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 rays from the sun that reach the earth are UVA rays, with the remaining 5% being UVB rays. The strength of the UV rays reaching the ground depends on a number of factors, such as:

  • Time of day: UV rays are strongest between 10 am and 4 pm.
  • Season of the year: UV rays are stronger during spring and summer months. This is less of a factor near the equator.
  • Distance from the equator (latitude): UV exposure goes down as you get farther from the equator.
  • Altitude: More UV rays reach the ground at higher elevations.
  • 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 can increase UV exposure. What’s important to know is that UV rays can get through, even on a cloudy day.
  • Reflection off surfaces: UV rays can bounce off surfaces like water, sand, snow, pavement, 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.

The amount of UV exposure a person gets depends on the strength of the rays, the length of time the skin is exposed, and whether the skin is protected with clothing or sunscreen.

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 UVA rays, with the rest being UVB.
  • Phototherapy (UV therapy): Some skin problems (such as psoriasis) are helped by treatment 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 give off UV rays (mostly UVA). The bulb also gives off some visible light, but it has a filter that blocks most of that out while letting the UV rays through. These bulbs have a purple glow and are used to view fluorescent material. Bug-zapping insect traps also use “black light” that gives off some UV rays, 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: 08/12/2015
Last Revised: 08/12/2015