When sunlight streams through a window, people often wonder if they receive the same benefits or risks as standing directly outdoors. Sunlight is not a single entity but a complex mixture of electromagnetic energy traveling in waves of various lengths. These different wavelengths interact uniquely with materials like standard window glass, which acts as a selective filter. Understanding this filtering process determines what part of the sun’s energy makes it indoors and what effects that energy has on the human body. The answer depends entirely on which specific component of solar radiation you are asking about.
The Different Types of Solar Radiation
Solar energy reaching the Earth’s surface is broadly categorized into three main parts of the electromagnetic spectrum: ultraviolet (UV) radiation, visible light, and infrared (IR) radiation. Each component has distinct physical properties and biological effects. UV radiation represents the shortest, most energetic wavelengths, making up approximately eight percent of the total solar energy, and is divided into UV-A and UV-B. Visible light occupies the middle portion of the spectrum, accounting for about 42 to 47 percent of solar energy, and is the light our eyes can perceive. IR radiation comprises the longest wavelengths, responsible for nearly 50 percent of the sun’s energy, and is sensed directly as heat.
How Window Glass Alters Sunlight
Standard architectural glass, typically made from soda-lime silicate, possesses a highly selective filtering property that fundamentally changes the composition of the sunlight passing through it. The glass acts as a nearly complete barrier to the shorter, high-energy UV-B wavelengths. Standard window glass blocks almost 100 percent of UV-B radiation because the molecular structure of the glass absorbs these specific short wavelengths, preventing them from penetrating the interior space.
In contrast, the longer UV-A wavelengths are far less affected by the glass barrier. Most common window glass is partially transparent to UV-A, allowing a significant portion to pass through unimpeded. The glass remains highly transparent to the entire range of visible light, which is why the room remains bright.
The interaction with infrared radiation is more complex, depending on the specific wavelength. Short-wave infrared radiation coming directly from the sun passes through the glass with minimal restriction. However, the long-wave infrared radiation, re-emitted by objects warmed indoors, is largely blocked or absorbed by the glass. This selective transmission and blockage drives the warming effect inside a closed space.
Vitamin D Synthesis and Indoor Exposure
The biological process of synthesizing Vitamin D in the skin is entirely dependent on exposure to a specific type of solar energy. Vitamin D production requires high-energy UV-B radiation to convert a cholesterol precursor, 7-dehydrocholesterol, into previtamin D3. This biochemical reaction is the primary way the human body naturally creates Vitamin D.
Because standard window glass is an extremely effective barrier against UV-B radiation, this necessary wavelength is almost entirely filtered out before it reaches the skin indoors. Without sufficient UV-B, the crucial conversion process cannot be initiated. Consequently, sitting behind a closed window will not lead to adequate Vitamin D production.
This is a direct biological consequence of the glass’s filtering properties. The perceived warmth or brightness of the sun indoors offers no benefit for Vitamin D status because the specific component required for this health benefit is missing.
Effects Beyond Vitamin D: Tanning and Heat
Even though UV-B is blocked, the UV-A radiation that successfully penetrates the glass can still affect the skin. Since UV-A passes through most standard window glass, it contributes to tanning, which is a sign of skin cells attempting to protect themselves. Prolonged exposure to this transmitted UV-A is also associated with photoaging, including wrinkling, and increases the risk of long-term skin damage.
The warmth felt when sitting by a sunny window is primarily due to the transmission of visible light and short-wave infrared radiation. Once these waves enter the room, they are absorbed by surfaces, furniture, and the skin. These warmed objects then re-radiate energy at longer infrared wavelengths. Because the window glass blocks the escape of this longer-wave infrared energy, the heat becomes trapped, causing the interior temperature to rise in a phenomenon known as the greenhouse effect.