Sunlight has been used therapeutically for various dermatological disorders from time immemorial. Knowledge of flux of ultraviolet (UV) radiation is essential for phototherapy and to advise on protection against harmful effects of UV radiation.

In our country (8º-33ºN), sunlight is a rich source of UV light and it shines brightly for at least 8 months in a year and hence it could be utilized with advantage for treatment of vitiligo, psoriasis and other dermatological disorders. Data on UVA irradiance are limited. Knowledge of UVB irradiance is useful to optimize the conditions for phototherapy of skin diseases and to minimize the harmful effects of the Sun.

UVA and UVB irradiance varies with the time of the day, season of the year, place and atmospheric conditions. The first three factors determine the solar zenith angle and hence the path-length of the radiation through the atmospheric ozone and other absorbers. Therefore, the UV irradiance is highest in the tropics, in the summer at noon. However, for a given location, the most important factors that affect the UV irradiance are ozone, clouds and aerosols. The effect of ozone depletion is negligible at lower (between 30ºN and 30ºS) latitudes, including India. Clouds have a variable effect on UV irradiance; depending upon the height and amount of clouds, the reduction in irradiance can vary from 7% to as much as 87% compared to a clear sky. Aerosols can also affect UV irradiance with reductions exceeding 50% in the presence of dust and smoke plumes and sulfates and other industrial pollutants absorb UV only weakly and reduce the surface irradiance by 10%- 20%.

UVA irradiance is more than 50% of the peak irradiance between 9.15 a.m. and 3.30 p.m. and the peak irradiance occurs between 12.30 p.m. and 12.45 p.m. and maximum UVA irradiance is 7.14 mw/cm². UVB irradiance is more than 50% between 10.15 a.m. and 2.45 p.m. with peak irradiance occurring between 12.30 p.m. and 12.45 p.m. and the maximum UVB irradiance recorded is 40.2 µw/cm². The peak UVB irradiance is lower up to 27%, 46%, 24% and 17% in the months of November, December, January and February respectively compared to the months of March to October. The peak UVA irradiance is lower up to 15%, 30%, 21% and 13% in the months of November, December, January and February respectively compared to the months of March to October. The seasonal variation is significant in Northern India with the irradiance being 4-6 times higher in the months of April to September compared to that in November, December and January. In Southern India, UV irradiance is lower by 43% in the month of December.

Globally, a parameter of UV Index is used to provide comprehensive information on the amount of erythemally effective UV irradiance. UV Index provides a convenient measure of expressing changes in UV irradiance by a single number (scalar 1-20). It provides an idea of variation of erythemally effective UV irradiance at different locations in India at different months. UVI of greater than 10 prevails in Southern India between February and October and in Northern India between April and September.

Based on these data it can be concluded that sunlight is an inexpensive source of UV radiation in India.

Every process necessary for life on earth is powered by the tiny fraction of solar energy that reaches earth’s surface, known as electromagnetic radiation. The electromagnetic spectrum ranges from very short wavelength gamma radiation through x-rays, ultra violet (UV), visible, infrared to radio waves.

Solar radiation outside the earth’s atmosphere is made of the entire spectrum. Our atmosphere (stratosphere) is very effective in absorbing and scattering much of this radiation. Essentially no radiation of wavelength shorter than 290nm reaches the earth’s surface. The atmosphere, especially the ozone layer, also absorbs a considerable quantity of the sun’s UVB energy so that the final make-up of UV radiation on earth is 94% UVA and 6% UVB with majority of total radiation comprising of visible and infrared radiation. Visible radiation and near-infrared radiation are implicated in raising the temperature of the skin. The UV radiation (UVR) is of special interest for the dermatologists.

Ultraviolet radiation spans the wavelength region from 400 to 100nm. It is divided into UVC (100-280), UVB (280-320) and UVA (320-400). Wavelength between 200 and 280 of UVC is absorbed by the DNA, RNA and cell proteins as well as the stratum corneum and can be lethal to the viable cells of the epidermis. Because of its germicidal action, it is often called germicidal radiation. UVB causes sunburn and is often referred to as sunburn spectrum. UVA is referred to as black light because it is not visible to the eye and causes certain substances to emit visible fluorescence.

The biological effects of UV radiation vary enormously with the wavelength. Photobiological response in the skin includes sunburn, pigmentation, aging and cancer.

