Index 8, Reported Lower

UV index 8 is a high-ultraviolet hour, not a mild one. By the World Health Organization’s definition, 1 index unit is 25 mW/m² of erythema-weighted ultraviolet, so index 8 is 200 mW/m², inside the band WHO calls very high. Sunglasses do not lower that number on the skin. They lower the number the eye is allowed to send. The eye is brain tissue, grown out of the forebrain. A dark UV-blocking lens cuts the ultraviolet before it reaches that tissue, and the central nervous system is signaled that the UV is lower than it is.

Skin protection that depends on the eye is then built for the smaller report. Ultraviolet B at the eye raises alpha-melanocyte-stimulating hormone from the pituitary. That hormone is an order to melanocytes: make pigment, and defend the cell. Keep UVB off the eye and the order is not given at the strength of index 8. The body inhibits a protection it would have mounted, because the eyes reported a dose the sky did not agree to. Bare skin is still standing in 200 mW/m².

In 2003, UVB aimed only at a mouse eye increased melanocytes in skin the lamp never reached. A UVB-blocking lens on that eye stopped the increase. The ultraviolet had not fallen. The report had.

Hiramoto, Yanagihara, Sato, and Inoue published that result in the Journal of Investigative Dermatology. UVB delivered to the eye, and not to the skin, raised α-MSH in plasma and increased DOPA-positive melanocytes in epidermis the beam had not touched. Take the pituitary out and the skin response died. Cut the ciliary ganglion and it died. Cut the optic nerve and it did not. Mice lacking inducible nitric oxide synthase also failed to answer. The report was not a picture on the retina. It left the front of the eye by the ophthalmic division of the trigeminal nerve, crossed a nitric-oxide step, and reached the hypothalamic–pituitary proopiomelanocortin system. The pituitary put the hormone in the blood. Melanocytes far from the eye obeyed it.

Hiramoto, Yanagihara, Sato, and Inoue published that result in the Journal of Investigative Dermatology.

The dose and the control came in the papers that followed. A sunlamp put 2.5 kJ/m² of UVB on the eye. In 2012 Hiramoto and Sato showed that this eye exposure lost its effect on epidermal melanocytes in mice missing gp91phox, a subunit of NADPH oxidase, while the same UVB aimed at the ear still worked. Skin that is hit directly can tan without that nerve. Skin that is depending on the eye cannot. In 2013 a UVB-blocking contact lens suppressed both the rise in plasma α-MSH and the rise in DOPA-positive melanocytes. A lens of similar darkness that let UVB through did not. Shade, as the wearer experiences it, was not the switch. Ultraviolet withheld from the eye was the switch. A further 2013 experiment removed the pituitary and again lost the pigmentation, and found the eye exposure a stronger melanocyte stimulus than the restraint stress run beside it.

That is the inhibition. At UV index 8 the sky is still delivering 200 mW/m² to uncovered skin. A sunglass lens that blocks UVB files a lower ultraviolet with the only instrument the brain has for that ocular report. The α-MSH order those experiments measured does not go out. Melanocytes that would have been recruited by the eye are not recruited. The body protects for the report it received. The report is lower than the index.

What the hormone does once a person has it

α-MSH is not a mouse curiosity. It is the ligand of the melanocortin 1 receptor on human melanocytes, and when people are given it, skin defense moves in the direction the receptor was built for. Barnetson and colleagues, in the Journal of Investigative Dermatology in 2006, gave 65 fair-skinned volunteers a synthetic α-MSH analog at 0.16 mg/kg, in 3 cycles of 10 days across 3 months. Melanin density rose at 8 skin sites. In the low-MED group it rose 41%, from 2.55 to 3.59. In the high-MED group it rose 12%, from 4.18 to 4.70. After a challenge of 3 minimal erythemal doses, sunburn cells in the low-MED volunteers fell by more than 50%. Thymine dimers in the basal layer fell 59%, with P = 0.002. Pigment went up. UV lesions went down. The hormone is photoprotection, measured in human skin.

Song, Abdel-Malek, Kadekaro, and colleagues showed the same hormone acting inside the cell, in Pigment Cell & Melanoma Research in 2009. Human melanocytes pretreated with α-MSH made less of the oxidative DNA lesion 8-oxodG after UV, and they raised catalase and ferritin. The protection required a working MC1R. Cells with a loss-of-function receptor did not get it, and an agouti analog blocked it. Strip the hormone away and that defense does not start. A lens that keeps UVB off the eye strips away the ocular source of the hormone at the hour the source is needed.

