Tanning is the skin’s defense mechanism, caused by the production of the pigment melanin in response to ultraviolet (UV) radiation. While the sun’s rays undeniably enter the vehicle, the material properties of automotive glass significantly alter which parts of the UV spectrum reach your skin. The true answer depends entirely on which window you are sitting next to.
Understanding UVA and UVB Rays
The sun emits ultraviolet radiation, which is broadly categorized into two types that affect our skin: UVA and UVB rays. Ultraviolet A (UVA) rays have a longer wavelength, allowing them to penetrate deeper into the skin’s layers, reaching the dermis. UVA is the primary trigger for melanin production, making it the main factor in tanning and long-term skin aging, such as wrinkles and sunspots.
Ultraviolet B (UVB) rays have a shorter wavelength and primarily affect the skin’s outermost layer, the epidermis. UVB is the main cause of sunburn and also plays a significant role in stimulating vitamin D synthesis in the skin. Both UVA and UVB damage the DNA in skin cells, contributing to the risk of skin cancer, but their differing wavelengths mean they are filtered differently by common materials.
The UV Filtering Power of Car Windows
The ability of a car window to block UV rays depends entirely on how the glass is constructed. Vehicles use two main types of glass: laminated and tempered. Laminated glass, which is universally used for windshields, consists of two layers of glass with a polyvinyl butyral (PVB) plastic film sandwiched between them.
This PVB interlayer is highly effective, blocking nearly 99% of both UVA and UVB radiation, essentially making it impossible to get a tan or sunburn through the front windshield. The PVB layer is included primarily for safety, as it holds shattered glass fragments together in a crash, but its UV-blocking properties are an important secondary benefit.
In contrast, the side and rear windows of most cars are made of tempered glass, which is a single, thinner layer. Tempered glass is designed to crumble into small, blunt pieces upon impact, reducing injury risk, but it lacks the PVB film. This means that while tempered glass blocks most of the shorter-wavelength UVB rays, it allows a significant portion of the longer-wavelength UVA rays to pass through. Therefore, sitting next to a side window exposes you to the UVA radiation that drives tanning and skin aging.
Why You Still Need Protection While Driving
The UVA radiation that penetrates the side windows poses a serious long-term health risk, even if it does not cause an immediate sunburn. Chronic, low-level exposure to these rays leads to photoaging, which is premature skin aging characterized by leathered texture, deep wrinkles, and dark spots. This effect is often visible in long-term drivers who frequently expose one side of their face and arm to the sun.
Case studies, such as the widely published image of a truck driver, demonstrate this asymmetrical skin damage, known as unilateral dermatoheliosis. Decades of sun exposure through the driver’s side window caused significantly more pronounced aging on that side of his face compared to the shielded side. This confirms that the UVA rays passing through side glass cause cumulative damage.
Furthermore, UVA exposure is strongly linked to an increased risk of melanoma and non-melanoma skin cancers. Even if you do not notice a visible tan, the radiation is causing damage to cellular DNA, increasing your risk over time. For regular drivers, proactive measures are recommended to mitigate this risk. Simple steps include wearing broad-spectrum sunscreen on exposed skin, utilizing UV-protective clothing, or having professional UV-blocking window film applied to the side windows.