Indirect sunlight—light that has been scattered, diffused, or reflected—carries a measurable risk of skin damage. The damage is caused by the invisible ultraviolet (UV) radiation component, not the visible light. UV radiation can penetrate the atmosphere and various barriers with ease, a fact often underestimated. Therefore, indirect sunlight can damage your skin, particularly due to the cumulative effects of daily exposure.
Understanding UVA and UVB in Indirect Light
Indirect sunlight poses a threat due to the distinct properties of UVA and UVB radiation. Ultraviolet B (UVB) rays have a shorter wavelength (290–320 nm) and primarily cause sunburn and affect the skin’s surface layers. Ultraviolet A (UVA) rays have a longer wavelength (320–400 nm), allowing them to penetrate much deeper into the skin’s dermal layer.
This longer wavelength makes UVA the primary concern in indirect light. UVA is not filtered as significantly by the atmosphere as UVB, remaining relatively constant throughout the day and year. Approximately 95% of the UV radiation reaching the Earth’s surface is UVA, meaning it presents an exposure risk even when the sun is not directly shining.
Pathways of Indirect Exposure
Indirect UV radiation reaches the skin through several common pathways.
Transmission Through Clouds
One major pathway is transmission through cloud cover, which filters visible light and heat but only moderately reduces UVA penetration. Up to 90% of UVA rays can still pass through clouds, meaning a cloudy day does not equate to a safe day for exposed skin.
Penetration Through Glass
Another significant pathway is exposure through standard window glass in homes, offices, or cars. While common window glass effectively blocks nearly all UVB rays, it allows a considerable portion of UVA to pass through unimpeded. Individuals who spend long hours near a window, such as while driving or working indoors, receive chronic, low-level UVA exposure. A higher proportion of UVA can pass through the side and rear windows of a car compared to the laminated windshield.
Reflection Off Surfaces
The third pathway involves the reflection of UV radiation off various surfaces, known as ambient UV. This significantly increases exposure even when a person is standing in the shade. Snow is particularly reflective, bouncing back up to 90% of UV radiation. Other common surfaces also contribute, including dry sand which reflects about 15% of UV rays, and concrete or water surfaces.
Cellular Effects of Chronic Low-Level UV
Constant, low-level exposure to indirect UV, predominantly UVA, causes cumulative damage to skin cells and the extracellular matrix. This chronic exposure leads to photoaging, which causes the premature development of wrinkles, laxity, and an uneven texture. UVA radiation penetrates the deep dermal layer where it attacks structural proteins.
Within the dermis, UVA induces reactive oxygen species (ROS) and free radicals, causing oxidative stress that damages DNA, proteins, and lipids. This oxidative cascade activates enzymes called matrix metalloproteinases (MMPs), which degrade collagen and elastin fibers that provide the skin’s strength and elasticity. The resulting inflammation also triggers cellular senescence, a state where cells stop dividing but remain metabolically active.
The accumulation of DNA damage from chronic exposure increases the risk of skin cancer over time. While UVB is traditionally linked to sunburn and acute damage, UVA plays a substantial role in the development of both non-melanoma skin cancers, such as basal cell carcinoma, and the more dangerous melanoma. Indirect UV damage rarely causes a noticeable sunburn, allowing subthreshold damage to progress silently.
Daily Strategies for Indirect Sun Protection
Mitigating the risk from indirect sun exposure requires consistent protective measures applied daily, regardless of the weather or location. The most effective strategy involves the consistent use of a broad-spectrum sunscreen, formulated to protect against both UVA and UVB radiation. Sunscreen should be applied to all exposed skin, even when spending the day indoors near windows.
Look for products with a Sun Protection Factor (SPF) of 30 or higher and apply a sufficient amount 15 minutes before exposure. Reapplication is necessary every two hours if outdoors, or every four to six hours if remaining indoors. Physical barriers also offer reliable protection against ambient and diffused UV radiation.
Wearing clothing with a high Ultraviolet Protection Factor (UPF) rating and wide-brimmed hats shields the skin from reflected rays. For indoor environments, installing UV-filtering window films or using curtains can significantly reduce UVA radiation transmitted through the glass. Incorporate these protective steps into a daily routine, recognizing that UV exposure is a continuous, year-round phenomenon.