Abstract
Background/Aim: The angular distribution of solar radiance and its spectral characteristics is required for the determination of vitamin D3 production in humans. Materials and Methods: The vitamin D3 weighted exposure can be calculated by integrating the incident solar spectral radiance over all relevant parts of the human body. A novel instrument allowing simultaneous measurements of spectral radiance from more than 100 directions has been developed. A large solar simulator for controlled experiments is described. Results: In summer it is relatively easy to obtain sufficient vitamin D because sun exposure times are short. In winter solstice vitamin D3 cannot be obtained with realistic clothing even if the exposure were extended to all daylight hours. Conclusion: Improved and controlled experiments to determine vitamin D3 production are required to assess the positive effects of solar UV radiation and to assess its natural variability.
Ultraviolet radiation from the sun (1) causes a considerable global disease burden including acute and chronic health effects on the skin, eye and immune system. On the other hand, UV is essential for vitamin D3 production in humans (1, 2). Emerging evidence suggests an association between vitamin D levels as an indicator of health risk (2) related to some cancers, multiple sclerosis among others, along with the established link with musculoskeletal health. The causal relationship between numerous diseases and vitamin D3 has been shown in a recent study (3). In the following vitamin D is used as a general term whereas we use the expression vitamin D3 to describe UV related issues.
Vitamin D synthesis in the human skin due to solar UVB (280-315 nm) radiation is the main source of vitamin D for humans, whereas dietary intake contributes only a small percentage (10%) to the necessary supply (4), at least according to current knowledge. Although vitamin D can be effectively produced by UVB radiation, there are large seasonal differences in its production (5, 6). As a result more than 50% of the German population has an insufficient vitamin D supply, especially during winter (7). This finding has been recently re-confirmed in a large group of patients (8).
The diffuse irradiance on a horizontal surface, that is used in most studies to date, does not take into account the complex geometry of the radiation field of the sky for different meteorological conditions. Therefore, our goal was to calculate the vitamin D3 weighted exposure of a human, represented by a 3D voxel model (9), using radiative transfer models of the sky radiance. In addition sky radiance can now be measured sufficiently fast by a newly-developed multi-directional spectroradiometer (MUDIS) (10). In combination with well-designed solar simulators (11) which have been developed for plant research already 30 years ago, these measurements and calculations are capable to trigger new insights into the production of vitamin D of humans.
Materials and Methods
Controlled environmental chambers had been developed and constructed already since 25 years ago for plant research (11). The major achievements were:
Irradiance up to 1,100 W/m2
Spectral distribution similar to sunlight
Diurnal variation of sunlight
Spatial inhomogeneity <10%
Reasonable operational costs
A comparison of the natural and simulated irradiance can be found in Figure 1.
In contrast to the simplification to using just irradiance to describe solar radiation, in reality, outdoor radiation is more complex and best described by the physical quantity “spectral radiance” (9). The spectral radiance is a complex function that depends on time of the day, day of the year, latitude, longitude, height above sea level, wavelength, ozone column and ozone profile, aeresols, clouds cover, cloud type, cloud distribution in the sky, air pressure, humidity and is a function of the zenith as well as the azimuth angles. Typical distributions of the sky radiance for cloudless skies are shown in Figure 2.
Clouds complicate the picture significantly, as can be concluded from Figure 3. In recent decades spectral sky radiance has been measured by scanning instruments (12). For accurate measurements both the wavelength spectrum and the sky distribution need to be measured. Since the necessary time to complete one wavelength scan could be up to 30 min with (fast) scanning instruments, it takes a day or more to complete one measurement of spectral sky radiance. Clouds, however, can move quite quickly and therefore can change the radiation field within seconds (13). Therefore we developed a novel instrument that is capable of measuring sky radiance in different directions simultaneously (10). With such an instrument (Figure 4) it is possible to acquire a spectrum from more than 100 directions (Figure 5) within one second. It is, therefore, well-suited to provide the necessary information on spectral sky radiance, that is required to measure the Vitamin D3 weighted exposure of humans.
To calculate a biologically-weighted exposure, the weighted radiance distribution of the sky and the geometry of the human are needed as input. The radiance distribution can be simulated with a radiative transfer model (Figure 1) or it is measured with a spectroradiometer like MUDIS mentioned before and seen in Figures 4 and 5. The procedure of weighting can be recognized in Figure 7. The geometry of a human is taken into account by using projectionareas (projections) of a 3D-Modell of a human as can be seen in Figures 6 and 8. With different clothing the projections of the human can vary and therefore can represent different atmospheric conditions and human behavior (Figure 6).
The weighted exposure of a human is calculated by multiplying the weighted radiance distribution with the projections of the human (Figure 7) and integrating over the whole hemisphere (9).
Results
With the assumption that an unclothed human can produce 1,000 IU Vitamin within just 1 min for an UV index of 10 and assuming linearity it has been possible to estimate the following exposure times for a healthy Vitamin status (1,000 IU per day).
Difficulties and uncertainties
To estimate required exposure times for synthesizing vitamin D several assumptions were made. In reality none of these assumptions are probably strictly fulfilled and at least the following assumptions need to be checked:
The exposure from solar radiation of a human in vertical posture experiencing a UVI of 10 is equivalent to 1,000 IU per minute.
Different parts of the human skin may have a different spectral transmission and a different sensitivity towards the incoming energy.
The weighting function for vitamin D production is correctly described by the CIE weighting function.
The accumulated vitamin D increases linearly with exposure.
The impact of reflected radiation from the ground due to higher albedo is negligible.
Further difficulties are the inclusion of specific obstructions (e.g. houses, trees) for a realistic outdoor exposure.
Conclusion
We were able to estimate the time required to achieve 1,000 IU per day (16), the assumptions that lead to these exposure times are still quite uncertain.
This is particularly true for the statement that a person can synthesize 1,000 IU in one minute with an UVI of 10 (16-18). Therefore our exposure times should be treated with caution.
Achieving a higher accuracy experiments under controlled conditions would be desirable. The phytochambers described above are still operational. However, they would need to be characterized with respect to spectral radiance. In addition new technologies that did not exist in the 1980s could be applied, such as radiation from LEDs. Meanwhile, there are several UV LED commercially available from different manufacturers (e.g. 19). With a combination of different LEDs it may be even possible to develop an improved solar simulator.
- Received January 15, 2016.
- Revision received February 16, 2016.
- Accepted February 18, 2016.
- Copyright© 2016 International Institute of Anticancer Research (Dr. John G. Delinassios), All rights reserved