There is a 2nd, local route, and it should be named so it is not confused with the route sunglasses cut. Cui and colleagues, in Cell in 2007, showed that UV striking keratinocytes makes those cells release their own α-MSH, because p53 turns on POMC in the epidermis. Mice without p53 did not tan at the site of the UV. That local order still fires in skin the rays actually reach. It is why a shoulder in the sun can darken behind a pair of lenses. It does not replace the order from the eye. The Hiramoto skin that responded had not been irradiated. Its melanocytes were waiting on the pituitary. Sunglasses leave that wait unanswered while index 8 continues on every uncovered surface.

How the eye reads light through a lens

A sunglass lens is a spectral filter set in front of central nervous tissue. Visible light, roughly 400–700 nm, is what the wearer calls brightness. Cut it and the world looks dimmer. The pupil, a reflex, opens. Retinal illuminance falls, and the brain’s estimate of the hour falls with it. A separate population of ganglion cells does not even need rods and cones to file that estimate. Berson, Dunn, and Takao showed in 2002 that ganglion cells projecting to the circadian clock still fire to light when rod and cone input is blocked. Hattar and colleagues, in the same issue of Science, put the pigment melanopsin in those cells and traced their axons to the clock and to the pupil center. The pigment is tuned near 480 nm, in the blue, not in the UVB band. A gray, green, or brown lens that cuts that blue files a dimmer day. The wearer feels a milder sun. The feeling is the visible report, and it is false about index 8.

The protection report is a different wavelength and a different nerve. UVB is about 280–315 nm. The adult cornea and lens stop most of it before the retina, which is why cutting the optic nerve did not stop the melanocyte response. The endings that carried it sit at the front of the eye and run with the trigeminal nerve into the brain. Sunglasses sit exactly there. A lens built to block UVB blocks the band those endings use. The visual darkening and the UVB block then agree with each other, and both disagree with the sky. The clock is told the day is dim. The pituitary is not told the ultraviolet is high. Index 8 is still index 8 on the skin.

The open pupil has been blamed for a different injury, and the measurement does not support the blanket version. Hoover, in Applied Optics in 1987, surveyed more than 400 retail sunglass lenses and found that each lens reduced UV at the crystalline lens and at the retina, dilation included. Ordinary dark glass is not how the inside of the eye gets a larger UV dose. The damage in this paper is upstream of that argument. The lens succeeds at keeping UV off the eye, and by succeeding it cancels the eye’s order for skin defense. A cheap lens that is dark in the visible and open in the UV is a 2nd failure: the pupil is wide, the side of a flat frame leaks, and the eye itself is the tissue left exposed. Either way the skin has not been given the index it is standing in.

Where the profit started

Arctic slit goggles of bone and wood were a slot against snow glare. They narrowed the field. They were not a filter that told the brain a high UV hour was a low one. In 1752 James Ayscough sold blue and green tinted lenses for sensitive sight. Comfort, not a day outdoors.

The object that inhibits the report is the mass-market pair. In 1929 Sam Foster sold sunglasses on the Atlantic City boardwalk for 10 cents, off a counter, with no optician in the sale. In 1936 Bausch & Lomb built the anti-glare prototype for the Army Air Corps. Public sale came in 1937. The metal Ray-Ban Aviator frame came in 1938. History.com, summarizing a 1938 Life magazine report, puts the prior year’s sales near 20 million pairs, and says only about 1/4 of wearers had a medical reason. Pilots had a glare problem at altitude. The rest of the country bought a darker world.

The trade scaled with the habit. EssilorLuxottica, owner of Ray-Ban, reported 2024 revenue of 26.51 billion euros. That is group sales, lenses and frames and sunglasses and stores, not a sunglasses-only profit and not a margin. It is large enough to fix the point. A 10-cent boardwalk lens became a global optical business, and the sentence sold beside it is that darker means safer. Darker means the eye received less. Less, at index 8, is the wrong instruction to a protection system.

