Ultraviolet (UV) Radiation

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How are people exposed to UV radiation?


Sunlight is the main source of UV radiation, even though UV rays make up only a small portion of the sun’s rays. About 95% of the UV radiation from the sun that reaches the earth is UVA, with the remaining 5% being UVB. The amount of UV radiation you may be exposed to at any point depends on a number of factors, such as:

  • Time of day: Almost a third of the day’s UV rays from the sun comes down between 11AM and 1PM, with three-quarters between 9AM and 5PM.
  • Season of the year: UV rays are strongest during summer months. This is less of a factor near the equator.
  • Distance from the equator (latitude): The amount of UV exposure per year goes down as you get further from the equator.
  • Altitude: People burn more easily at higher elevation because more UV rays get through.
  • Clouds: The effect of clouds can vary – sometimes cloud cover blocks some UV from the sun and lowers UV exposure, while some types of clouds can reflect UV and so can increase UV exposure. What is important to know is that UV can get through, even on a cloudy day.
  • Reflection off surfaces: UV rays can bounce off surfaces like water, sand, snow, or grass, leading to an increase in UV exposure.
  • Contents of the air: Ozone in the upper atmosphere, for example, filters out some UV radiation.

Man-made sources of UV rays

Man-made sources of UV rays can also be important. These include:

  • Sunlamps and sunbeds (tanning beds and booths): The amount and type of UV radiation someone is exposed to from a tanning bed (or booth) depends on the specific lamps used in the bed, how long a person stays in the bed, and how many times the person uses it. Most modern UV tanning beds emit mostly (more than 95%) UVA rays, with the rest being UVB.
  • Phototherapy (UV therapy): Some skin problems (psoriasis, for example) can be treated with UV light. For a treatment known as PUVA, a drug called a psoralen is given first. The drug collects in the skin and makes it more sensitive to UV. Then the patient is treated with UVA radiation. Another treatment option is the use of UVB alone (without a drug).
  • Black-light lamps: These lamps use bulbs that emit UV. The bulb may also make some visible light, but it is made with a filter that blocks most of that out, letting through UV rays (mostly UVA). They have a purple glow and are used to view fluorescent material. Bug zapping insect traps also use “black light,” but the bulbs use a different filter that causes them to glow blue.
  • Mercury-vapor lamps: Mercury vapor lamps can be used to light large public areas such as streets or gyms. They do not expose people to UV rays if they are working properly. They are actually made up of 2 bulbs - an inner bulb that emits light and UV rays, and an outer bulb that filters out the UV. UV exposure can only occur if the outer bulb is broken. Some mercury vapor lamps are designed to turn themselves off when the outer bulb breaks. The ones that don’t have this feature are only supposed to be installed behind a protective layer or in areas where people wouldn’t be exposed if part of the bulb breaks.
  • High-pressure xenon and xenon-mercury arc lamps, plasma torches, and welding arcs: Xenon and xenon-mercury arc lamps are used as sources of light and UV rays for many things, such as UV “curing” (of inks, coatings, etc), video projection, fiber optics, disinfection, to simulate sunlight (to test solar panels, for example), and even in some car headlights. Most of these, along with plasma torches and welding arcs, are mainly of concern in terms of workplace UV exposure.

Last Medical Review: 07/02/2013
Last Revised: 05/30/2014