Everyone loves a sunny day. Sunshine provides us with UV light, but this is both good and bad. Ultraviolet radiation is a classic example of a double-edged environmental exposure. A modest, well-timed slice of UVB yields essential vitamin D and can treat certain skin conditions. Yet, the same spectrum in higher amounts drives the world’s most common malignancy, skin cancer, as well as cataract blindness, photo-aging, and immune suppression.
Ultraviolet (UV) radiation sits just beyond visible violet light on the electromagnetic spectrum (100-400 nm) and is commonly divided into UVA (315-400 nm), UVB (280-315 nm), and UVC (100-280 nm). Almost all UVC, and most UVB, are absorbed by the ozone layer, so our exposure is dominated by UVA with a smaller, biologically potent fraction of UVB. Human‐made sources—welding arcs, germicidal lamps, tanning beds—can emit intense UV across all three bands, creating additional risk in workplaces and salons.
UVB photoconvert 7-dehydrocholesterol in the epidermis to pre-vitamin D-3, a crucial step in calcium homeostasis, skeletal growth, and immune modulation. The World Health Organization (WHO) notes this is the only indisputable health benefit of solar UV; all other cutaneous effects are either cosmetic or harmful.
Even brief, unprotected exposure (about 10–15 minutes for fair skin, longer for dark skin) can meet daily vitamin-D needs. Dermatologic societies still prefer dietary or supplemental sources because cutaneous synthesis of Vitamin D is accompanied by DNA damage.
The skin damage is both acute and chronic. Acutely, it can cause sunburn, which can be quite painful. Chronic exposure to UV radiation can cause hidden damage even without sunburn, including collagen fragmentation, thickened elastin fibers, and generation of reactive oxygen species – all of which accelerates aging. These changes start decades before the aging becomes apparent. Even more importantly, chronic exposure to UV photons initiates mutations in tumour suppressor genes, enhancing skin cancer risk.
UV light also affects the eyes. The cornea and lens absorb nearly all UVB and much UVA, protecting the retina but putting anterior tissues in the firing line. The WHO estimates up to 10 percent of the world’s cataracts—a leading cause of blindness—are attributable to chronic UV exposure.
Short-term high doses (e.g., welding without a visor, spring skiing on fresh snow) can cause photokeratitis—a painful, reversible “sunburn” of the cornea. Long-term cumulative exposure is linked to pterygium and pinguecula, fibrovascular growths that can encroach on vision, and to ocular melanoma, a rare but aggressive tumour.
The risk to your eyes is why UV blocking sunglasses should be worn year round outdoors. When choosing sunglasses look for a label that states: “UV 400” or “100 % UVA + UVB”. It is interesting to note that a “polarised” label alone says nothing about UV protection. It’s also a myth that darker lenses are safer, since UV blocking depends on an invisible coating or the base lens material, not the tint.
Look for larger wrap around sun glasses with UV protection even if you wear contacts. Cheap lenses can darken vision, dilate pupils, and increase retinal UV unless they meet UV 400. Remember even on cloudy days, up to 80% of UV can penetrate clouds. Car windshields filter most UVB but not much UVA. So wear UV blocking sunglasses for driving too.
Just remember even UV blocking sunglasses will not protect you from tanning beds, snow glare and arc welding or staring directly at the sun, even an eclipse. Special lenses are required for those extreme situations.
UV has a paradoxical relationship with immunity. Low, controlled doses are exploited in dermatology clinics to quell overactive skin conditions such as psoriasis and atopic dermatitis. Conversely, excessive UV exposure suppresses immune response, potentially heightening susceptibility to infections and reducing vaccine efficacy. Epidemiological work also ties high lifetime UV load to reactivation of latent viruses like herpes simplex.
So what is recommended regarding sun exposure giving the potential benefits and risks? Does sunscreen actually lower vit D levels? Well, in theory it would, but, from a practical standpoint, no it doesn’t. People generally don’t apply sunscreen as heavily or frequently as recommended, and they tend to spend even more time in the sun when wearing sunscreen. A 2023 narrative review summarised >70 papers and concluded that “normal, real-life sunscreen use rarely produces vitamin-D deficiency.”
Fair-skinned individuals should obtain vitamin D from diet or supplements and use year-round sun screen protection, even brief unprotected exposure can burn. For darker-skinned individuals: melanin gives some natural SPF, but not immunity; cumulative UV damage still causes cancers, though at lower rates. Short controlled sun exposure (20–30 min, UVI < 3) can help maintain vitamin D when dietary intake is low.
For children, 50–80% of lifetime UV dose accrues before age 18, so early sunburns double adult melanoma risk. Protective habits must start in childhood.
References
Boyd, Kierstan. How to choose the Best Sunglasses to Avoid Sun damage. American Academy of Ophthalmology. May 29, 2024. https://www.aao.org/eye-health/glasses-contacts/sunglasses-3?utm_source=chatgpt.com
Accessed Jul 23, 2025
Gatta, Elisa, Cappelli, Carlo. Sunscreen and 25-Hydroxyvitamin D Levels: Friends or Foes? A Systematic Review and Meta-Analysis. Endocrine Practice. 2025 June; 31(2): 839–848. https://www.sciencedirect.com/science/article/abs/pii/S1530891X25001223?utm_source=chatgpt.com Accessed Jul 23, 2025
Hart, PH, Norval, M. Are there differences in immune responses following delivery of vaccines through acutely or chronically sun-exposed compared with sun-unexposed skin? Immunology. 2020 Feb;159(2):133–141. https://pmc.ncbi.nlm.nih.gov/articles/PMC6954713/
Accessed Jul 22, 2025
Marks, R, Foley, PA, Jolley, D, Knight, KR, Harrison, J, Thompson, SC. The Effect of Regular Sunscreen Use on Vitamin D Levels in an Australian Population: Results of a Randomized Controlled Trial. Arch Dermatol. 1995;131(4):415–421.
https://jamanetwork.com/journals/jamadermatology/article-abstract/556563?utm_source=chatgpt.com Accessed Jul 22, 2025
Tang, Xiaoyou, Yang, Tingyi, Yu, Daojiang, Xiong, Hai, Zhang, Shuyu. Current insights and future perspectives of ultraviolet radiation (UV) exposure: Friends and foes to the skin and beyond the skin. Environment International. Volume 185, 2024.
https://www.sciencedirect.com/science/article/pii/S0160412024001211?utm_source=chatgpt.com Accessed Jul 23, 2025
World Health Organization. Ultraviolet Radiation.
https://www.who.int/news-room/fact-sheets/detail/ultraviolet-radiation?utm_source=chatgpt.com Accessed Jul 22, 2025