Set beside the cancer rates

Melanoma of the skin rose across the same decades the lens became ordinary clothing. In the SEER 8 registries the observed age-adjusted incidence was 8.80 per 100,000 in 1975 and 28.64 per 100,000 in 2022, a 3.25-fold rise. The National Cancer Institute puts the 2026 US estimate at 112,000 new cases and 8,510 deaths. Indoor tanning, intermittent burning, fair skin, age, and better detection all sit on that curve. So does a device whose measured effect, in the contact-lens experiment, is to suppress the hormone later shown in people to raise melanin and cut thymine dimers. A shared calendar is not a verdict by itself. A shared calendar plus a hormone that defends skin, plus a lens that stops the eye from calling for that hormone, is a mechanism with a body count worth taking seriously. The rise was not subtle. The product was not rare.

Warning

At UV index 8, or near it, sunglasses inhibit the body’s ability to protect the skin. The sky is at 200 mW/m². The lens tells the eye, and through the eye the brain, that the ultraviolet is lower. The pituitary does not release the ocular α-MSH signal at the level that index would have produced. Melanocytes that take their order from that signal are left under-defended in a very high UV hour. The darker the view, the more complete the understatement. A lower report is not a lower dose. The dose is on the skin. The damage is the protection that was never ordered.

Sources

1. Hiramoto K, Yanagihara N, Sato EF, Inoue M. Ultraviolet B irradiation of the eye activates a nitric oxide-dependent hypothalamopituitary proopiomelanocortin pathway and modulates functions of α-melanocyte-stimulating hormone-responsive cells. J Invest Dermatol. 2003;120(1):123–127.
2. Hiramoto K, Sato EF. Ultraviolet B radiation to the eye induces pigmentation in the epidermis via gp91phox. Clin Exp Dermatol. 2012;37(1):65–67. Eye dose cited as 2.5 kJ/m².
3. Hiramoto K, Kobayashi H, Yamate Y, Ishii M, Sato T, Inoue M. UVB-induced epidermal pigmentation in mice and UVB-blocking contact lenses. Cont Lens Anterior Eye. 2013. doi: 10.1016/j.clae.2012.10.072.
4. Hiramoto K, et al. UVB irradiation of the mouse eye induces pigmentation of the skin more strongly than stress loading. Clin Exp Dermatol. 2013;38(1):71–76.
5. Barnetson RS, et al. [Nle4-D-Phe7]-α-melanocyte-stimulating hormone significantly increased pigmentation and decreased UV damage in fair-skinned Caucasian volunteers. J Invest Dermatol. 2006;126(8):1869–1878.
6. Song X, Mosby N, Yang J, Xu A, Abdel-Malek Z, Kadekaro AL. α-MSH activates immediate defense responses to UV-induced oxidative stress in human melanocytes. Pigment Cell Melanoma Res. 2009;22(6):809–818.
7. World Health Organization, World Meteorological Organization, United Nations Environment Programme, International Commission on Non-Ionizing Radiation Protection. Global Solar UV Index: A Practical Guide. 2002. Index 8–10 is very high. 1 index unit is 25 mW/m² erythemal irradiance, so index 8 is 200 mW/m².
8. Cui R, Widlund HR, Feige E, et al. Central role of p53 in the suntan response and pathologic hyperpigmentation. Cell. 2007;128(5):853–864.
9. Berson DM, Dunn FA, Takao M. Phototransduction by retinal ganglion cells that set the circadian clock. Science. 2002;295(5557):1070–1073.
10. Hattar S, Liao HW, Takao M, Berson DM, Yau KW. Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science. 2002;295(5557):1065–1070.
11. Hoover HL. Sunglasses, pupil dilation, and solar ultraviolet irradiation of the human lens and retina. Appl Opt. 1987;26(4):689–695.
12. National Cancer Institute. Cancer Stat Facts: Melanoma of the Skin. SEER. SEER 8 observed incidence 8.80 per 100,000 in 1975 and 28.64 per 100,000 in 2022. 2026 estimates: 112,000 new cases, 8,510 deaths.
13. EssilorLuxottica, 2024 annual revenue, 26.51 billion euros, as reported from the company release. Figure is group sales, not sunglasses profit.
14. History.com. Who invented sunglasses? Foster Grant, 1929, 10 cents, Atlantic City. Life magazine, 1938, as summarized there: about 20 million pairs the prior year, about 1/4 worn for a medical reason. Ray-Ban Aviator sequence: 1936 prototype, 1937 public sale, 1938 metal frame.