Ultraviolet (UV) light is a form of electromagnetic radiation that falls between X-rays and visible light on the spectrum. It is the primary agent responsible for tanning the skin, whether from the sun or specialized indoor equipment. The UV light used in indoor tanning devices is intentionally manipulated to achieve a cosmetic effect by focusing on specific wavelengths. Understanding which parts of the UV spectrum are utilized clarifies how indoor tanning works and how it compares to natural sun exposure.
Defining the Three Types of UV Radiation
The ultraviolet spectrum is traditionally divided into three categories based on wavelength, which determines how deeply the radiation penetrates the skin. Ultraviolet A (UVA) has the longest wavelength, ranging from 315 to 400 nanometers (nm). UVA penetrates the skin the deepest, reaching past the outer layer (epidermis) into the dermis.
Ultraviolet B (UVB) has a medium wavelength, typically between 280 and 315 nm. UVB radiation is largely absorbed by the epidermis, the skin’s outermost layer. It is responsible for stimulating new melanin pigment production, triggering the redness associated with sunburn, and synthesizing Vitamin D.
Ultraviolet C (UVC) has the shortest and most energetic wavelength, falling between 100 and 280 nm. Virtually all UVC radiation from the sun is filtered out by the Earth’s ozone layer. Consequently, UVC light has no role in natural tanning and is typically only encountered in artificial sources like germicidal lamps, though modern tanning devices filter it out.
The Dominant UV Spectrum in Tanning Beds
Tanning beds are engineered to produce a UV output overwhelmingly dominated by UVA radiation. Lamps in most commercial beds emit a spectrum composed of 90% to 98% UVA light and a small, controlled percentage of UVB light. This ratio achieves the desired cosmetic outcome while reducing the immediate risk of a visible burn.
UVA light causes an immediate darkening of existing melanin pigment, allowing a tan to appear quickly during a session. This rapid bronzing effect is appealing because it provides a visible tan without the delay or intense redness caused by high doses of UVB. The small amount of UVB included stimulates the production of new melanin, leading to a tan that develops more gradually and lasts longer.
The design of the lamps, typically low-pressure fluorescent bulbs, incorporates filters to ensure that the shortest UVC wavelengths are blocked. This manipulation of the UV spectrum focuses energy on the wavelengths that produce a cosmetic tan. High-pressure tanning beds use smaller, higher-wattage quartz bulbs and may even emit over 99% UVA, further minimizing the UVB component.
Comparing Artificial UV Intensity to Natural Sunlight
The UV radiation from a tanning bed differs significantly from natural sunlight, not just in the UVA to UVB ratio, but also in overall intensity. Natural sunlight delivers a mix of about 95% UVA and 5% UVB, a spectral balance similar to the tanning bed output. However, a tanning bed concentrates this radiation, delivering a dose in a short session that is far more intense than midday sun.
Many commercial tanning beds emit UV radiation that is between three and six times stronger than the midday summer sun. Some high-pressure beds can emit UVA doses 10 to 15 times higher than the sun’s natural output. This concentration means a short tanning session can expose the skin to UV amounts that would take hours to accumulate outdoors.
The consequence of this unnaturally concentrated dose is the rapid, deep penetration of high-intensity UVA into the dermis. This intense exposure accelerates cellular damage linked to premature aging and increased risk of skin cancer, even without causing an immediate sunburn. The high UV index delivered by a tanning bed, which can be equivalent to 13, is significantly higher than a typical summer sun at noon.