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UV index

UV index

Overview
The ultraviolet index or UV Index is an international standard measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 of the strength of the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 (UV) radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 from the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 at a particular place on a particular day. It is a scale primarily used in daily forecasts aimed at the general public.

Its purpose is to help people to effectively protect themselves from UV light, of which excessive exposure causes sunburn
Sunburn
A sunburn is a burn to living tissue, such as skin, which is produced by overexposure to ultraviolet radiation, commonly from the sun's rays. Usual mild symptoms in humans and other animals include red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV...

s, eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 damage such as cataract
Cataract
A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...

s, skin aging, and skin cancer
Skin cancer
Skin neoplasms are skin growths with differing causes and varying degrees of malignancy. The three most common malignant skin cancers are basal cell cancer, squamous cell cancer, and melanoma, each of which is named after the type of skin cell from which it arises...

 (see the section health effects of ultraviolet light).
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Encyclopedia
The ultraviolet index or UV Index is an international standard measurement
Measurement
Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 of the strength of the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 (UV) radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 from the sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 at a particular place on a particular day. It is a scale primarily used in daily forecasts aimed at the general public.

Its purpose is to help people to effectively protect themselves from UV light, of which excessive exposure causes sunburn
Sunburn
A sunburn is a burn to living tissue, such as skin, which is produced by overexposure to ultraviolet radiation, commonly from the sun's rays. Usual mild symptoms in humans and other animals include red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV...

s, eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

 damage such as cataract
Cataract
A cataract is a clouding that develops in the crystalline lens of the eye or in its envelope, varying in degree from slight to complete opacity and obstructing the passage of light...

s, skin aging, and skin cancer
Skin cancer
Skin neoplasms are skin growths with differing causes and varying degrees of malignancy. The three most common malignant skin cancers are basal cell cancer, squamous cell cancer, and melanoma, each of which is named after the type of skin cell from which it arises...

 (see the section health effects of ultraviolet light). Public-health organizations recommend that people protect themselves (for example, by applying sunscreen
Sunscreen
Sunblock is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn...

 to the skin and wearing a hat) when the UV index is 3 or higher; see the table below for complete recommendations.

Description


The UV index is a linear scale, with higher values representing the risk level of skin damage due to UV exposure. An index of 0 corresponds to zero UV irradiation, as is essentially the case at nighttime. While an index of 10 corresponds roughly to midday sun and a clear sky, indices greater than 11 are quite common in the southern hemisphere where the Ozone layer is depleted
Ozone depletion
Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere , and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon...

. The numbers are related to the amount of UV radiation reaching the surface of the earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, measured in W
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

/m2
Square metre
The square metre or square meter is the SI derived unit of area, with symbol m2 . It is defined as the area of a square whose sides measure exactly one metre...

, but the relationship is not simple (see below for definition) because UV power is spread unevenly over a spectrum of wavelengths; the shorter wavelengths are much more damaging. By weighting
Weighting
The process of weighting involves emphasizing the contribution of some aspects of a phenomenon to a final effect or result — giving them 'more weight' in the analysis. That is, rather than each variable in the data contributing equally to the final result, some data are adjusted to contribute...

 their effect the index aims to create a number that cannot be expressed in physical units, but is a good indication of likely skin damage.

While the UV index can be calculated from a direct measurement of the UV spectral power at a given place, the value given in weather forecasts is usually a prediction based on a computer model. While this may be in error, as with all predictions, it is essentially the same index as we would expect to measure.

The UV index announced in weather forecasts
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...

 is a prediction of how strong the actual UV intensity will be at the sun's highest point in the day, which typically occurs during the four-hour period surrounding solar noon. The prediction is made by a computer model that accounts for the effects of sun altitude
Solar azimuth angle
The solar azimuth angle is the azimuth angle of the sun. It is most often defined as the angle from due north in a clockwise direction.It can be calculated in various way. In different times, it has been explained in different ways. It can be calculated, to a good approximation, using the following...

, air mass, and inclement weather (clouds), which increase or decrease, the amount of UV radiation that will reach the surface. The calculations are weighted in favor of the UV wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

s to which human skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

 is most sensitive according to the McKinlay-Diffey Erythema action spectrum. Providing the public with an easy-to-understand daily forecast of UV intensity is the main purpose of the UV index.

Some inexpensive portable devices also measure actual UV index.

Definition


The UV index is a number linearly related to the intensity
Intensity (physics)
In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...

 of UV radiation reaching the surface of the earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 at a given point. It cannot be simply related to the irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...

 (measured in W
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

/m2
Square metre
The square metre or square meter is the SI derived unit of area, with symbol m2 . It is defined as the area of a square whose sides measure exactly one metre...

) because the UV of concern occupies a spectrum of wavelength from 295 to 325 nm and shorter wavelengths have already been absorbed a great deal when they arrive at the Earth's surface. Skin damage, however, is related to wavelength, the shorter wavelengths being much more significant. The UV power spectrum (strictly expressed in watts per square metre per nanometre of wavelength) is therefore weighted according to a weighting curve known as the McKinlay-Diffey Erythema action spectrum, and the result integrated over the whole spectrum. This typically gives a figure of around 250 in mid-day sun and so is arbitrarily divided by 25 to generate a convenient index value, which becomes essentially a scale of 0 to 10 (though ozone depletion is now resulting in values above ten as commented above). Because the scale is linear and not logarithmic, as is often the case when measuring things such as sound level or brightness, it is reasonable to assume that one hour of exposure at index ten is approximately equivalent to two hours at index 5, although other factors like the body's ability to repair damage over a given time period could detract from the validity of this assumption.

