Uv Index Vitamin D Production

Uv Index Vitamin D Production

Abstract

UV radiation contains erythemally weighted UV, as well as UV that synthesizes vitamin D₃. Here, we attempted to determine the relationship between these factors by numerical simulation of atmospheric parameters, such as total ozone, using a simplified "SMART2" model for radiative transfer. Both forms of UV were almost linearly correlated with each other for a comparably large UV radiation exposure, larger than UV Index ~1.6. If erythemally weighted UV, which carries a risk of sunburn, is known, the amount of UV exposure needed for vitamin D synthesis in the epidermis can be estimated using this relationship. The production of 10 μg (400 IU) of vitamin D per day takes approximately 1/3 of the time needed to reach the minimal erythemal dose (MED) for an effective skin area of 600 cm² for skin phototype III. For an area of 1200 cm², 1/6 of that exposure time suffices. From a UV Index that is commonly used, the risks and benefits can be evaluated using this linear relationship, which will enable people to effectively manage their UV exposure and consider the risks and benefits to optimize health outcomes.

Introduction

Since depletion of the ozone layer was recognized in the 1980s, many articles have cited the destruction of the ozone layer and the harmful effects of UV radiation. These reports have been commented on by the World Meteorological Organization (WMO) and the World Health Organization (WHO), as well as others 1-5. Some publications disseminated information about excessive exposure to UV radiation being hazardous to human health and showed links to skin cancer, cataracts, suppression of the immune system, etc., as well as providing information on how to avoid these. In many countries, organizations responsible for health and the environment relayed information about the harmful effects of UV through mass media via television programs and weather forecasts. In Japan, UV information is broadcast daily to help people avoid exposure, except during the winter months. These warnings advise people to avoid UV exposure and provide advice on protection. Prior to the time when the health risks of UV exposure were highlighted, as a result of fears of ozone depletion, many people believed that exposure to the sun and a tan was preferable for pale skin, and people often went out into the sun without protection. After recognizing ozone layer depletion, most Japanese people regard UV as a villain rather than it being healthy and have become very nervous about exposure. It is a fact that excessive exposure to UV is hazardous to human health, although many women avoid exposure for cosmetic reasons.

On the other hand, UV in solar radiation is actually needed to synthesize previtamin D in the epidermis (UV_VitD), which is metabolized to 25-hydroxyvitamin D, and then to 1α,25-dihydroxyvitamin D via the liver and kidneys 6. UV radiation plays a vital role in maintaining our health 7-13, especially in the prevention of diseases caused by deficiencies in serum vitamin D concentrations, and it has been reported that vitamin D can reduce the incidence of a number of cancers, such as rectal, breast, prostate and colon 14. However, there is no definitive concentration of vitamin D in the serum needed to maintain a healthy body, including that synthesized by UV in solar radiation. It has been proposed that 10–25 μg a day is needed 6, 15, but this figure differs according to the researchers and their respective countries. The Ministry of Health, Labour and Welfare of the Japanese Government has recommended a dietary intake of vitamin D for Japanese people of 5.5 μg per day for adults older than 18 years. For younger ages, an intake of 2.5–6.0 μg, with adequate serum parathyroid hormone, that is, those with 25-hydroxyvitamin D levels >20 ng mL⁻¹, will prevent vitamin D deficiency 16. However, the amount of vitamin D needed for a healthy body cannot be accurately estimated because of many unknown factors, including the duration of UV exposure in each season. It is therefore impossible to estimate the amount of vitamin D needed by the Japanese on a daily basis, including that synthesized by UV radiation. If the daily requirement for Japanese people is 15 μg, then a large discrepancy exists between the amount of vitamin D needed from dietary intake and the daily requirement. This vitamin D shortfall might be made up by UV exposure 17. It is much simpler to estimate dietary intake of vitamin D than the amount of the vitamin D synthesized in the epidermis.

