Sunlight reaches us directly or indirectly. Indirect sunlight is light that has been scattered or reflected before it reaches an observer. A common misunderstanding suggests indirect sunlight is free from ultraviolet (UV) rays, implying complete safety in shaded areas or on cloudy days. However, indirect sunlight does contain UV radiation, which can still affect human health.
The Nature of UV Radiation
Ultraviolet (UV) radiation is a form of electromagnetic energy emitted by the sun, invisible to the human eye. This radiation is categorized into three main types based on wavelength: UVA, UVB, and UVC. Each type has different energy levels and abilities to penetrate the Earth’s atmosphere and human skin.
UVA rays have the longest wavelengths, making up approximately 95% of UV radiation reaching Earth. They penetrate deeply into the skin, contributing to premature aging and some skin cancers. UVB rays have shorter wavelengths, comprising about 5% of ground-level UV. They are primarily responsible for sunburn and are strongly linked to most skin cancers. The shortest and most energetic, UVC rays, are almost entirely absorbed by the Earth’s ozone layer and atmosphere, preventing them from reaching the surface.
Indirect Sunlight and UV Exposure
Ultraviolet radiation remains a concern even in indirect sunlight due to scattering and reflection. UV rays can scatter off atmospheric particles like clouds and dust, and reflect off various surfaces, reaching areas not in direct view of the sun.
On cloudy days, a common misconception is that UV exposure is minimal. While clouds can filter some UV radiation, light or thin clouds may even increase UV levels due to scattering, allowing substantial UV to reach the ground. Being in the shade reduces direct UV exposure but does not eliminate it. UV rays can reflect off surrounding surfaces like buildings, sand, water, and snow, reaching individuals even under an umbrella or tree. Fresh snow, for instance, can nearly double a person’s UV exposure due to its reflective properties.
Indoors, standard window glass offers some protection, but not complete. Most conventional window glass effectively blocks UVB rays. However, a significant portion of UVA rays can still pass through glass. Prolonged indoor exposure can therefore contribute to skin aging and other long-term damage from UVA radiation.
Protecting Yourself from Indirect UV
Protective measures are important because indirect sunlight still carries UV radiation. Sunscreen application should extend beyond just direct sun exposure. Using broad-spectrum sunscreen, which protects against both UVA and UVB rays, is advisable even on cloudy days or in shaded environments.
Wearing protective clothing, such as long-sleeved shirts and pants made from tightly woven fabrics, offers a physical barrier against UV radiation. Sunglasses that block 99% or 100% of UVA and UVB rays are also important for eye protection. UV-filtering window films for homes and cars can help reduce UVA penetration.
While seeking shade is beneficial, combine it with other protective measures. Be mindful of peak UV hours, typically between 10 AM and 4 PM, even in the shade, as UV intensity is higher then. These approaches ensure protection against UV radiation from both direct and indirect sources.