Ultraviolet

• 1. Introduction

  ›

  Ultraviolet radiation occupies a strange position in our world. It makes up roughly 10% of the total electromagnetic output of the Sun, yet human eyes cannot see it at all. It builds vitamin D in our bodies and sterilizes hospital surfaces. It also tears apart DNA, fades museum paintings, and would make life on dry land impossible if the atmosphere did not absorb most of it. Ultraviolet sits in the electromagnetic spectrum between visible light and X-rays, spanning wavelengths of 100 to 400 nanometers. That range is short enough that the photons carry energies of roughly 3.1 to 12 electron volts. At the lower end, that is just enough to start breaking chemical bonds. At the upper end, it overlaps with ionizing radiation. The questions worth holding as this story unfolds are these: how was something invisible discovered in the first place, who divided this wide spectrum into the subtypes we rely on today, and what does ultraviolet actually do when it meets living tissue, glass, a painting, or a strand of DNA?

• 2. Discovery and Naming

  ›

  In February 1801, the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the spectrum darkened silver chloride-soaked paper more quickly than violet light itself. He announced the discovery in a brief letter to the Annalen der Physik. Ritter called what he had found "de-oxidizing rays" to stress their chemical reactivity, deliberately distinguishing them from the "heat rays" that had been identified the previous year at the opposite end of the visible spectrum. The simpler term "chemical rays" caught on soon afterward and remained the common name throughout most of the 19th century, though the physicist John William Draper broke ranks and preferred "tithonic rays."

  The current name traces directly to Latin. "Ultra" means "beyond," so ultraviolet simply means "beyond violet," reflecting the fact that violet occupies the highest-frequency position a human eye can register. The discovery of the sub-200-nanometer region, later called vacuum ultraviolet because oxygen in air absorbs it completely, came in 1893 from German physicist Victor Schumann. In 1878, researchers established that short-wavelength light could kill bacteria by sterilization. By 1903, the most effective germicidal wavelengths were known to center around 250 nanometers. The landmark 1960 finding that ultraviolet radiation damages DNA gave later generations of researchers a mechanistic foundation for everything from sunburn to evolutionary biology. The formal division of the spectrum into UVA, UVB, and UVC was settled unanimously by a committee of the Second International Congress on Light on the 17th of August 1932, at the Castle of Christiansborg in Copenhagen.

• 3. The Spectrum Divided

  ›

  UVA spans 315 to 400 nanometers and is what most people absorb every day. It passes through the ozone layer almost entirely unimpeded. More than 95% of the ultraviolet radiation that reaches the Earth's surface is UVA. It produces a quick tan by oxidizing melanin that is already present in the skin, and the effect lasts for days. UVB occupies 280 to 315 nanometers and is mostly absorbed by the ozone layer, with only a small fraction reaching the ground. The tan it produces takes roughly two days to develop because the body must synthesize new melanin. UVC covers 100 to 280 nanometers and is described by the ISO standard ISO 21348 as "completely absorbed by the ozone layer and atmosphere."

  Beyond those three everyday categories, the ISO standard recognizes further subdivisions. Near ultraviolet runs from 300 to 400 nanometers and is visible to birds, insects, and fish. Extreme ultraviolet, or EUV, occupies 10 to 121 nanometers and is entirely ionizing radiation by some definitions. A particularly significant marker in the EUV range is a prominent He+ spectral line at 30.4 nanometers, which sets the long end of the EUV band. At wavelengths longer than about 30 nanometers, radiation interacts mainly with outer valence electrons of atoms; shorter wavelengths interact with inner-shell electrons and nuclei. The boundary between "hard" and "soft" ultraviolet is used differently across fields: in astrophysics it often falls at the Lyman limit of 91.2 nanometers, while at least one applied-physics publication placed it at 190 nanometers.

• 4. What the Atmosphere Does

  ›

  Sunlight at the top of Earth's atmosphere consists of roughly 50% infrared light, 40% visible light, and 10% ultraviolet light, for a total intensity of about 1,400 watts per square meter in vacuum. By the time that light reaches the ground with the Sun at zenith, the atmosphere has absorbed about 77% of the UV. What remains shifts dramatically in composition: visible light accounts for 44% of ground-level sunlight, ultraviolet drops to just 3%, and infrared accounts for the rest. Almost no UVC survives the journey at all.

  The ozone layer is the main filter. Shorter bands of UVC, along with even more energetic solar UV, are absorbed by oxygen in the upper atmosphere. That absorption process is not passive storage: single oxygen atoms released by the UV photolysis of dioxygen react with additional dioxygen to generate ozone. The layer that forms then blocks most remaining UVB and the fraction of UVC that ordinary oxygen did not catch. Extreme ultraviolet below 121 nanometers ionizes air so strongly that it is entirely absorbed before reaching the ground. Without this system, the Earth could not sustain life on dry land. On partly cloudy days, patches of blue sky between clouds still deliver scattered UVA and UVB through Rayleigh scattering. Cloud cover during total overcast does absorb UV, but the degree of absorption varies with cloud thickness and latitude, with no consistent empirical relationship clearly established between thickness and the specific fraction of UVA or UVB blocked.

• 5. Biology and Damage

  ›

  UVB radiation excites DNA molecules in skin cells, triggering the formation of aberrant covalent bonds between adjacent pyrimidine bases and producing dimers. Most UV-induced pyrimidine dimers are removed by nucleotide excision repair, a process that calls on about 30 different proteins. Dimers that escape repair can induce programmed cell death, known as apoptosis, or cause replication errors that lead to mutation. The most deadly form of skin cancer, melanoma, is mostly caused by DNA damage independent from UVA radiation, as shown by the absence of a direct UV signature mutation in 92% of all melanomas. Occasional overexposure and sunburn are probably greater risk factors for melanoma than long-term moderate exposure.

