_Visible_Light_Induces_Reactive_Oxygen_Species_and_Matrix-Degrading_Enzymes
