Yes, indirect sunlight contains ultraviolet (UV) rays, meaning exposure risk is not limited to standing directly in the sun. This indirect radiation results from sunlight scattering off molecules and particles in the atmosphere, allowing UV to reach you even when you are in the shade or indoors. UV radiation is categorized into three types based on wavelength. Ultraviolet A (UVA) has the longest wavelengths, penetrating deepest into the skin and comprising about 95% of the UV radiation that reaches the Earth’s surface. Ultraviolet B (UVB) has shorter wavelengths, primarily affecting the outer layer of the skin and causing sunburn, while Ultraviolet C (UVC) is completely absorbed by the Earth’s ozone layer.
The Science of Scattered Light
The process that converts direct sunlight into indirect, or diffuse, light is known as atmospheric scattering. This scattering is the primary mechanism for indirect UV exposure, allowing the radiation to reach areas not in the sun’s direct line of sight.
This scattering is explained by two models: Rayleigh and Mie. Rayleigh scattering occurs when light interacts with tiny particles, such as nitrogen and oxygen molecules, which are much smaller than the light’s wavelength. Shorter wavelengths, like UVB, are scattered far more effectively than longer ones, which is why the sky appears blue and why UVB is heavily scattered.
Mie scattering involves larger particles, such as dust, water droplets, and aerosols, which are similar in size to the UV wavelengths. This type of scattering is less dependent on wavelength and results in the white appearance of clouds and haze. UVA rays are scattered less effectively by the atmosphere compared to UVB, allowing them to maintain a relatively high intensity in the diffuse light that reaches the Earth’s surface. This explains why UVA is a constant presence in indirect sunlight, posing a risk for photoaging and long-term skin damage.
UV Transmission Under Outdoor Conditions
The presence of UV rays in indirect sunlight means that seeking shade outdoors offers only partial protection from solar radiation. While shade protects you from the sun’s direct rays, you are still exposed to diffuse UV radiation scattered by the atmosphere and reflected off surrounding surfaces. Studies show that being in the shade can reduce UV exposure by approximately 50%, but this reduction is highly variable. Reflective surfaces like concrete, sand, and water can scatter up to 80% of UV radiation, allowing rays to bounce under a shade structure and onto your skin. For instance, UV levels under a beach umbrella may still be as high as 84% of full sun exposure due to reflection off the sand and water.
Cloud cover is also an unreliable shield against UV. While thick, dark storm clouds can block substantial radiation, thin or scattered clouds may only reduce UV exposure by 20% to 40%. The “broken-cloud effect” occurs when scattered clouds enhance surface UV levels by reflecting and scattering the radiation, sometimes resulting in higher UV exposure than on a clear day. Even on a completely overcast day, up to 80% of UV rays can still penetrate the cloud layer.
UV Rays and Window Glass
The protection provided by window glass against UV rays depends entirely on the type of glass installed. Standard, untinted window glass is highly effective at blocking UVB rays, which is why you are unlikely to get a sunburn while sitting indoors next to a window. However, this common glass allows a substantial amount of UVA radiation to pass through, with transmission rates often ranging from 50% to 75%. Because UVA rays are responsible for photoaging and long-term skin damage, prolonged exposure near a window can still contribute to these effects. Specialized glass, such as laminated glass used in car windshields or modern energy-efficient windows with Low-E coatings, can significantly reduce UV transmission, blocking up to 85% to 95% of both UVA and UVB.
Protecting Yourself From Diffuse UV
Protecting yourself from diffuse UV requires a comprehensive strategy that accounts for this pervasive, indirect exposure. The most effective measure is the daily application of a broad-spectrum sunscreen with an SPF of at least 30, regardless of cloud cover or time spent in the shade. Broad-spectrum protection filters both the aging UVA rays and the burning UVB rays present in indirect light. Physical barriers provide an additional layer of defense against scattered and reflected radiation. Wearing tightly woven clothing, wide-brimmed hats, and sunglasses that offer 100% UV protection helps shield your skin and eyes from all angles of exposure.