Outside the earth’s atmosphere solar UV radiation is remarkably constant, varying less than 1%. Surface irradiance is affected, however, by many factors: solar zenith angle (varying with the latitude, time of the year and time of day), ozone, clouds and aerosols, altitude, ground reflection and scattering by air molecules (Rayleigh scattering).

The variation in UV irradiance with the time of the day, seasons and the latitude is due to changes in the elevation of the sun above the horizon. Solar Zenith Angle (SZA) is often used in the place of solar elevation. It is the angle between the sun and the local vertical. SZA determines the path-length the radiation has to travel through the atmospheric absorbers (ozone) and scatterers before reaching the earth.

Higher the elevation (SZA=zero, at noon, in summer, near the equator), shorter the path-length, lesser the attenuation and therefore higher the irradiance. With increase in the solar zenith angle (at dawn and dusk, during winter and higher latitudes), the path-length increases, resulting in greater attenuation and lesser irradiance (Fig. 1). Shorter the wavelength, greater the attenuation and therefore, variation in SZA affects UVB more than UVA. Fig. 1: Solar Zenith Angle–Influence on the path-length

The SZA also determines the dose per unit area; increasing angle spreads the same amount of energy over a larger area and thereby reduces the dose per unit area.

Ozone is by far the most important absorber of UVR. It is produced naturally in the stratosphere by reaction between oxygen and UVR of lesser than 240nm. It is mainly produced over the tropics due to higher levels of solar radiation and large scale weather transports it to higher latitudes during winter.

All of UVC and most of UVB radiation is absorbed by stratospheric ozone. With ozone depletion, UV irradiance, particularly UVB, increases at the earth’s surface. Ozone depletion is greater at higher latitudes towards the north and south poles and negligible at the lower latitudes between 35ºS and 35ºN. With a decrease of 1% in the ozone value, the irradiance increases by approximately 1.2%.

Effect of clouds on UVR is as variable as clouds themselves. Normally clouds attenuate UVA and UVB to the same extent. In certain conditions and for short times a small amount of cloud may even enhance UV irradiance compared to the fully clear skies.

The effect of clouds depends upon the height, thickness, amount of clouds and in-cloud absorbers (aerosols). For overcast skies, with cloud ceiling above 7kms (high level clouds), the attenuation is estimated to be only about 10%; between 3-7kms (mid-level clouds), attenuation is up to 50% and for lower level clouds (stratus clouds), attenuation is up to 80%. If the irradiance for clear skies is known, the irradiance in the presence of clouds can be calculated or estimated by multiplying the irradiance with the cloud modification factor (CMF) which ranges from 0.2 to 1 depending upon the height and the amount of the cloud.

The CMF for different levels and amount of clouds is as follows:

Aerosols:

Aerosols are tiny particles suspended in the air that can scatter or absorb UVR. The effect of aerosols is independent of wavelength. Dust and smoke plumes can attenuate more than 50% of UVR. Sulfates and other industrial pollutants reduce UVR by 10-20% and this factor is important near industrial areas.

Aerosols also have an indirect effect on the climate by changing the properties of the clouds. Clouds with high aerosol content have small water droplets and scatter more UVR leading to greater attenuation. On the other hand, clouds with low aerosol content have large water droplets and therefore scatter less and allow much of the sunlight to pass through and reach the surface.

UV irradiance increases with altitude because the amount of absorbers and scatterers in the overlying atmosphere decreases. An increase in altitude by 1000m increases UV irradiance by 6-8%. Therefore places at higher altitude receive more UVR.

Reflection of sunlight from the terrain is of little importance, except from snow. Fresh snow may reflect 80% or more and may expose parts that are normally shaded; this is important at higher latitudes and altitudes. Sand reflects about 25%, so that sitting on the beach can lead to sunburn. Water reflects only 5%, but transmits up to 40%; so swimming in sea or pool does not prevent UVR.

This depends strongly on the wavelength and light of shorter wavelength is scattered more. UVR, particularly UVB, is scattered so much that at the surface of the earth the UVB is roughly composed of 1:1 mixture of direct and diffuse radiation (Sky radiation).

Adequate amount of UV radiation is present in India (8º4'-37º6'N) for at least 10 months a year. Treatment of various dermatologic disorders using sunlight requires a thorough knowledge of the UVA and UVB irradiance in a given area and given time to optimize the dosage and minimize adverse effects.

Diurnal variation: Measurement of UVR for one year at Coimbatore has shown that both UVB and UVA irradiance increase progressively from 8 a.m. until its peak value by mid-day (between 12 noon and 12.45 p.m.) and then declines (Fig. 2). 

