Ultraviolet (UV) radiation (UVR) has well-known adverse effects on the skin and eyes. Little attention is given to physical means of photoprotection, namely glass, window films, sunglasses, and clothing. In general, all types of glass block UV-B. For UV-A, the degree of transmission depends on the type, thickness, and color of the glass. Adding window films to glass can greatly decrease the transmission of UV-A. Factors that can affect the transmission of UVR through cloth include tightness of weave, thickness, weight, type of fabrics, laundering, hydration, stretch, fabric processing, UV absorbers, color, and fabric-to-skin distance.
Key points
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Physical methods of photoprotection include glass, window films, sunglasses, and clothing.
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All types of glass block UV-B.
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UV-A transmission depends on the type of the glass.
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Window films added to glass greatly decrease the UV-A transmission of the glass.
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Fabric characteristics can greatly affect the level of photoprotection offered by clothing.
Introduction
All people are exposed to ultraviolet (UV) radiation (UVR) from the sun. UVR is divided into UV-C (100–290 nm), UV-B (290–320 nm), and UV-A (320–400 nm). All of UV-C and most UV-B (approximately 90%) are absorbed by the ozone layer, which results in mainly UV-A and a small percentage of UV-B reaching the surface of the earth. UVR has known effects on the skin and eyes. With regards to the skin, the acute effects include erythema, edema, pigment darkening, delayed tanning, epidermal hyperplasia, and vitamin D biosynthesis. The chronic effects include immunosuppression, photoaging, and photocarcinogenesis. On the eyes, UVR exposure has been strongly associated with the development of pterygium, photokeratitis, climatic droplet keratopathy, and cortical cataracts. Photoprotection in the form of seeking shade between 10 am and 2 pm , wearing a wide-brimmed hat, and applying sunscreen is widely promoted to the public. However, little attention is given to educate the public on physical methods of photoprotection, which include clothing, glass, and sunglasses.
Introduction
All people are exposed to ultraviolet (UV) radiation (UVR) from the sun. UVR is divided into UV-C (100–290 nm), UV-B (290–320 nm), and UV-A (320–400 nm). All of UV-C and most UV-B (approximately 90%) are absorbed by the ozone layer, which results in mainly UV-A and a small percentage of UV-B reaching the surface of the earth. UVR has known effects on the skin and eyes. With regards to the skin, the acute effects include erythema, edema, pigment darkening, delayed tanning, epidermal hyperplasia, and vitamin D biosynthesis. The chronic effects include immunosuppression, photoaging, and photocarcinogenesis. On the eyes, UVR exposure has been strongly associated with the development of pterygium, photokeratitis, climatic droplet keratopathy, and cortical cataracts. Photoprotection in the form of seeking shade between 10 am and 2 pm , wearing a wide-brimmed hat, and applying sunscreen is widely promoted to the public. However, little attention is given to educate the public on physical methods of photoprotection, which include clothing, glass, and sunglasses.
Photoprotection by glass
Glass is a combination of sand or silica and other components, which are melted together at a very high temperature. At room temperature, it becomes a solid, whereas at higher temperatures, it softens to become a liquid. Flat glass is the basic material used to make industrial and automotive glass. It is produced by the float process. During the float process, sand, limestone, soda ash, dolomite, iron oxide, and salt cake are mixed together with broken glass (cullet). These ingredients are melted at 1600 °C to produce a flat glass. The flat glass is then treated in different ways to produce the different types of glasses that are discussed in the next section ( Table 1 ).
| Types of Glass | Comments |
|---|---|
| Annealed glass | Basic flat glass Breaks into large pieces |
| Toughened (tempered) glass | Withstands more compression than annealed glass Breaks into small regular pieces |
| Coated glass | Made by applying a surface coating to the glass Decreases the transmission of light and heat, resistance to scratch, or resistance to corrosion |
| Laminated glass | Made up of ≥2 layers of glass with interlayers of plastic Decreases transmission of UVR, transmission of sound, and improves security Broken pieces are held together by the interlayer |
| Patterned glass | Flat glass with a surface that shows a regular pattern |
Main Types of Glass
Annealed glass is the basic flat glass, which is the first result of the float process. It is the starting material in the glass industry to produce more advanced glass. It breaks into large pieces.