To illustrate the weighting principle, the incident power density in mid-day sun is typically 0.6 mW/(nm m2) at 295 nm, 74 mW/(nm m2) at 305 nm and 478 mW/(nm m2) at 325 nm. (Note the huge absorption that has already taken place in the atmosphere at short wavelengths.)
The weighting factors applied to these figures are 1.0, 0.22, and 0.03 respectively. (Also note the huge increase in damage caused by the shorter wavelength, i.e., 305 nm is 22% as damaging as 295 nm, and 325 nm is 3% as damaging as 295 nm.) Integration of these values using all the intermediate weighting values over the spectral range of 305 nm to 325 nm produces a figure of 264, which is then divided by 25 to give an index of 10.6.

History


In 1992, three scientist
Scientist
A scientist in a broad sense is one engaging in a systematic activity to acquire knowledge. In a more restricted sense, a scientist is an individual who uses the scientific method. The person may be an expert in one or more areas of science. This article focuses on the more restricted use of the word...

s from Environment Canada developed the UV index, and made Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 the first country in the world to broadcast forecasts of the predicted daily UV levels for the next day. Several other countries followed suit with their own UV indices, among them the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 in 1994. Until recently, the methods of calculating and reporting a UV index varied from country to country. Today, a worldwide UV index, standardized by the World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 (WHO), has replaced the inconsistent regional methods. The international UV index not only specifies a uniform calculation method but also standard colors and graphics for printed media. In the USA, the WHO index replaced the original US index in 2004. In 2005, the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 launched the UV Alert. While the two countries have different baseline UV intensity requirements before issuing an alert, the goal is to raise awareness of the dangers of overexposure to the sun on days with intense UV radiation.

How to use the index


Recommendations for protection when the day's predicted UV index is at various values are:
UV Index Description Media Graphic Color Recommended Protection
0–2 No danger to the average person Green
Green
Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...

 
Wear sunglasses
Sunglasses
Sunglasses or sun glasses are a form of protective eyewear designed primarily to prevent bright sunlight and high-energy visible light from damaging or discomforting the eyes. They can sometimes also function as a visual aid, as variously termed spectacles or glasses exist, featuring lenses that...

 on bright days; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin.
3–5 Little risk of harm from unprotected sun exposure Yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...

 
Wear sunglasses and use SPF 30+ sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense.
6–7 High risk of harm from unprotected sun exposure Orange
Orange (colour)
The colour orange occurs between red and yellow in the visible spectrum at a wavelength of about 585–620 nm, and has a hue of 30° in HSV colour space. It is numerically halfway between red and yellow in a gamma-compressed RGB colour space, the expression of which is the RGB colour wheel. The...

 
Wear sunglasses and use SPF 30+ sunscreen, cover the body with sun protective clothing
Sun protective clothing
Sun protective clothing is clothing specifically designed for sun protection and is produced from a fabric rated for its level of ultraviolet protection. A novel weave structure and denier may produce sun protective properties...

 and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 11:00 AM to 4:00 PM during summer in zones that observe daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

).
8–10 Very high risk of harm from unprotected sun exposure Red
Red
Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 630–740 nm. Longer wavelengths than this are called infrared , and cannot be seen by the naked eye...

 
Wear SPF 30+ sunscreen, a shirt, sunglasses, and a hat. Do not stay out in the sun for too long.
11+ Extreme risk of harm from unprotected sun exposure Violet
Violet (color)
As the name of a color, violet is synonymous with a bluish purple, when the word "purple" is used in the common English language sense of any color between blue and red, not including either blue or red...

 
Take all precautions, including: wear sunglasses and use SPF 30+ sunscreen, cover the body with a long-sleeve shirt and trousers
Trousers
Trousers are an item of clothing worn on the lower part of the body from the waist to the ankles, covering both legs separately...

, wear a very broad hat, and avoid the sun from two hours before to three hours after solar noon (roughly 11:00 AM to 4:00 PM during summer in zones that observe daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

).


Cautionary notes


Be aware when interpreting the UV index and recommendations that:
  1. The intensity of UV radiation reaching the surface of the earth varies depending on how high the angle of the sun is in the sky. The sun reaches its highest angle at solar noon, which rarely corresponds to 12:00 on clocks. This is because of the differences between solar time
    Solar time
    Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

     and local time in a given time zone
    Time zone
    A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

    .
  2. The recommendations given are for adults with pale to lightly tan skin. Children and particularly fair-skinned people or those who have sun sensitivity for medical reasons need to take extra precautions.
  3. Damage from sun exposure is cumulative over one's lifetime. Cumulative exposure to the sun imparts damage to the epidermis (the outer layer) and the dermis
    Dermis
    The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...

    , (the deeper layer where the skin's framework exists). Damage to the dermal layer changes the structural components, causing elastin fibers to thicken and become more numerous. Collagen
    Collagen
    Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

     is damaged and degraded and reticulin fibers appear throughout the dermis rather than outlining the specific dermal-epidermal junction.

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