The two main types of ultraviolet light are UVA and UVB. All tanning units use a combination of these two rays, which contribute differently to the tanning process. Though different units use
different ratios of UVA and UVB, each type is needed.
The epidermis, the skins outermost layer, is where tanning takes place. Exposure to UVB light forces melanocytes, located within the epidermis, to produce melanin, which causes the
skin to redden. Exposure to UVA light forces the melanin to oxidize and darken the skin to a deeper golden brown color. This is all just your skins natural defense mechanism for protecting
itself against UV ray overexposure.
Normally, the outer of layer of skin exfoliates itself about every 28 days, but when exposed to high amounts of UVB rays, the exfoliation process is accelerated to about 5-10 days. Therefore, the higher the UVB percentage, as found in low-pressure beds, the sooner your tan fades.
High Pressure beds use Quartz lamps, which are smaller, and produce very little UVB, as opposed to the fluorescent tube type bulbs found in low-pressure beds. Thanks to the low UVB amount, your skin keeps its color much longer, and tans much faster. Most low-pressure beds require about 10-12 sessions before you are considered having a "Base Tan", but it is approximately 3 sessions in a High Pressure Bed!