Toughened or tempered glass is produced by heating the flat glass, followed by rapid cooling of the glass surface by air, which results in glass that withstands more compression than flat glass. It breaks into small regular pieces.
Coated glass is made by applying a surface coating to the glass that results in additional advantages, such as decrease in the transmission of light or heat, resistance to scratch, or resistance to corrosion.
Laminated glass comprises 2 or more layers of glass with 1 or more interlayers of polymeric material (plastic) such as polyvinyl butyral. It is used in car windshields and facades of buildings. Technology can be incorporated in laminated glass to provide UV filtering, decreasing of sound transmission, adding colors, or resistance to fire. When it breaks, the broken pieces are held together by the interlayer, which provides safety.
Patterned glass is a type of flat glass with a surface that shows a regular pattern.
UV Transmission Through Residential Glass
Almost all glass blocks UV-B radiation, regardless of type or properties of the glass. However, the transmission of UV-A radiation varies according to the type, thickness, and color of the glass. Duarte and colleagues looked at the transmission of UV-A through various types of residential glass (annealed, patterned, tempered [toughened], and laminated). In most studies on glass, UV-A transmission is measured up to 380 nm, because at wavelength greater than 380 nm, glass would have to be opaque or heavily tinted to provide photoprotection. The transmission was measured by a photometer (UV-A-400C, NBC, OH) at zero distance from the UV-A source. These investigators showed that annealed and tempered glass transmit 74% and 72% of UV-A, respectively; patterned glass transmits less UV-A (45%), whereas laminated glass blocks all UV-A. The color of the glass also plays a major role in the transmission of UV-A. Of 5 different colors of patterned glass (green, yellow, wine, blue, and colorless), green was found to be the most protective, followed by yellow. Colorless and wine have similar properties, whereas blue glass offered the least photoprotection. The thickness of the glass has a small effect on the UV-A transmission. Increasing the thickness of the glass 5 times (from 0.2 cm to 1.0 cm) resulted in a modest decrease in the transmission of UV-A, from 76% to 51%.
UV Exposure in Automobiles
People spend approximately 1 to 2 hours per day in their automobiles, based on an epidemiologic study that evaluated 169 individuals from different parts of the United States. When Kimlin and colleagues evaluated UV exposure in cars both with open and closed windows, they found that the exposure is high enough to be considered in calculating the total lifetime UV exposure.
There is some clinical evidence to suggest that UV exposure in automobiles might have a biological significance. Hampton and colleagues found that a 30 to 60 minutes exposure to solar radiation in midday summer in the United Kingdom through automobile tempered glass can reach a dose of 5 J/cm 2 , which is sufficient to induce eruptions in patients with severe photosensitivity disorders. Two studies from Australia, which separately evaluated actinic keratosis and lentigo maligna, found these 2 conditions to be more common on the right side (the driver’s side in Australia). Skin cancers such as basal cell carcinomas, squamous cell carcinomas, melanomas, and Merkel cell carcinomas were found to be slightly more common on the left side from 2 retrospective studies performed in the United States.
Automobile Glass
Of the flat glass industry market, 15% to 20% goes to the production of automobile glazing. Automobile glazing is made up of laminated or tempered (toughened) glass. Either type can be tinted to improve the comfort and decrease visible light and infrared transmission. Most automobile glasses are tinted. The most commonly used tint is green, although gray and blue have also been used. Windshields are made up of laminated glass for safety reasons to prevent ejection of the passengers in the event of frontal impacts, whereas side and back windows are usually made up of tempered (toughened) glass.