More than 50% of the peak irradiance of UVA is present between 9.15a.m.-3.30p.m. and that of UVB between 10a.m.-2.45p.m. UVA irradiance is one fourth to one fifth of the peak irradiance at 8a.m. and 5p.m., where as UVB is less than one tenth of the peak in the early hours of morning and late afternoon. The mean peak UVA irradiance ranges from 4.7mw/cm2 to 6.59mw/cm2 and UVB irradiance ranges from 19.5µw/cm2 to 33.84µw/cm2 in different months, both being lowest in the month of December (Fig. 3). Fig. 2: Diurnal variation of UVA and UVB

Seasonal variation: The UVB irradiance increases in March-April, without a corresponding rise in UVA irradiance. In the month of December, the UVB irradiance decreases by 46%, while the UVA irradiance decreases by 30%. Thus UVB irradiance shows a significant seasonal variation compared to UVA irradiance (Fig. 3). Fig. 3: Seasonal variation of UVA and UVB

Seasonal variations are more pronounced at higher latitudes; in Northern India, the irradiance in winter is one third to one fourth of that in the months of April to September (Fig. 4). Fig. 4: Variation of UV irradiance at different latitudes in India UVB is lower by 55% at 48ºN (Paris) and by 15% at 28ºN (Delhi) compared to that at 11ºN (Coimbatore). On the other hand, UVA is lower by just 19% at 48ºN. Thus the UV irradiance, particularly UVB, decreases as one moves northwards from the equator.

Ozone, clouds and aerosols: For a given location and time, the important factors that affect UVR are ozone, clouds and aerosols. Ozone depletion is negligible over India. The attenuation of UV irradiance due to clouds varies from a minimum of 7% to maximum of 87% and can last from as short as 15 minutes to as long as whole day depending on the height, thickness and amount of cloud (Fig. 5). Fig. 5: Effect of clouds on UVA and UVB

		UVA irradiance recorded in an industrial locality of Mumbai is 24-50% lesser compared to that recorded in Coimbatore (Fig.6), attributable to pollution. Fig 6: UVA Irradiance in Mumbai and Coimbatore

The variations in UV irradiance influence the duration of exposure to sunlight for UVA and UVB phototherapy. For UVB phototherapy, the ideal time is 11a.m.-2 p.m. PUVASOL during this time leads to unwanted exposure to UVB; therefore the ideal time for PUVASOL is 9.30a.m.-11a.m. and 2p.m.-3.30p.m.

Table 2: Duration of exposure for UVB phototherapy

Table 3: Duration of exposure for PUVASOL

The UVA and UVB irradiance being lower in the months of November-January, the duration of exposure will be correspondingly longer. Phototherapy is possible even in the presence of clouds.

UV Index is a parameter used to provide comprehensive information on the amount of erythemally effective UV irradiance. It is the integral of the UV irradiance weighted by the CIE Erythemal Action Spectrum. It provides a convenient way of expressing changes in the UV irradiance by a single number (scalar, from 1-20) and UVI of 1 unit = 2.5µw/cm² (25mW/m²). It is usually given for local solar noon when the sun is highest in the sky and for clear sky conditions. 

UV Index is obtained by using the formula

where El is solar spectral irradiance expressed in w/m² at wavelength l ; dl is the wavelength interval used in the summation, Ser (l )is the erythema reference action spectrum and ker is a constant equal to 40m²/w.

Table 4: UV Index for India (Source: GOME data)

UVI can be used to estimate the duration of exposure for UVB phototherapy and for advice on photoprotection.

		The graph (Fig. 7) shows the duration of exposure for Type IV, Type V and Type VI skin with MED of 45millijoules, 60millijoules and 90millijoules respectively. Figure 7: UV Index and duration of exposure for UVB

UV irradiance varies with the time of the day, season of the year and latitude and these variations affect UVB irradiance more than UVA irradiance. Seasonal variation is significant in Northern India compared to Southern India. Adequate UV irradiance is present for almost 10 months in a year in most parts of India. Effect of clouds is significant only with the overcast skies with low level clouds.

Ideal time for UVB phototherapy is between 11a.m.-2 p.m. and for PUVASOL between 9.30 a.m.-11a.m. and 2 p.m.-3.30 p.m. when the incidental UVB exposure and heat due to infrared are minimal.

Information on UV Index for different locations is available in weather forecasts and can be helpful as a guide on UV irradiance.

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