Solar UV Index

The solar UV Index (UVI) describes the level of solar UV radiation relevant to human sunburn (erythema). The values of the UV index extend from zero upwards – the higher the index value, the greater the potential for damage, and the less exposure time it takes for harm to occur. The UVI was originally used in Canada, and was defined so the maximum value in the South of the country was 10 at midday in the summer, and about 1 at midday in the winter. For fair skinned individuals, a UVI of 10 or more is usually considered as “extreme” because under those conditions, skin damage can occur after less than 15 minutes of exposure. The UVI scale is an open-ended scale. When the Sun is below the horizon the UVI is zero, and at its maximum on the Earth’s surface, for example in the Altiplano region of South America in summer, the UVI can exceed 25. In the tropics at sea level the UVI can exceed 16. Since the mid 1990s, information has been provided to the public about UV intensities in terms of the internationally adopted UVI scale along with appropriate health warnings. However, the UVI also depends strongly on the cloud cover. Other factors include the seasonally varying Sun- Earth separation, the altitude, atmospheric pollution, and surface reflection. When the surface is snow-covered, the UVI can be up to 90% greater than for snow-free surfaces.

Category

         UVI Range

         Low < 2

         Moderate 3 – 5

         High 6 – 7

         Very High 8 – 10

         Extreme 11+

  UV index vary with location and time

The combination of total ozone, aerosols, clouds, pollution, altitude, surface reflectivity and solar zenith angle (that is determined by the geographical position, season and time of the day) are the main factors resulting in variation in the UV Index. A global picture of the UVI can be derived by instruments on board satellites. They indicate that the index varies with the latitude and the time of the year, as can be seen from the two figures for September and March presented below. UV irradiation increases with altitude, and therefore the UVI is higher at mountain locations. The presence of “patchy clouds” or snow-covered ground can result in very large UV indices. A good example is the high altitude desert Puna of Atacama in Argentina, where a UV index of 18 is common in January and December, with a maximum of 20 and even more on occasional days. A combination of small solar zenith angle near noon, high altitude, a naturally low total ozone column and a very clean atmosphere cause these exceptionally high values.

 Effects of solar UV radiation exposure on the human eye 

The effects of UV radiation on the eye may be acute (occurring often after a short, intense exposure usually after a latent period of several hours) or long-term after an acute exposure. There are also long-term effects following chronic exposure of the eye to levels of UV radiation below those required for the acute effects. The commonest acute effect, photo-keratitis (snow blindness) leaves few or no permanent effects, whereas cataract due to chronic exposure is irreversible and ultimately leads to blindness. Avoidance of sun exposure is an effective but impractical means of avoiding exposure of the eyes to UV radiation. Despite these, additional protection is frequently needed under conditions of high ambient UV irradiation and/or reflective surfaces. Appropriate glass and plastic lenses absorb all UV-B and much of the incident UV-A Even clear spectacle lenses provide protection from UV-B. However, in the case of non-wrap around spectacles there is potential for ambient UV radiation to enter the eye from the side. This effect can be exacerbated by tinted sunglass lenses which encourage a wider opening of the eye. UV radiation-blocking soft contact lenses, that cover the entire cornea, effectively shield the cornea and ocular lens against UV radiation incident from all angles. They offer a UV protection alternative in those situations where the wearing of sunglasses is not practical or convenient.

Effects of solar UV-B exposure on the human skin 

Acute exposure of the skin to solar UV radiation causes sunburn and in the long term skin cancers The amount of UV radiation required to produce sunburn depends on the absorption in the superficial layers (varying with the amount of pigment) of the skin and on other genetic factors. The efficacy with which sunlight produces sunburn depends on the amount of UV-B radiation present; more UVB is present at high altitudes and more is present in noontime sun than at earlier of later hours. Chronic exposure of the skin to UV radiation causes photo ageing (including wrinkling, thinning, and loss of elasticity); however, UV-A may be more important than UV-B in causes these latter changes. Basal and squamous cell carcinomas occur most often and with high frequency in fair skinned individuals living in sunny climates. Fortunately, most of these skin cancers are readily treated and rarely fatal. Cutaneous melanoma is considerably more dangerous, but occurs with much lower frequency than the other types of skin cancer.

The relationship between melanoma and UV-B radiation is not well understood, but exposure in early life seems to be an important factor in its development. Dark skinned persons have natural protection in their skin against sunlight. Although melanoma does occur in darker skinned persons, such cancers are often not related to sun exposure. Methods of decreasing sun exposure of the skin include remaining indoors during the peak UV-B hours around midday and seeking shade at other times. Broad brimmed hats, sunglasses and full-body clothing that reduce the area of exposed skin are also effective. Sunscreens are designed to protect against sunburn and can be highly. There is evidence that they reduce the incidence of squamous cell carcinoma and pre-cancerous lesions in the skin. Obtaining a suntan will not usually help to prevent UV-B induced skin cancer and the UV radiation exposure needed to acquire the tan adds to the skin cancer risk.

KP/DT