UV transmission through automobile glass
Similar to residential glass, both laminated and tempered glass used for automobiles block UV-B. The transmission of UV-A depends on the type and color of the glass.
Laminated glass, which is used in windshields, blocks most UV-A radiation. Bernstein and colleagues measured the UV irradiance (by ELSEC UV monitor [Littlemore Scientific Engineering, Dorset, UK]) of solar radiation in a sunny September day in New Jersey both directly and through a laminated glass (windshield of a 2006 Volvo) and found that laminated glass blocked 98% of the UVR. Moehrle and colleagues examined green-tinted, blue-tinted, and infrared reflective glass of the laminated types used in windshields of 3 Mercedes-Benz models for their UV transmission characteristics and found that all of them blocked UV-A up to 380 nm. Comparing 3 different colors of laminated glass used in windshields, gray was the most protective, with transmission of only 0.06% of UV-A, followed by green, which transmits 9% of UV-A, and clear laminated glass, which transmits 9.7% of UV-A radiation. Laminated glass used in windshields allows minimal transmission of UV-A radiation (up to 380 nm), and gray has been shown to be the most effective.
Tempered glass is used in most side and back windows. Similar to other types of glass, it blocks UV-B but transmits variable amounts of UV-A radiation. Bernstein and colleagues found that the side window (tempered glass) of a 2006 Volvo transmitted 79% of the UV-A radiation. Hampton and colleagues evaluated 4 different colors of tempered glass used in side windows. These investigators found the transmission of UV-A as follows: clear (63%), light green (36%), dark green (23%), and gray (11%). Another study evaluated 3 different types of tempered glass: double-glazed green, double-glazed blue, and double-glazed infrared reflective glass. The study showed that the average amount of UV-A transmission was 17.5% in double-glazed green glass, 22.4% in double-glazed blue glass, and 0.8% in double-glazed infrared reflective glass. From these studies, the amount of UV-A transmission through tempered glass depends on the color of the glass. In addition, the thickness of the glass offers additional protective benefit, with double-glazed glass transmitting less UV-A than a single pane of the same color. Using infrared reflective technology in the tempered glass adds great advantage in regards to UV-A protection and decrease of infrared transmission.
These studies show how the characteristics of glass affect the transmission of UVR, but how can this be applied to day-to-day life? Mean UVR exposure to a car passenger is 3% to 4% of the ambient UVR, with the highest UV exposure to the left arm (in those countries with left-sided driver’s seats) and lateral head of the driver. Whether the UV-A exposure through the tempered glass is sufficient to induce rash in photosensitive patients is dependent on the distance of the body part from the window and time spent in the car. In the worst-case scenario, when the arm is elevated and thus in direct contact with sunlight, exposure of 30 minutes through a tempered, clear, side window resulted in measurements of 5 J/cm 2 of UV-A. This dose is sufficient to induce clinical lesions in highly photosensitive patients with conditions such as polymorphous light eruption or chronic actinic dermatitis.
Window films
The use of window films started in the 1960s. They were used for the reflection of solar radiation but allowing vision. During the energy crisis of the 1970s, interest also developed in reducing heat loss to the outside.
Film is composed of polyester substrate with adhesive layer on 1 side and scratch-resistant coat on the other side. All the components of the film must have a high optical quality to allow for vision through the film.
A standard film has the following components: (1) protective release layer, which is a thin polyester layer, which must be removed before application; (2) adhesive layer to bind the film to the glass; (3) multiple layers of polyester with laminating adhesive in between; (4) hard acrylic coating to protect the film from scratching; and (5) dyes, metals, alloys, or UV filters which are added to offer specific advantages to the film.