  UVA causes harm through a different mechanism. It generates reactive chemical intermediates, including hydroxyl and oxygen radicals, which then damage DNA indirectly. The DNA damage from UVA consists mostly of single-strand breaks, while UVB damage includes direct formation of thymine and cytosine dimers and double-strand DNA breakage. UVA is also immunosuppressive for the entire body, accounting for a large part of the immunosuppressive effects of general sunlight exposure. Sunburn itself, paradoxically, is not triggered by DNA damage. UVB damages messenger RNA, which triggers a rapid ribotoxic stress response through a protein called ZAK-alpha in ribosomes. This surveillance response leads to inflammatory signaling and immune cell recruitment long before the slower detection of DNA damage would activate anything.

  The eye faces its own distinct risk. The structure most sensitive to UV damage is the eye at wavelengths in the lower UVC band, specifically 265 to 275 nanometers. That radiation is nearly absent from ordinary sunlight at ground level but is emitted by electrical arcs in arc welding. Unprotected exposure to those arcs can produce photokeratitis, a condition sometimes called "welder's flash" or "arc eye," and can lead to cataracts. Mountaineers face elevated UV from both reduced atmospheric filtering and reflection from snow and ice, making full-coverage eye protection especially important at altitude.

• 6. Sunscreen and Skin Defense

  ›

  Melanin is the body's primary UV defense. Its photochemical properties allow it to absorb UV radiation and dissipate the energy as harmless heat. Sunscreen ingredients cannot dissipate energy from excited states as efficiently as melanin can. An experiment by Hanson and colleagues, published in 2006, measured reactive oxygen species in untreated skin and in sunscreen-treated skin. In the first 20 minutes, the sunscreen film protected, reducing reactive oxygen species. After 60 minutes, however, the amount of absorbed sunscreen was high enough that reactive oxygen species were greater in the treated skin than in the untreated skin. The study concluded that sunscreen must be reapplied within two hours to prevent UV from penetrating to sunscreen-infused live cells.

  SPF ratings specifically measure protection against UVB and are also referred to as UVB-PF, for "UVB protection factor." They provide no data about protection against UVA. Several studies suggest that the absence of UVA filters in some sunscreens may explain the higher incidence of melanoma observed in certain sunscreen users compared to non-users. Inorganic blockers including titanium dioxide and zinc oxide help protect against UVA, as does the organic compound avobenzone. Five sunscreen ingredients have been shown to protect mice against skin tumors, though some sunscreen chemicals produce potentially harmful substances when illuminated while in contact with living cells. The amount that penetrates through the stratum corneum is a continuing area of scrutiny.

• 7. Machines and Materials

  ›

  Argon-fluoride excimer lasers operating at 193 nanometers are routinely used in integrated circuit production by photolithography. The process exposes a photoresist chemical through a mask, triggers chemical reactions, and allows selective etching. Research is pushing toward 13.5-nanometer extreme ultraviolet lithography for even finer circuit features. UV-diode-pumped solid-state lasers using cerium-doped lithium strontium aluminum fluoride crystals, a technology developed in the 1990s at Lawrence Livermore National Laboratory, provide wavelengths shorter than 325 nanometers commercially.

  UV LEDs reached a notable milestone in 2019, when UVA LEDs at 365 nanometers and longer became available with efficiencies of 50% at 1.0 watt output, following significant advances over the preceding five years. The most common types are at 395 nanometers and 365 nanometers, both within the UVA spectrum. For materials, UV exposure degrades polymers through chain degradation and loss of strength. Aramid rope must be shielded with a thermoplastic sheath to retain its strength outdoors. For museum artifacts, the concern runs in the opposite direction: many pigments and dyes absorb UV and change color, so institutions often place black curtains over watercolor paintings and ancient textiles. Common window glass passes about 90% of light above 350 nanometers but blocks over 90% of light below 300 nanometers. A study found that car windows allow 3 to 4% of ambient UV to pass through, particularly at wavelengths greater than 380 nanometers. Fused quartz and certain crystals such as calcium fluoride and magnesium fluoride, by contrast, can be transparent down to vacuum UV wavelengths of 150 to 160 nanometers, making them essential for scientific instruments that need to work in that range.

• 8. Life Evolved Around It

  ›

  Current evolutionary models attribute the development of some of the most fundamental cellular machinery to UV radiation itself. Before the ozone layer formed, early prokaryotes that approached the ocean surface were almost invariably killed by UV-induced thymine dimers. The few that survived had developed enzymes capable of monitoring genetic material and removing dimers through nucleotide excision repair. Many proteins involved in modern cell division, in both mitosis and meiosis, are believed to be evolved modifications of those original repair enzymes.

  Animals that exist today still navigate a UV-shaped world in ways that go beyond sunburn. Birds have a fourth color receptor specifically for ultraviolet, and structural features of the eye that transmit more UV give smaller birds genuine UV vision. Colias eurytheme butterflies rely on UV-reflecting color cues from female hind wings for mate identification, while Pieris napi females in northern Finland, where less UV radiation reaches the environment, have evolved stronger UV signals to attract males, compensating for reduced ambient UV. Mantis shrimp such as Neogonodactylus oerstedii can sense UV wavelengths. Even scorpions glow a yellow to green color under UV illumination, a property useful to researchers monitoring their populations. Elevated levels of UV-B radiation have been proposed as a contributing factor in mass extinction events in the fossil record, suggesting that the relationship between UV radiation and life has been decisive at the largest scales of biological history.