Quick answer
Ultraviolet light is non-ionizing — its photons are too weak to strip electrons from atoms the way X-rays do. But it is far from harmless: UV is absorbed by molecules in your skin and eyes (above all by DNA) and drives photochemical damage. The crucial point, and the one most people get wrong, is that the hazard depends on the wavelength, not simply on "how much UV".
Shorter wavelengths carry more energy per photon — but they also penetrate less deeply into tissue. So the danger of any UV source is set by the interplay of wavelength × penetration depth × the biological action spectrum × dose, never by photon energy alone. That is why germicidal far-UVC at 222 nm can be highly potent against microbes yet comparatively gentle on human skin, while 254 nm reaches living cells and 280–315 nm UV-B is the band that drives sunburn, cataracts, and skin cancer.
This article explains, band by band, what UV actually does to the skin and the eye — acute and long-term — and where the genuine benefits lie. The companion article UV & Occupational Safety (forthcoming) translates this biology into exposure limits, lamp risk classes, and the controls an employer must put in place.
:::visual{name="uv-tissue-penetration"}
1. The spectrum, and why wavelength governs the biology
UV occupies 100–400 nm, between visible light and X-rays. By convention it splits into four working bands, and each behaves differently in tissue:
| Band | Wavelength | Penetration into skin | What it drives |
|---|---|---|---|
| UV-A | 315–400 nm | reaches the dermis (deep) | photoaging, oxidative stress |
| UV-B | 280–315 nm | epidermis to the basal layer (~30 µm) | sunburn, DNA damage, vitamin D, skin cancer |
| UV-C | 200–280 nm | upper epidermis only (254 nm) | germicidal; acute eye/skin effects at the surface |
| Far-UV-C | 200–235 nm (esp. 222 nm) | the dead stratum corneum + tear film only | germicidal, much shallower human reach |
Penetration depth increases with wavelength: the highly energetic short wavelengths are scattered and absorbed more strongly near the surface. Roughly 70 % of UV-B is absorbed already in the stratum corneum, ~20 % reaches the living epidermis, and only ~10 % the upper dermis; UV-A, by contrast, drives 20–30 % of its energy into the deep dermis.
:::callout{variant="insight" title="The core idea: hazard is not monotonic with photon energy"} A 222 nm photon is more energetic than a 254 nm photon, yet 222 nm is less hazardous to human skin — because it is absorbed in the dead, outermost layer before it can reach living, dividing cells. "Shorter equals more dangerous" is simply false. The hazard is what reaches a vulnerable target and how strongly that target absorbs it. :::
A second source of confusion is the action spectrum — the wavelength-weighting of an effect. There is not one UV curve but several distinct ones, and they must never be conflated:
- the germicidal action spectrum (microbial DNA inactivation), peaking around 260–265 nm;
- the human actinic hazard spectrum used for exposure limits (ICNIRP), peaking around 270 nm;
- the erythema (sunburn) reference spectrum standardized by the CIE (ISO/CIE 17166), where UV-B is orders of magnitude more effective per unit dose than UV-A.
:::detail{title="Why mixing these spectra is a real error, not a pedantic one"} The germicidal spectrum describes how efficiently a wavelength kills a microbe; the actinic hazard spectrum describes how efficiently it injures human tissue. They peak at different wavelengths and have different shapes. Designing a safety case from the germicidal curve — or judging disinfection efficacy from the hazard curve — produces wrong numbers. Throughout this article, "how good at disinfection" and "how harmful to people" are kept strictly separate. :::
2. The skin — acute effects
Sunburn (erythema)
Erythema is the familiar acute response: delayed redness peaking 4–24 hours after exposure, driven overwhelmingly by UV-B. Because DNA's absorption closely tracks the erythema action spectrum, DNA is considered a major chromophore for sunburn — the redness is, in part, a visible readout of molecular damage underneath.
DNA photodamage — the molecular core
When UV-B or UV-C is absorbed by DNA, it produces two signature lesions between adjacent pyrimidine bases: cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs). UV-A produces CPDs too, but at a much lower yield and essentially no 6-4PPs — its damage is more indirect (oxidative, via photosensitization). Cells repair these lesions through nucleotide excision repair; when repair fails or is overwhelmed, the lesion can become a permanent mutation.
:::callout{variant="warning" title="No warning signals"} UV exposure produces no heat and no pain while it is happening — the symptoms (sunburn, "welder's flash") arrive hours later. There is therefore no natural feedback to make you step away. With artificial UV sources this is decisive: protection has to be engineered in advance, not judged by how it feels. :::
3. The skin — long-term effects
Skin cancer
The epidemiological and mechanistic evidence here is as strong as it gets. In 2009 the IARC classified solar radiation, ultraviolet radiation (UV-A, UV-B and UV-C), and UV-emitting tanning devices as Group 1 — "carcinogenic to humans", its highest certainty category (Monograph 100D).
The mechanism is well mapped for non-melanoma skin cancer: UV-B creates CPDs at adjacent pyrimidines; mis-repair leaves a characteristic "UV signature mutation" (C→T and CC→TT) in the tumour-suppressor gene p53. Strikingly, the mutational hotspots in p53 found in these cancers coincide with the sites where UV-B most readily forms CPDs.