According to a survey taken in Tokyo (Japan), 60% of 284 healthy pregnant Japanese women had severe vitamin D deficiency, with levels of 25-hydroxyvitamin D < 10 ng mL⁻¹ 18. One suggested reason for this was reduced exposure to sunlight. In Kyoto (Japan), there was a high incidence of craniotabes, which correlated with the month of birth 19. Thus, in Japan, it is believed that vitamin D deficiency is due to reduced UV exposure. As excessive exposure is hazardous, it is important to know the appropriate UV exposure time to obtain the amount of vitamin D needed. Some studies have aimed to estimate the optimal exposure time 20-22. Most have discussed the relationship between the time needed for effective exposure for vitamin D synthesis and that which would lead to a harmful dose of erythemally weighted UV (UV_Ery). To effectively inform people on how to manage their UV exposure, the amount of UV_Ery and UV_VitD, which is hazardous to humans minimal erythemal dose (MED), and the amount of vitamin D synthesized in the epidermis, must be known. This could be calculated if the UV spectra are known 23, which is possible by observing solar radiation levels. However, it is difficult to observe the UV spectra continually at any one location. In this study, we developed a numerical method to obtain UV_VitD from UV_Ery information, such as the UV Index, to yield appropriate exposure times.

The data needed for the analyses, such as UV spectra observed by Brewer (MkII) spectrophotometers, total ozone as measured by Dobson spectrophotometers and other meteorological parameters were obtained by the Japan Meteorological Agency at three observation sites in Japan: Sapporo, located in the northern region (43°04′ N, 141°20′ E, 26.3 m above sea level); Tsukuba, in central Japan (36°03′ N, 140°08′ E, 31.0 m); and Naha, in southwestern Japan (26°12′ N, 127°41′ E, 27.5 m), as shown in Fig. 1. The climates differ greatly at these three sites. The spectral aerosol optical depth was calculated using sun photometers and direct solar radiation to obtain data for cloudless conditions, which was measured at Tsukuba. The calculations used here were developed based on the "SMARTS2" model 24, which was applied to all three representative observation sites mentioned above.

Locations of the Sapporo, Tsukuba and Naha sites where the Japan Meteorological Agency conducts observations of UV spectra and total ozone. The other sites are Cape Ochi-ishi, Cape Hedo and Tsukuba where UV_Ery is observed by the National Institute for Environmental Studies.

Materials and Methods

The quantity of vitamin D synthesized in the epidermis is expressed as follows:

(1)

where q _e, S _type, S _der and t _ex are the production rate of vitamin D synthesis in the epidermis per unit energy of UV_VitD, the effect of skin type, the effective exposed skin area and the solar exposure time, respectively. Details are described in Miyauchi et al. 23. UV_VitD is obtained by a second equation:

(2)

where E (λ) is the UV irradiance of the wavelength λ, reaching the surface of the ground, and S _Vd is the International Commission on Illumination (CIE) action spectrum 25 proposed by MacLaughlin et al. 26 for vitamin D synthesis. Following evaluation, we adopted a production rate, q _e, for white people whose skin phototype (SPT) might be II obtained by Davie et al. 27, 28. It is one of the most important parameters used to calculate the amount of vitamin D synthesized in the epidermis, by UV_VitD from 7-dehydrocholesterol to vitamin D via previtamin D, which is estimated to be 0.00101 ± 0.00054 μg mJ⁻¹ for vitamin D synthesis 23 using the alternative CIE action spectrum instead of an action spectrum, which Davie et al. 28 applied. Webb and Engelsen 21 applied Holick's rule while Dowdy et al. 20 discussed the discrepancy between the amount of vitamin D produced under sunny conditions in Boston and Holick's rule because of the difference between the UV spectra they used. McKenzie et al. 22 evaluated the benefits in a different way, although it was similar to Holick's rule. On the other hand, Davie et al. 28 proposed two production rates of vitamin D from UV_VitD for adults. One of these rates was adopted in this study, because it is reliable without the need for an oral supplement of cholecalciferol, which might introduce an uncertainty of assimilation. It should also be noted that the efficiency of vitamin D production in the epidermis depends on age 29 as well as medical conditions. Further studies of vitamin D production rates in the human body are needed. Most of the Japanese population are skin phototype (SPT) III, but there is a general range from SPT II to IV. Here, the value of S _type was inferred to be 0.83 for SPT III compared to SPT II considering the irradiance to 1 MED 1, 21. It is possible to obtain S _type values for the other SPTs by estimating the time taken to reach the MED. UV_VitD can be obtained by applying Eq. 2 and the SMART2 model using the S _Vd action spectrum. On the other hand, UV_Ery for the CIE action spectrum S _Er, which is determined for erythemal UV, is defined as follows 30:

