"But I Work Indoors": So Why Are There Spots on My Hands?
Your job is in an air-conditioned office. You commute by bus and MTR. You don't sunbathe on weekends. By every logic, you should be among the least likely to get age spots.
Then you look at the backs of your hands.
This confusion is common, and the answer is a fact many people never hear: ordinary glass blocks most UVB, the rays that burn you, but lets a large share of UVA through. UVA is the type most linked to photoaging and pigmentation. No warmth and no redness doesn't mean your skin isn't being billed.
This article explains UVA versus UVB, how window seats, car cabins, cloudy days and short walks quietly add up, and why the backs of the hands are often the first place to tell on you.
🔬 UVA vs UVB: Are You Only Guarding Against One?
Two types of UV mainly reach the ground, and they behave differently.
Why "I didn't burn" is misleading
UVB gives clear warnings: redness, pain, peeling. UVA doesn't. It accumulates quietly and shows up years later as spots, wrinkles and loss of firmness.
A sunburn is a one-time reminder. UVA offers no reminder, only a record.
Two things worth remembering
UVA stays relatively steady across the day and year. UVB peaks at summer noon. UVA remains meaningful in winter, morning and late afternoon.
UVA reaches deeper. It penetrates into the dermis, affecting collagen and elastic fibers, and is also linked to pigmentation.
🪟 What Does Glass Actually Block?
This is the heart of the article.
A practical way to think about it
Ordinary window glass largely blocks UVB but lets a meaningful share of UVA through.
Dark doesn't mean UV-proof. A dark tint without UV-blocking properties mainly cuts visible light and heat, and may not block UVA.
Glass or film with UV-blocking properties can stop most UV. Look for a stated UV-blocking rate and ask the supplier for test data.
Many Hong Kong buildings use low-E or reflective glass. Their UV blocking varies, so don't assume.
🧮 The Daily Bill of Small Exposures
Each item is short. Together, they aren't.
How much does it add up to?
Imagine a typical working day:
A window-side desk, around 8 hours
Walking and waiting at either end of the commute, about 20–30 minutes in total
A lunchtime round trip, about 15–20 minutes
A bus, minibus or car ride, about 1–2 hours
Over 250 working days a year and ten years, that's thousands of hours of UVA, almost all of it without a single sensation.
(This is an illustrative scenario for the cumulative effect, not a measured dose.)
🚗 Driving: One Side of the Face, One Hand
Car windows are a useful example.
Windshields are usually laminated glass and block UV more strongly.
Side and rear windows are mostly tempered glass. UV blocking varies by vehicle, and some research suggests certain side windows let a substantial amount of UVA through.
While driving, the hands stay on the wheel for long periods, and the window-side face, neck and arm receive more exposure.
A real case
A 2012 case report in The New England Journal of Medicine described a long-haul truck driver whose window-side face was visibly more aged and thickened than the other side. The case is often cited to show that years of exposure through glass can leave visible marks.
Hong Kong scenarios
Driving a private car or taxi to work
Sitting on the sunny, window side of a bus
Resting a hand on the wheel or window ledge at red lights
☁️ Cloudy, Early, Late: UVA Doesn't Take Days Off
"It's cloudy, so no sunscreen" is one of the most common mistakes.
Cloud blocks much visible light and infrared (so you don't feel hot), but thin cloud and haze block UV only partly, and a significant share still reaches the ground.
Morning and late afternoon: UVB weakens, but UVA remains.
Winter: Hong Kong's UV index is lower, not zero. Pay attention when the index is "very high" or above.
Check the Hong Kong Observatory's daily UV index forecast as a guide.
🖐️ Why the Backs of the Hands?
Among all body sites, the backs of the hands are among the first to show it.
Handwashing washes the protection away
Many people overlook this. Hongkongers wash hands often, and use alcohol rubs. Each wash or rub removes sunscreen from the backs of the hands. What you applied in the morning is mostly gone by noon.
🛡️ Practical Solutions: No Need to Move Into a Cave
The aim isn't to avoid daylight. It is to shrink the biggest small exposures.
At the office
Adjust your seat. If you can, avoid the sunniest window seat, or move your desk away from direct sun.
Blinds or curtains. Close them in the strongest hours. They cut both UV and glare.
UV-blocking film. If you manage the space, ask a professional supplier for film with UV-blocking test data.
Keep hand sunscreen on your desk. If it's in sight, you'll use it.
In the car
Apply sunscreen to hands, face and neck before driving. Don't skip the window side.
Consider thin UV-protective gloves, especially on long drives.
UV-blocking film: ask the manufacturer or a professional shop, and check local rules on window tint.
On the bus, choose the shaded side when you can.
On short walks
Apply even for short trips. It isn't only for the beach.
Carry an umbrella. It covers face, neck and arms.
Wear a wide-brim hat and sunglasses.
Protecting the hands
💭 Five Common Myths
"Glass blocks UV." It blocks most UVB. A meaningful share of UVA passes.
"Dark glass or film is always protective." Dark isn't UV-blocking. Check for a stated UV rating.
"No UV on cloudy days." Cloud doesn't block it completely.
"A high SPF is enough." SPF mainly reflects UVB protection. For UVA, look for a PA rating or "broad-spectrum" label.
"Screens and indoor lights give me spots." Current evidence suggests ordinary screens and indoor lighting emit very little UV and aren't a main cause. The effect of visible light on pigmentation is still being studied, and there isn't enough evidence that everyday screen use causes age spots.
❓ Frequently Asked Questions
Disclaimer: This article is for general education only. It can't diagnose and doesn't replace professional medical advice. If you have a spot you're unsure about or notice a change, consult a registered doctor, such as a dermatologist.
Source directions: Hong Kong Observatory UV index information; Department of Health sun-protection resources; WHO UV information; American Academy of Dermatology; studies on UVA transmission through window and car glass; the 2012 NEJM case of unilateral dermatoheliosis.