:::callout{variant="safety" title="Stated carefully"} UV-B is the principal driver of basal- and squamous-cell carcinoma through this DNA-damage pathway. Melanoma is more complex — UV-A and intermittent, intense exposure are implicated, and the causal picture is still being refined. We do not attribute melanoma to any single wavelength. What is settled is the Group 1 classification: UV radiation causes skin cancer. :::
Photoaging
Distinct from cancer, photoaging is driven mainly by the deeper-penetrating UV-A. In the dermis it generates oxidative stress that switches on matrix metalloproteinases (chiefly MMP-1, a collagenase), which degrade the type-I and type-III collagen that gives skin its structure — a roughly threefold rise in collagen breakdown within 24 hours of exposure. Repeated over years this produces wrinkles, loss of elasticity, and the leathery texture of chronically sun-exposed skin.
4. The eye
Acute — photokeratitis ("welder's flash", snow blindness)
The cornea and tear film absorb UV-C and far-UV-C strongly, so at 254 nm the lens and retina are normally spared — but the surface pays the price. Photokeratitis and photoconjunctivitis are painful inflammations of the cornea and conjunctiva, appearing 6–12 hours after exposure, fully reversible, and easily prevented with proper eyewear.
The eye is, in fact, the most sensitive UV target of all: acute photokeratitis can occur at radiant exposures as low as 4–6 mJ/cm² at 270 nm, the peak of the eye's actinic action spectrum — well below the threshold for most skin effects.
Chronic — cataract and pterygium
Over years, UV-B is implicated in cortical cataract (a dose-dependent association with short-wavelength UV, damaging the lens proteins) and in pterygium (a growth on the conjunctiva, linked to cumulative UV exposure). The WHO estimates that of the ~15 million people worldwide blinded by cataract, on the order of 10 % may be attributable to UV exposure.
:::callout{variant="warning" title="Don't carry the UV-C rule across to the chronic case"} "254 nm doesn't reach the lens" is true for acute UV-C exposure at the surface. It does not mean UV is harmless to the lens: chronic, solar UV-B is precisely the band that drives cortical cataract over a lifetime. Acute-UV-C reasoning and chronic-solar reasoning are different problems. :::
5. The far-UV-C story (222 nm) — why wavelength changes everything
Far-UV-C around 222 nm (from filtered KrCl excimer lamps) is the clearest demonstration of the spine of this article. It is germicidally potent, yet it reaches living human cells far less than 254 nm does, because it is absorbed in the dead stratum corneum of the skin and the tear film of the eye. Microbes — under a micrometre across — are fully exposed; human dividing cells, sitting 10–20 µm beneath dead layers, are largely shielded.
:::visual{name="far-uvc-penetration"}
The human-tolerance data are encouraging but must be read with care: filtered 222 nm has shown no eye irritation up to ~50 mJ/cm² and corneal integrity to ~600 mJ/cm² in controlled work, with a 36-month occupational eye-safety observation reporting no signal. Reflecting this, in 2022 the ACGIH raised its occupational exposure limits at 222 nm substantially (the detailed limits belong in the Occupational Safety companion).
:::callout{variant="safety" title=""More tolerant" is not "safe""} Two caveats are safety-critical, not footnotes. First, the gentleness holds only for filtered lamps — an unfiltered KrCl source emits a longer-wavelength tail (>230 nm) that does penetrate. Second, 222 nm generates ozone, an independent inhalation hazard that has to be managed. Long-term human data are still limited. Far-UV-C is more tolerant of human exposure than 254 nm — it is not a licence for unlimited exposure. :::
6. The other side — benefits, in context
UV is not only a hazard. UV-B (290–315 nm) converts 7-dehydrocholesterol in the skin into pre-vitamin D3, which isomerizes to vitamin D3 — the body's main natural route to vitamin D. And medical phototherapy uses UV deliberately: narrowband UV-B at 311 nm is a first-line treatment for psoriasis, eczema and vitiligo, with response rates above 70 % in plaque psoriasis, and is considered safer than PUVA because it needs no photosensitizing drug.
The lesson is not "UV is good after all" but the oldest one in toxicology: the dose makes the poison, and it is wavelength-specific. Narrowband UV-B phototherapy is medically supervised and dosed against an action spectrum; it is not a reason to seek out artificial UV for "a vitamin D boost".
Working with UV sources
Everything above is the biology. If you operate or install UV equipment — germicidal lamps, curing systems, far-UV-C fixtures — the practical question becomes: which exposures matter, what are the legal limits, and what controls keep people below them? That is the subject of the companion pillar, UV & Occupational Safety (forthcoming), covering the ICNIRP/ACGIH exposure limits, IEC 62471 photobiological risk classes, the OStrV/TROS framework, and the hierarchy of controls.
Sources
This article is built on peer-reviewed photobiology, ICNIRP guidance, IARC and WHO assessments; key anchors include Meinhardt et al. 2008 (penetration depths), the IARC Monograph 100D (UV as a Group 1 carcinogen), Fisher et al. 1997 (photoaging/MMP), the WHO UV fact sheet (ocular burden), and the far-UV-C safety literature (Buonanno, Kousha, Sugihara). Full source list attached to the article record.