(3)

Figure 2 shows a comparison between the irradiances for erythema, UV_Ery observed wavelengths of 290–325 nm, which represents the limits of the Brewer (MkII) spectrophotometer, and UV_Ery calculated by the model for the years 2005–2007 for 124 cloudless skies at Tsukuba for 12:00 Japanese standard time (JST), selected from records of direct solar radiation taken by a pyrheliometer at 12:00 JST. The data were accepted in cases where the records were stable within one hour between 11:30 and 12:30 JST. To calculate UV_Ery, total ozone and the optical thickness of aerosols (obtained from the Japan Meteorological Agency) were combined. Some data were omitted because they were over the 95% confidence limit from the relationship between the calculated and observed UV_Ery. This calculation improved on previous methods 23 by the addition of data on the NO₂ absorption effect and observed surface pressure. Figure 3(a) shows a comparison of the spectral UV_Ery between the observed and calculated Tsukuba data at 12:00 JST for spring, summer and winter, and Fig. 3(b), for UV_VitD for the same time period as in Fig. 3(a). In the figure, the observed spectra are shown every 0.5 nm due to the specifications of the Brewer (MkII) spectrophotometer, but the calculated spectra are shown every 1.0 nm. Despite some unavoidable observation errors for the UV spectra, total ozone, spectral aerosol optical depth and some calculation suppositions, the data are closely correlated. Because the calculated spectra are very similar to the observed spectra, we presumed that the calculated spectra could be used as a proxy for UV_Ery and UV_VitD.

Comparison between irradiances at 290–325 nm for observed and calculated UV_{E ry} during 2005–2007 for 124 cloudless skies at Tsukuba at 12:00 JST.

Comparisons of spectral erythemally weighted UV (UV_Ery) and UV for vitamin D synthesis (UV_VitD) between the observed and the calculated at Tsukuba, 12:00 JST in the three typical seasons.

Figure 4(a) shows the relationships between UV_Ery and UV_VitD calculated for 124 cloudless skies at Tsukuba, for all the data of 2005–2007 at the representative times of 09:00, 12:00 and 15:00 JST, with known conditions for total ozone and spectral aerosol optical depth. Figures 4(b) and (c) are the same as 4(a), at Sapporo and Naha, but with the assumption of cloudless skies. However, aerosol data from Sapporo and Naha were not available and were substituted with data from Tsukuba. For the UV_Ery calculations, integration was between wavelengths of 290 and 400 nm, and for UV_VitD, it was between 290 and 330 nm. In the figure, the dispersions are caused mainly by the slant pass lengths of the atmosphere due to the solar zenith angle (SZA) at each time point and the total ozone amount (Fig. 5). The larger the SZA is, the more absorption by ozone occurs, and the relationship between UV_Ery and UV_VitD changes through spectrum differences between the CIE action spectra S _Er and S _Vd. The seasonal variation of total ozone at Naha is smaller than that at the other sites. This is because the dispersion at Naha is smaller than elsewhere. In contrast, at Sapporo, the large variation of total ozone has a greater effect on the UV_VitD/UV_Ery ratio than that at the other sites. This is especially the case in winter at Sapporo, where the ratio varies less than during the other seasons because of the high levels of total ozone and larger zenith angle, which results in the dispersion, as seen in Fig. 4(b). The spectral aerosol optical depth changes on a daily basis, independently of other atmospheric parameters. However, UV_VitD/UV_Ery at Naha, as shown in Fig. 4(c), remained almost constant compared to the other sites. This means that aerosols do not affect the relationship between UV_VitD and UV_Ery. Furthermore, at Naha, the seasonal variations of total ozone were lower than those at the other sites throughout the year. Similar relationships were reported by McKenzie et al. 22 and Downs et al. 31 by measurements.

