The common belief is that glass provides a complete barrier against the sun’s harmful rays. This is a partial truth that overlooks a significant component of ultraviolet (UV) radiation. Understanding the distinct properties of the sun’s energy and how different types of glass interact with it is necessary to assess the risk. Skin damage does not always manifest as an immediate sunburn, making the risk of indoor exposure easy to ignore.
Understanding UVA and UVB Radiation
The sun emits two primary types of ultraviolet radiation that impact the skin: UVA and UVB. These two forms of energy are differentiated by their wavelength and the depth to which they penetrate the skin’s layers. UVB rays have a shorter wavelength and are the primary cause of sunburn, causing damage mainly to the epidermis, the outermost layer of the skin. This type of radiation is most intense during the middle of the day, particularly between 10 a.m. and 4 p.m.
UVA rays possess a longer wavelength, allowing them to penetrate much deeper into the skin, reaching the dermis layer. This deeper penetration damages collagen and elastin fibers, contributing to long-term issues like premature aging, wrinkles, and dark spots. UVA exposure is consistent throughout the day and year, and it poses the main risk for people spending extended time indoors. Both UVA and UVB radiation contribute to the risk of skin cancer.
Glass Permeability in Homes and Vehicles
Standard clear glass, commonly used in residential and office windows, filters out most UVB radiation. This is why sitting near an unshaded window does not typically result in a visible sunburn. However, this same clear glass offers limited protection against the longer wavelength UVA rays. Typical single-pane window glass allows a significant amount of UVA to pass through, with transmission rates as high as 75% to 85%.
The type of glass in vehicles also presents varying levels of protection. The front windshield is almost always made of laminated glass, which consists of a plastic interlayer sandwiched between two sheets of glass. This construction effectively blocks 98% to 99% of all UV radiation, including both UVA and UVB. Conversely, the side and rear windows are typically made of tempered glass, which allows a considerable amount of UVA to penetrate the interior, similar to standard home windows. This difference explains why drivers often show more sun damage on the side of their face and arm nearest the side window.
Determining When Indoor Protection is Necessary
Protection is necessary when there is a risk of prolonged, direct exposure to UVA rays through a window. If you spend several hours each day sitting within a few feet of an unshaded window, the cumulative exposure warrants daily protection. The same need for protection applies during daily commuting or long drives, especially for skin exposed through the side windows.
The most effective protection involves a broad-spectrum sunscreen with a Sun Protection Factor (SPF) of at least 30, which defends against both UVA and UVB. Since SPF primarily measures UVB protection, it is important to look for a product that also specifies UVA protection, often indicated by a PA rating (e.g., PA+++ or PA++++). Applying a broad-spectrum sunscreen in the morning to all exposed areas, like the face, neck, and hands, is a simple way to mitigate the indoor risk.
Non-sunscreen alternatives can also significantly reduce indoor UV exposure.
Non-Sunscreen Alternatives
- Installing a clear, UV-blocking film on existing windows is an effective solution that can block up to 99% of UV radiation.
- Adjusting window coverings, such as closing blinds or shades, particularly during peak daylight hours.
- Repositioning furniture to move seating areas away from the direct path of sunlight streaming through a window.