2025-12-30 · 4 min read
Basics of sun protection
The three UV bands, what overexposure does to skin, eyes and immune function, and the practical measures that reduce dose: clothing, timing, shade and SPF 30+.
Sunlight sustains essentially all life, and for humans it does one job nothing else does as efficiently: UV-B striking skin converts 7-dehydrocholesterol into the precursor of vitamin D, which the body needs to absorb calcium and maintain bone. Modest, regular exposure is useful.
The same radiation causes harm at higher doses. Ultraviolet is invisible, and it divides into three bands by wavelength. UV-A has the longest wavelengths, 400 down to 320 nanometres. UV-B follows at 320 to 290 nm. UV-C is shortest, 100–280 nm, and it is absorbed by atmospheric ozone and oxygen; almost nothing below about 290 nm reaches the ground. Only UV-A and UV-B matter outdoors (see the three UV bands compared). Between them they cause sunburn, photo-ageing, wrinkling, photosensitive reactions and elevated skin cancer risk, and they can aggravate existing skin conditions.
Skin does defend itself. Melanocytes in the epidermis produce melanin, which absorbs UV and disperses its energy. Sustained exposure drives more melanin production, which is what tanning is: a damage response, not a sign of health. As new keratinocytes reach the surface the pigmented cells are shed and the tan fades.
Overexposure to sunlight can lead to:
Proper exposure has benefits; excess does not. Here are the main consequences.
Skin changes
UV drives melanocytes to cluster and proliferate, producing freckles, solar lentigines and moles. Most remain benign, but changes in an existing mole (asymmetry, irregular border, colour variation, diameter over 6 mm, or evolution over time) warrant a dermatologist's assessment.
Early ageing
UV, and UV-A especially, degrades dermal collagen and elastin. The result is wrinkles, laxity, leathery texture and dark spots: photo-ageing, which accounts for most of what people read as aged skin on the face, neck and hands.
Suppressed immune function
UV exposure depletes epidermal Langerhans cells (the skin's resident antigen-presenting cells) and shifts cytokine signalling, producing both local and systemic immunosuppression. This is part of why UV-damaged skin is more prone to malignancy, and why strong sun exposure so often precedes a cold sore.
Eye damage
UV damages ocular tissue. Acute overexposure causes photokeratitis, a painful corneal burn. Cumulative exposure raises the risk of cataract and pterygium, and contributes to macular degeneration. UV-rated sunglasses are not a cosmetic choice.
Skin cancer
Cumulative UV exposure is the principal modifiable risk factor for basal cell carcinoma, squamous cell carcinoma and melanoma. Risk varies with phototype but is not zero at any skin tone, and melanoma on darker skin is more often diagnosed late.
Best practices for sun protection:
Limit exposure at peak intensity. UV is strongest roughly between 10am and 4pm, though this shifts with latitude, altitude and season. Reflective surfaces raise the effective dose considerably: fresh snow can nearly double it, and water and pale sand add substantially.
Take a break
Some work cannot be moved indoors (farming, fishing, construction). Where the hours are fixed, shade breaks and an umbrella or canopy still cut the cumulative dose meaningfully.
Proper clothing
Covering skin is the most reliable protection there is, because it does not wash off or need reapplying. Wear tightly woven or UPF-rated fabric, a wide-brimmed hat and UV-rated sunglasses. This matters most for infants, young children and older adults.
Sunscreen
Apply broad-spectrum SPF 30 or higher about 15–30 minutes before going out, and reapply every two hours and after swimming or heavy sweating. Current dermatology guidance is SPF 30 as the floor; older advice citing SPF 15 has been superseded. Use enough: roughly a teaspoon for face and neck, about 35 ml for a whole adult body. See application techniques for sunscreens.
When to see a doctor
Have any new, changing or non-healing skin lesion assessed promptly. Early detection is decisive in skin cancer outcomes, and periodic screening is worthwhile for anyone with high cumulative exposure, fair skin or a family history.
Frequently asked
What SPF is recommended?
Broad-spectrum SPF 30 or higher. Older guidance citing SPF 15 as sufficient has been superseded; the American Academy of Dermatology and most national bodies now recommend 30 as the minimum.
Does UV really affect the immune system?
Yes. UV exposure depletes epidermal Langerhans cells and alters cytokine signalling, producing local and systemic immunosuppression. This is one reason UV-damaged skin is more susceptible to malignancy, and why sun exposure can trigger cold sore recurrence.
When is UV strongest?
Broadly between 10am and 4pm, but it varies considerably with latitude, altitude and season, and reflective surfaces raise the effective dose. Snow can nearly double it, and water and pale sand add substantially.