(a–c) show the relationships between UV_{E ry} and UV_{V itD} calculated for 124 cloudless skies at Tsukuba, Sapporo and Naha for all of the data for 2005–2007 at representative times of 09:00, 12:00 and 15:00 JST, respectively, with known conditions for total ozone and spectral aerosol optical depth. (b) and (c) are assumed to be cloud free. (d) Shows data for all stations.

Relationships between UV_{V itD}/UV_{E ry} and the slant pass ozone (relative ozone mass) observed at each site for 2005–2007 at 09:00, 12:00 and 15:00 JST. In the figure, slant pass ozone greatly expands causing the dispersion of UV_{V itD}/UV_{E ry} especially for Sapporo. On the other hand, at Naha, the situation is reversed.

Considering the locations of the three sites, the relationship between UV_Ery and UV_VitD shown in Fig. 4(d) could be applied to any location in the Japanese Archipelago, except for the highlands. Thus, if UV_Ery is known, UV_VitD can be estimated by application of the following equations, which are obtained by a curve fitting, allowing for some errors:

(4)

(5)

This means that UV_VitD is strongly and linearly correlated with UV_Ery for UV Index larger than 1.6 (0.04 W/m²) at the three sites described above. This estimation contains some errors (see the Appendix S1).

Results

This method enables an instant conversion of harmful UV levels to those needed for vitamin D synthesis. Table 1 shows the time required for skin damage (i.e. for 1 MED (300 J m⁻²) for SPT III) and the corresponding times required to produce 10 μg of vitamin D for effective skin areas of 600 and 1200 cm² as a function of the UV Index. Even for an effective skin area of 600 cm², it takes several minutes to reach a UV Index >10. In this case, the time taken to produce 10 μg of vitamin D is ~1/3 of the time to reach the MED for a comparatively larger UV Index. For a skin area of 1200 cm², 10 μg of vitamin D could be produced in half the time for a skin area of 600 cm². In summer, often >1200 cm² of skin is exposed, and this is preferable to avoid any risk of UV overexposure and to gain the maximum benefit. The time calculated here is longer than that determined by McKenzie et al. 22, who considered the skin type to be SPT II, producing 1000 IU of vitamin D in one minute, with a UV Index of 10 for a full body, and the area of the face and hands to constitute 1/10 of a full body exposure. The exposed area is not always 600 cm²; for example, if 1/10 of a full body is ~900 cm², applying average Japanese adults according to the "Du Bois formula" and the "Lund and Browder chart" as to which regional areas of the body are to be calculated, the results would be almost the same as those obtained by McKenzie et al. 22 shown in table 2, for the Japanese population with a SPT III skin type.

Table 1. Time taken to reach MED and produce 10 μg of vitamin D for effective skin areas of 600 and 1200 cm² for the SPT III according to the UV Index

UV Index	UVEry (W/m2)	Estimated UVVitD (W/m2)	Time to MED (min)	Time to produce 10 μg
				600 cm2 (min)	1200 cm2 (min)
1	0.025	0.031	200	108	53.9
2	0.050	0.079	100	42.2	21.1
3	0.075	0.131	66.7	25.2	12.6
4	0.100	0.184	50.0	18.0	9.0
5	0.125	0.237	40.0	14.0	7.0
6	0.150	0.290	33.3	11.4	5.7
7	0.175	0.342	28.6	9.7	4.8
8	0.200	0.395	25.0	8.4	4.2
9	0.225	0.448	22.2	7.4	3.7
10	0.250	0.501	20.0	6.6	3.3
11	0.275	0.553	18.2	6.0	3.0
12	0.300	0.606	16.7	5.5	2.7
13	0.325	0.659	15.4	5.0	2.5
14	0.350	0.712	14.3	4.7	2.3
15	0.375	0.764	13.3	4.3	2.2

