UVA and UVB in sunlight, Optimal Utilization of UV rays in Sunlight for phototherapy

Introduction

Sunlight is an inexpensive and natural source of ultraviolet light. In PUVASOL, the exposure time to sunlight is arbitrarily determined.[2] As the quantum of UV in sunlight varies from place to place at different times, the arbitary exposure may result in excessive or inadequate dose of UV light.[2] To overcome this, we recorded the UV radiation from sunlight from January 1999 to December 1999 between 8.00 a.m. to 5 p.m. and based on the irradiance recorded we recommend the exposure time to sunlight for phototherapy.

Materials and Methods

PMA (Personal Measurement Assistant) Model 2100 from Solar Light Co., Philadelphia, a multipurpose measuring instrument with advanced data logging capabilities, was used to record the irradiance. It is provided with 2 detectors, one each for UVA (PMA 2110) and UVB (PMA 2101). The information is stored in a nonvolatile memory built into the detector. UVA and UVB data was recorded from 8.00 a.m. to 5.00 p.m. twice a week from January to December. The data was transferred over a RS232 Ink into a structured database for organization into charts. The organized data was then transferred to Microsoft Excel spreadsheet for the following:

1. Anaysis of UVA and UVB irradiance with relation to time of the day, in different months. The mean values were calculated for all the months. After excluding the values affected by environmental factors, corrected mean irradiance was calculated for all the months.

2. Calculation of UVA: UVB Ratio using the following formula:

[(UVA at a given time/UVB at that time) x 1000]

3. Calculation of exposure time for 10 mJ of UVB and 1 Joule of UVA using the following formula:

Exposure Time= 1000xDose (in Joules) - 60 x Irradiance (mW/cm2)

4. Calculation of incidental UVB exposure during PUVASOL using the formula:

[(UVB at a given time/1000) x 60 x exposure time for 1 joule of UVA at that time]

Result

The UVA and UVB increased progressively from 8.00 a.m. to mid-day then declined [Figure - 1] and [Figure - 2]. The Uva and UVB were found to reach the peak between 11.30 a.m. and 1.30 p.m. The highest peak UVB irradiance recorded was 40.2µw/cm2 in April and the highest peak UVA irradiance recorded was 6.59 mW/cm2 in July [Table - 1]. The peak UVB irradiance varied between 19.50µw/cm2 in December to 40.2µw/cm2 in April and that of UVA between 4.70/cm2 in December to 6.59mw/cm2 in July. The ratio of UVA:UVB in the early hours of morning and late afternoon was 1:500 to 1:600 and at mid day was 1:200.

When using sunlight as the source for phototherapy, we deliver UVB in increments of 10mJ and UVA in increments of 1 joule. The duration of exposure to sunlight for 10mJ of UVB and for 1 joule of UVA based on the irradiance that we recorded are shown in the tables 2 and 3. The duration of exposure is longer in the months of November, December, January and February.

Incidental UVB exposure while delivering 1 joule of UVA at the peak time ranges from 4.73µw/cm2 to 5.19µw/cm2. It is lesser in the early hours of morning and late afternoon.

Discussion

The time of the day, the season of the year, and the observer′s latitude on earth′s surface alter the solar intensity and spectrum.[3],[4] All these factors are related to the height of the sun in the sky and hence the pathway of beam of sunlight through the atmosphere. At mid-day the sun would be almost directly overhead and therefore the pathway of sunlight through the atmosphere is half the length compared with the morning and evening [Figure:3]. Increase in the pathway through the atmosphere leads to attention of sunlight and shorter the wavelength, greater the attenuation.[3]

The Present study (at Coimbatore) shows that UVA radiation varies with time, being higher at mid-day and lower in morning and evening and these findings are similar to the previous study.[1] In addition, the present study shows that UVB is much lower than UVA in morning and evening as evidenced by the woder ratio. In a French study, in Paris (48 degrees North) the maximum UVB irradiance of 15 µw/cm2 and UVA of 5.4 mW/cm2 was observed at noon in early July.[5] At Coimbatore (11 degrees North and 77 degrees East) the maximum UVB irradiance recorded in July at noon was 32.5 µW/Cm2 and UVA was 6.42 mW/cm2. Compared to the months of March to October, UVB was lower by 24%, 40%, 19% and 12% in the months of November, December, January and February respectively and UVA was lower by 13%, 22%, 18% and 13% in the same months. At Isehara (139 degrees East and 35.5 degrees North) Japan, outdoor experiments throughout one year showed that mean amount of solar UV light in summer was 6 fold larger than in winter.[6] [Table - 2], [Table - 3]

To conclude

1. UVa and UVB irradiance and hence the duration of exposure for phototherapy vary with time, place and season.

2. Exposure time for 10mJ of UVB al peak time (11.30 am.-1.30 p.m.) is 4 min. 55 sec. to 11 min. 22 sec.

3. Exposure time for 1 joule of UVA at peak time (11.30 a.m.-1.30 p.m.) is 2 min. 32 sec. to 4 ruin. 31 sec.

4. Incidental UVB exposure is maximum at peak time and lower in the morning and evening.

5. Ideal time for PUVASOL will be therefore between 9.15 a.m.-11.15 a.m. and 2.30 p.m.- 3.30 p.m. which will minimize unwanted exposure to UVB and infrared radiation.

6. Duration of exposure is longer in the months of NOvember, December, January and February.