The UV blocking ability of films can be achieved by different ways. The metals, alloys, and dyes, which are incorporated into films to reflect the solar radiation, can offer some UV protection by reflection or absorption of UVR. However, the primary method of UV protection is by either adding UV inhibitors to the adhesive layer or having a separate layer of UV inhibitors. Having a separate layer of UV inhibitors may offer better performance and longevity, but there is no evidence to support this statement. Most manufacturers claim UV protection of up to 99%; however, this claim is based on the measurement of UVR up to 380 nm. A study evaluated 40 different films from 8 different brands and found that UVR blocking of the film when applied to glass ranged from 86% to 99%. Most of the tested products blocked more than 90% of UVR up to 400 nm, but only 2 products blocked 99% of UVR.
Applying film to glass results in decrease in the transmission of UVR. Two studies evaluated window films on tempered glass and found more than 99% reduction in the transmission of UVR. Duarte and colleagues applied a sunlight control film (G5, Insulfilm, Brazil) to a tempered glass, which resulted in total blockage of the transmission of UV-A as measured by a UV-A photometer (NBC, OH). Bernstein and colleagues measured the UV transmission through the windshield, side window, and side window plus Formula One UV absorbing film (Formula One ® , Solutia, St. Louis, MO, USA). The measurement was made on a sunny day in New Jersey by a UV monitor (ELSEC UV, Dorset, UK). These investigators found that the windshield blocked 98% of UVR, whereas the side window blocked only 21% of the UVR, which increased to 99.6% after the addition of window film.
Photoprotection with sunglasses
UV Exposure and the Eye
UV-B is absorbed mainly by the cornea, whereas UV-A is absorbed by the cornea, aqueous, and the lens. The time of maximum UV exposure to the eyes is from 8 to 10 am and 2 to 4 pm , when the rays of the sun are parallel to the eyes, in contrast to the time of maximum UV exposure to the skin, which is from 10 am to 2 pm .
Exposure to UV-A and UV-B can affect the eyes. There is strong evidence to support the association of UVR exposure and the following eye conditions: pterygium, photokeratitis, climatic droplet keratopathy, and cortical cataract. Several studies showed an association between UV-B and UV-A exposure and the formation of pterygium in a dose-dependent fashion. Acute exposure to UV-B can result in photokeratitis, which is a painful, superficial burn of the cornea similar to skin sunburn, whereas chronic exposure to UV-A and UV-B is associated with the formation of climatic droplet keratopathy. UV-B has also been strongly associated with the development of cortical cataracts; doubling the UV-B exposure resulted in 60% increase in the risk of developing cortical cataracts. A recent meta-analysis has suggested an association between sunlight exposure and the development of age-related macular degeneration. However, this association is most likely caused by blue light rather than UV-A or UV-B. On the other hand, there is limited evidence to support the association of UVR and the development of pinguecula, nuclear and posterior subcapsular cataract, and ocular melanoma.
Sunglasses Guidelines
Three national standards for sunglasses exist: (1) the American standard ANSI Z80.3, last updated in 2010; (2) the European standard EN 1836:2005; and (3) the Australia/New Zealand standard AS/NZS 1067:2003. The European and the Australian are similar with regards to UVR requirements, but they differ in the definition of UV-A and the maximum allowed UV-B transmission ( Table 2 ). The American standard is summarized in Table 3 . In the American standard, the allowed UV transmission depends on whether it is for normal use (such as inside cars, or from car to home or office) or for prolonged use.
| Lens Category | Luminous Transmittance (LT) (%) | UV-B (% LT) | UV-A | ||
|---|---|---|---|---|---|
| AS/NZS 1067 and EN 1836 | EN (280–315 nm) | AS (280–315 nm) | EN (315–380 nm) | AS (315–400 nm) | |
| 0 (very light tint) | 80–100 | 10 | 5 | LT | LT |
| 1 (light tint) | 43–80 | 10 | 5 | LT | LT |
| 2 (medium tint) | 18–43 | 10 | 5 | LT | LT |
| 3 (dark tint) | 8–18 | 10 | 5 | 50% LT | 50% LT |
| 4 (very dark tint) | 3–8 | 10 | 5 | 50% LT | 50% LT |
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