Unexpected Places to get a Sunburn: How Easily Does Sunlight Get To You?

Sunlight consists of three major subtypes of electromagnetic radiation: ultraviolet radiation, visible light, and infrared radiation.¹ Ultraviolet (UV) radiation is responsible for causing damage to the skin that leads to skin cancer, the most common type of cancer in light skinned populations in the world.¹

Types of Ultraviolet Radiation

There are three types of UV radiation: ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC). Approximately 90-99% of UVA radiation reaches the earth’s surface, compared to 1-10% of UVB and no UVC.¹

• UVA radiation has a long wavelength and low energy, so can penetrate deeper into the skin. It is responsible for causing aging of the skin and inducing immediate and persistent pigmentation (i.e. tanning).¹
• UVB radiation is short wavelength and high energy so penetrates the upper layers of the epidermis only. It is a thousand times more effective at causing sunburns than UVA radiation.
• UVC is filtered by the stratospheric ozone layer in the atmosphere so does not reach the earth. If exposed to an artificial source, such as a germicidal lamp, it can cause burns and skin cancer.¹

This article will explore a few unexpected places where the skin is susceptible to damage by UV radiation. 

In The Car: UV Penetration Through Glass

Skin cancers on the left side of the face are more common where drivers sit on the left side of the car. This is consistent with studies that show that side windows do not adequately block UVA radiation, the only type of ultraviolet radiation that can pass through side glass windows.² Windshields, on the other hand, do offer consistent protection to driver’s eyes and faces against UVA light. A study performed in 2016 showed that the average front-windshield UVA blockage was 96% (ranging from 95.7-96.3%), compared to 71% by side windows (ranging from 44-96%).²

The increased protection offered by windshield windows is due to their design. These windows consist of two panes of glass with a clear layer of plastic between them, which contains UVA protection factors.² Side windows do not have this plastic layer and are only a single pane of glass, which may or may not have a high level of UVA blocking polymers within it. The amount of UVA protection is dependent on the glass type, color, coating, and other factors, which can vary greatly depending on the manufacturer.²

People often request tinted car windows due to the presumed high level of UV protection that they offer. There are several types of tinted windows commercially available, with all cars on the U.K. market now required to have tinted windows. A study by Hampton et al. showed that tinting does decrease UVA transmission through glass. Clear glass transmitted 62% of UVA while dark-tinted glass transmitted only 11.4%.³ However, darker tints may be restricted by local laws. Light green tinting is allowed, but permits an average of 35.7% UVA transmission.³ Other studies have shown that tinted windows actually range significantly in the degree of UVA blockage, so one cannot assume that they are protected. In fact, films that effectively block UVA light are essentially clear.² When UV radiation encounters a regular pane of glass, it is simultaneously transmitted and absorbed. Quality UV-protective films will decrease the amount of radiation that is transmitted by increasing the amount that is absorbed, allowing only 0.4% transmission of UVA.⁴ Use of these films in automobile production is not yet industry standard, so one should never assume that their side car window is offering full sun protection.⁵

Protective Tips

1. Wear sun protective clothing (UPF >40)^a and broad brimmed hat, while traveling in your car.
2. Wear a sun protective sleeve on the left arm for left-sided driven cars or the right arm for right-sided driven cars.
3. Use sunscreen daily with broad spectrum (UVA and UVB) protection, at least SPF 30.⁶
4. Consider installing a clear protective film in your side car windows.

1. Clothing with an ultraviolet protection factor (UPF) of at least 15-20 is required for good protection, though European standards recommend a UPF of 40+. UPF is affected by the weave density, composition, color, fabric thickness, stretch, moisture, and fabric condition. Thick, heavy, dark clothing seems to provide the most protection. Garments that are wet or have been stretched or worn thin by numerous washes have decreased UPF values, so offer lower UV protection.⁶

Swimming: UV penetration Through Water

Many believe that the increased sunburn risk on the coast is due to the high reflectance of light off of water and sand. While it is true that UV light may reflect significantly, the level of reflection from water can actually vary significantly depending on the water contents. In fact, UV radiation penetrates through water more than it is reflected, making a person just as likely to experience burns under the water as above it. This is especially true for very clear water where chlorophyll concentrations are low (such as coral reefs, alpine lakes, and swimming pools).⁷ While the clarity of the water does seem to impact the underwater UV index, surface roughness has no effect.⁷ The increased risk for sunburn on the seaside seems to be more a product of lack of shade and direct exposure, rather than UV reflection.

Some studies have shown that wetting the skin with tap water or 5% saline solutions can decrease the threshold needed for UVB induced erythema (i.e. sunburn).^8,9 It has been proposed that this effect may be due to enhanced immunomodulatory effects of UV radiation after exposure to water, such as decreased antigen-presenting function of Langerhans cell, and a reduction in interleukin (IL)-12 and increase of IL-10.⁸ This increased sun sensitivity after soaking the skin may account for the increased suburn risk after bathing and while enjoying a day at the beach. 

Protective Tips

1. Wear sun protective clothing (UPF >40) and broad-spectrum sunscreen (SPF>30) when outside.
2. Reapply sunscreen every 2 hours, more often if going in the water.
3. Pat dry after submerging in an outdoor body of water.
4. Be sure to protect yourself when snorkeling, as UV can easily penetrate these clear waters. Wear UV protective swim wear (UPF >40) and reapply sunscreen as often as possible.
5. Seek shade whenever possible.
6. Avoid being outside when the UV index is highest, between 10 AM and 4 PM.

On a Cloudy Day: UV Penetration Through Clouds

The total global UVA radiation that makes it way to the earth’s surface consists of a direct and diffuse component.¹⁰ The direct component comes straight from the sun, while the diffuse component is comprised of radiation scattered by the atmosphere and clouds. The shorter the wavelength of the radiation, the more it scatters, making UV light more likely to scatter than visible light.¹⁰ Due to this high diffuse component of UV radiation, clouds cannot reduce global UVB and UVA radiation as much as visible light.¹⁰ While an increased degree of cloud cover can decrease the amount of UV radiation, this varies widely based on different cloud types, cloud optical depths, and the relative position of the sun with respect to the clouds.¹¹ In some cases, clouds can actually have an enhancing effect on UV radiation, called the “enhancement effect.”¹¹ Thus, even on a cloudy day, it is possible to get a high degree of UV exposure.

Protective Tips

1. Wear sun protective clothing (UPF>40) and broad-spectrum sunscreen (SPF>30) when outside, even if it is a cloudy day.
2. Reapply sunscreen every 2 hours, more often if going in the water.
3. Seek shade whenever possible.
4. Avoid being outside when the UV index is highest, between 10 AM and 4 PM.