The National Institute for Environmental Studies has a monitoring network for harmful UV using broadband UV radiometers for UV-B (MS-212W) and UV-A (MS-212A), manufactured by EKO Instruments Co. Ltd. (Tokyo, Japan), with the help of a Brewer spectrophotometer. Three sites in the network, Cape Ochi-ishi (43°10′ N, 145°30′ E, 50 m above sea level) near Sapporo, Tsukuba (36°03′ N, 140°07′ E, 40 m) and Cape Hedo (26°52′ N, 128°16′ E, 65 m) near Naha (Fig. 1), were used to evaluate the most appropriate exposure time to UV from the observed levels of harmful UV. Precision of the instruments is maintained by calibrations performed once every 2 years for the MS-212W, and once every 5 years for the MS-212A; however, UV_Ery data contained some unavoidable measurement errors, which were estimated to be 7.5–9.5% for system errors, and 7.45–8.21% for running errors 32. As an example, Fig. 6(a) shows the times taken to reach the MED (t _med) and to produce 10 μg of vitamin D (t _10 μg) during midsummer for effective exposure areas of 600 and 1200 cm² at Tsukuba, Cape Ochi-ishi and Cape Hedo. The gray zone indicates times over 60 min. In the figures, t_10 μg is always shorter than t _med, and much shorter for an exposure area of 1200 cm². However, that conclusion does not necessarily apply for smaller skin area exposures, as reported by McKenzie et al. 22. Our calculation shows that a crossover between the times t _10 μg and t _med occurred for a skin area of 324 cm² at the UV Index of 1. At a small UV_Ery, which generally occurs when a large zenith angle is present, 10 μg of vitamin D needs a longer t _10 μg. This means that it cannot be applied during periods such as early morning and the late evening, even in midsummer, especially for a skin area of 600 cm², when the UV reaching the ground is limited. Even people living in the northern part of the Japanese Archipelago, such as in Hokkaido, should avoid excessive UV exposure during the summer season (Cape Ochi-ishi); however, they would be able to produce vitamin D after a short exposure period, especially for people with an SPT of III or less. At Cape Hedo, which lies in the most southern part of the Japanese Archipelago, people should avoid excessive exposure to UV during the day. Figure 6(b) shows t _med and t _10 μg for the winter season, similar to Fig. 6(a). In the figure, both t _med and t _10 μg at noon at all three sites are much longer than those in the summer, especially at Cape Ochi-ishi and Tsukuba. However, it should be noted that the differences in t _med and t _10 μg at Cape Hedo between the winter and summer are less than those at the other sites, due to the location. Thus, t _med and t _10 μg vary greatly depending on the season and location. The data should enable people to understand how to safely manage the UV risks and benefits according to their location and the season.

The time taken to reach MED (t _med) and to produce 10 μg vitamin D (t _10 μg) during midsummer (a) and winter (b) for in situ measurements of effective skin area exposures of 600 and 1200 cm² at Tsukuba, Cape Ochi-ishi and Cape Hedo, as examples. The gray area shows times of more than 60 min.

Discussion

UV is needed to produce vitamin D; however, it is difficult to know how much vitamin D is produced, and what constitutes an appropriate, safe exposure time to solar radiation. The UV Index is widely used as a measure of UV risk. However, the amount of UV exposure needed for vitamin D synthesis is not well known. In this study, we suggest that knowledge of harmful UV levels that pose a risk to health can be used to provide immediate estimates of the amount of beneficial UV exposure needed to produce vitamin D, although this is currently restricted to the Japanese Archipelago. The risk to human health is assessed as the time to reach MED, which is a symptom of excessive exposure. This system should be used in conjunction with other factors, such as skin type, exposed area and the amount of vitamin D needed.

Exposure area

An exposure area of 600 cm² corresponds to a human face and the backs of both hands 27 although it depends on an individual's age. However, McKenzie et al. 22 defined it as 1/10 of the full body area. In practice, calculating the exposure area is very important, but arriving at a clear definition has proved difficult. It is preferable to expose a larger area, such as the shoulders and some parts of the arms or legs, rather than just the face and hands. For example, exposure of 1200 cm² (2 × 600 cm²) is preferable to produce more vitamin D at a reduced risk. UV is generally defined as spectral irradiance on a horizontal plane. However, bare skin area is not always horizontal; it is inclined and varies with the person's posture as investigated in detail by Seckmeyer et al. 33. The exposure area used in this study, as defined as in Eq. 1, assumes horizontal exposure. UV on the surface of the ground contains direct and diffusive irradiances. Direct UV has an effect according to the incidence angle for a tilted skin surface and the irradiance of UV from the sun. However, for UV wavelengths of ~300 nm, diffusive UV exceeds that of direct UV according to Rayleigh's theory for air molecules 34, although it is not always isotropic. Therefore, the incident energy of UV_VitD to a tilted skin area due to Rayleigh scattering could be larger than that received by a horizontal surface. However, the details of this process are too complex to be discussed here.

