Sunlight passing through a window is not truly “indirect,” which is defined as light scattered by atmospheric particles. Instead, glass acts as a powerful, selective filter that fundamentally alters the solar spectrum. This solar energy is broadly categorized into Ultraviolet (UV), Visible light, and Infrared (IR) radiation, each having different effects on human biology. Understanding how glass changes the composition of this energy is key to understanding light exposure indoors.
How Standard Window Glass Alters the Solar Spectrum
Standard residential windows are typically made of soda-lime glass, a material that interacts with the spectrum in a specific manner. This glass is essentially transparent to visible light, transmitting over 90% of the light we see. However, the invisible portions of the spectrum are treated very differently, which is key to understanding light exposure indoors.
The glass acts as a near-total barrier to the shortest, highest-energy wavelengths, blocking almost 100% of UV-B radiation. This is due to trace elements within the soda-lime composition. This filtering is an important distinction, as UV-B rays are the primary cause of sunburn.
In sharp contrast, the longer-wavelength UV-A rays pass through standard glass readily, with transmission rates often exceeding 50% to 75%. While glass filters out the sunburn-causing UV-B, it permits a large amount of the deeply penetrating UV-A radiation to enter indoor spaces. This selective filtering process profoundly changes the biological impact of the remaining sunlight.
The glass also affects the Infrared (IR) radiation, which we feel as heat. Solar IR is broadly divided into near-infrared and far-infrared. While much of the near-infrared portion of the solar spectrum is transmitted, standard window glass is largely opaque to the longer-wavelength, or thermal, infrared radiation. This characteristic contributes to the well-known “greenhouse effect” inside a closed room, where the glass allows solar energy in but traps the heat re-radiated from surfaces inside.
The Effect on Vitamin D Synthesis
The body’s ability to synthesize Vitamin D is directly dependent on exposure to the high-energy UV-B radiation component of sunlight. The process requires UV-B photons within a narrow wavelength range, typically between 270 and 300 nanometers, to convert 7-dehydrocholesterol in the skin into pre-vitamin D3. This is the only way the body can produce the “sunshine vitamin.”
Because standard window glass absorbs virtually all of the UV-B radiation, the sunlight that passes through is rendered biologically inert for this function. Sitting in a sunny office window or a car will not stimulate the necessary chemical reaction for Vitamin D production. The glass effectively removes the specific energy signature required to start the synthesis pathway.
Attempting to boost Vitamin D levels by sunbathing through a window is an ineffective strategy. The skin requires direct exposure to the unfiltered solar spectrum to gain this health benefit. While the sunlight remains bright and warm, its critical biological function is completely neutralized by the glass barrier.
Skin Damage and Aging from Filtered Sunlight
Even though the UV-B rays are blocked, the transmitted UV-A radiation still poses a risk for long-term skin damage and photoaging. UV-A rays have longer wavelengths and penetrate deeper into the skin’s layers than UV-B. This deep penetration allows UV-A to reach the dermal layer, where it causes significant damage to the structural components of the skin.
Prolonged exposure to this filtered light can accelerate the breakdown of collagen and elastin fibers, leading to the formation of fine lines, wrinkles, and skin laxity. The cumulative effect of UV-A exposure can also result in hyperpigmentation, manifesting as sunspots or age spots. This process happens gradually and without the warning sign of a sunburn, which is caused by the blocked UV-B rays.
The heat from the transmitted Infrared (IR) and visible light also contributes to photoaging. The combination of UV, visible, and IR light can synergistically increase the production of reactive oxygen species (ROS) within skin cells, which are unstable molecules that damage cellular structures. This heat-induced stress further degrades collagen and can suppress the skin’s immune function.