Pope and Godar 35 proposed the geometric conversion factor (GCF), which assumes that a human body consists of cylindrical parts that vary depending on the latitude of the location and the season, and showed that the GCF corresponds to roughly half that of the horizontal plane. Downs and Parisi 36 measured the mean exposure fraction (MEF) with a manikin model standing in an upright position using polysulfone dosimeters. The MEF depended on the SZA and was smaller for lower zenith angles, and for certain body sites. Liu et al. 37 analyzed the angular distribution of UV irradiated on a surface inclined from 0 to 90° at every solar elevation angle (SEA, ~80°) and azimuthal angle (~180°) under a clear sky by taking spectral UV measurements corresponding to UV_Ery and UV_VitD irradiated on an area of skin that was not always horizontal. For example, for a 10° incline of the skin toward the sun and a low SEA of 18.8°, the UV_VitD would be 2.7 times higher than that on a horizontal plane, and for a 30° incline, it would be 1.59 times higher at a SEA of 29.4°. McKenzie et al. 38 compared the erythemal UV irradiances on a surface normal to the sun and on a horizontal surface by measurements throughout the day in summer at Lauder, New Zealand. On cloud free days, the irradiance on a surface normal to the sun exceeded that on a horizontal surface, with the difference being generally less than 10% over a day, but the exact difference was dependent on the SZA. However, on cloudy days, the normal-incidence UV can be less than 50% of the horizontal-incidence UV. The results are suggestive in estimations of effective exposure skin area.

In this study, a horizontally converted area of bare skin could not be obtained for every possible situation. Thus, an accurate exposure area could not be evaluated. Therefore, we substituted skin area for "effective exposure skin area," which is used in the definition of UV irradiance on the horizontal plane in the atmosphere. Further consideration needs to be given to this subject.

Conclusions

The risks of UV exposure are known, especially after the depletion of the ozone layer, which has led to an increase in the amount of UV-B that reaches the ground; additionally, the mass media have played a role in informing the general public about the harmful effects of UV radiation, but have neglected to point out the merits of UV. This study makes it possible to calculate UV exposure to enable people to weigh the risks and benefits if the amount of erythemal UV is known. The UV environment on the ground differs according to the location, the season and the time of the day, as well as meteorological conditions such as cloud cover. It would be possible for people to find their personal "Goldilocks" zones of exposure time 22, which could be applied in their daily lives. In this study, we aimed to identify these zones measured on our Web site, although the observation sites were restricted 39. This method is valid for any location in Japan, except for the highlands.

Acknowledgements

The authors would like to thank M. Shimizu, one of our staff, for processing data, calculations and drawing pictures. We also thank T. Machida of the Center for Global Environmental Research at the National Institute for Environmental Studies, and N. Ohkawara of the Japan Meteorological Agency for their assessments of this study and for making valuable comments.

Supporting Information

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Description

php12651-sup-0001-FigsS1-S4.docxWord document, 219.5 KB

Figure S1. The differences in UVVitD estimated by Eqs. (4) and (5) (see text), and those calculated by the model, are shown as a function of UVEry for all the data under cloudless skies. Figure S2. The 30-min averages for UVVitD at Tsukuba on July 14th, 2015, with very small error bars. Figure S3. UVVitD/UVEry calculated by the model as a function of UVEry for all data under cloudless skies. Figure S4. The errors for the estimate of UVVitD,cloud with clouds for UVEry,cloud in the case UVEry,clear = 0.3 W/m2, using the equation ∆UVVitD,cloud = –0.027 (1 − UVEry,cloud/UVEry,clear) (see text).

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

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Source: https://onlinelibrary.wiley.com/doi/full/10.1111/php.12651