Ann R. Webb

Calculated Ultraviolet Exposure Levels for a Healthy Vitamin D Status

Abstract The dangers of overexposure to sunlight have been well publicized, but less attention has been given to an acknowledged benefit of exposure to UV radiation; that being the cutaneous synthesis of vitamin D3. Here we define a standard vitamin D dose on the basis of recently recommended requirements for vitamin D that take account of its risk reduction role in a variety of diseases, and present a web-based tool that enables the reader to calculate associated exposure times for any time and place using either default values or user-selected conditions. Either it is not possible to synthesize vitamin D3 at high latitudes in winter, or the exposure time required to reach a standard dose is sometimes impractical. Where solar UV is sufficient, a risk-benefit analysis of sunburn vs. vitamin D3 synthesis shows that the best time for brief sun exposure is in the middle of the day. For low solar elevation angles common at high latitudes, a fine line exists between adequate UV exposure for vitamin D3 synthesis and a risk of sun burn.

The role of sunlight exposure in determining the vitamin D status of the U.K. white adult population.

Background  Vitamin D is necessary for bone health and is potentially protective against a range of malignancies. Opinions are divided on whether the proposed optimal circulating 25‐hydroxyvitamin D [25(OH)D] level (≥ 32 ng mL−1) is an appropriate and feasible target at population level.

Recommended summer sunlight exposure levels can produce sufficient (?20 ng ml-1) but not the proposed optimal (?32 ng ml-1) 25(OH)D levels at UK latitudes

Recommendations on limitation of summer sunlight exposure to prevent skin cancer may conflict with requirements to protect bone health through adequate vitamin D levels, the principal source being UVB in summer sunlight. We determined whether sufficient (> or =20 ng ml(-1)) and proposed optimal (> or =32 ng ml(-1)) 25(OH)D levels are attained by following UK guidance advising casual short exposures to UVB in summer sunlight, and performed the study under known conditions to enhance the specificity of future recommendations. During wintertime, when ambient UVB is negligible, 120 white Caucasians, aged 20-60 years, from Greater Manchester, UK (53.5 degrees N) received a simulated summers sunlight exposures, specifically 1.3 standard erythemal dose, three times weekly for 6 weeks, while wearing T-shirt and shorts. The baseline winter data predict that 5% (confidence interval (CI): 2.7-8.6) of Greater Manchester white Caucasians have deficient (<5 ng ml(-1)) 25(OH)D, 62.5% (CI: 55.2-69.4) have insufficient, and only 2.9% (CI: 1.4-5.6) have proposed optimal levels. After the simulated summer exposures, 90 (CI: 84.9-93.7) and 26.2% (CI: 20.1-33.2) reached 20 and 32 ng ml(-1) 25(OH)D, respectively. Assuming midday UVB levels, sufficient but suboptimal vitamin D status is attained after a summers short (13 minutes) sunlight exposures to 35% skin surface area; these findings will assist future public health guidance on vitamin D acquisition.

The Convective Storm Initiation Project

The Convective Storm Initiation Project (CSIP) is an international project to understand precisely where, when, and how convective clouds form and develop into showers in the mainly maritime environment of southern England. A major aim of CSIP is to compare the results of the very high resolution Met Office weather forecasting model with detailed observations of the early stages of convective clouds and to use the newly gained understanding to improve the predictions of the model. A large array of ground-based instruments plus two instrumented aircraft, from the U.K. National Centre for Atmospheric Science (NCAS) and the German Institute for Meteorology and Climate Research (IMK), Karlsruhe, were deployed in southern England, over an area centered on the meteorological radars at Chilbolton, during the summers of 2004 and 2005. In addition to a variety ofground-based remote-sensing instruments, numerous rawinsondes were released at one- to two-hourly intervals from six closely spaced sites. The Met Office weather radar network and Meteosat satellite imagery were used to provide context for the observations made by the instruments deployed during CSIP. This article presents an overview of the CSIP field campaign and examples from CSIP of the types of convective initiation phenomena that are typical in the United Kingdom. It shows the way in which certain kinds of observational data are able to reveal these phenomena and gives an explanation of how the analyses of data from the field campaign will be used in the development of an improved very high resolution NWP model for operational use.

The role of sunlight exposure in determining the vitamin D status of the UK white Caucasian adult population.

Background  Vitamin D is necessary for bone health and is potentially protective against a range of malignancies. Opinions are divided on whether the proposed optimal circulating 25‐hydroxyvitamin D [25(OH)D] level (≥ 32 ng mL−1) is an appropriate and feasible target at population level.

Variability of UV Irradiance in Europe

The diurnal and annual variability of solar UV radiation in Europe is described for different latitudes, seasons and different biologic weighting functions. For the description of this variability under cloudless skies the widely used one‐dimensional version of the radiative transfer model UVSPEC is used. We reconfirm that the major factor influencing the diurnal and annual variability of UV irradiance is solar elevation. While ozone is a strong absorber of UV radiation its effect is relatively constant when compared with the temporal variability of clouds. We show the significant role that clouds play in modifying the UV climate by analyzing erythemal irradiance measurements from 28 stations in Europe in summer. On average, the daily erythemal dose under cloudless skies varies between 2.2 kJ m−2 at 70°N and 5.2 kJ m−2 at 35°N, whereas these values are reduced to 1.5–4.5 kJ m−2 if clouds are included. Thus clouds significantly reduce the monthly UV irradiation, with the smallest reductions, on average, at lower latitudes, which corresponds to the fact that it is often cloudless in the Mediterranean area in summer.

UK Food Standards Agency Workshop Report: an investigation of the relative contributions of diet and sunlight to vitamin D status.

The UK Food Standards Agency (FSA) convened an international group of scientific experts to review three Agency-funded projects commissioned to provide evidence for the relative contributions of two sources, dietary vitamin D intake and skin exposure to UVB rays from sunlight, to vitamin D status. This review and other emerging evidence are intended to inform any future risk assessment undertaken by the Scientific Advisory Committee on Nutrition. Evidence was presented from randomised controlled trials to quantify the amount of vitamin D required to maintain a serum 25-hydroxy vitamin D (25OHD) concentration >25 nmol/l, a threshold that is regarded internationally as defining the risk of rickets and osteomalacia. Longitudinal evidence was also provided on summer sunlight exposure required to maintain 25OHD levels above this threshold in people living in the British Isles (latitude 51 degrees-57 degrees N). Data obtained from multi-level modelling of these longitudinal datasets showed that UVB exposure (i.e. season) was the major contributor to changes in 25OHD levels; this was a consistent finding in two Caucasian groups in the north and south of the UK, but was less apparent in the one group of British women of South Asian origin living in the south of the UK. The FSA-funded research suggested that the typical daily intake of vitamin D from food contributed less than UVB exposure to average year-round 25OHD levels in both Caucasian and Asian women. The low vitamin D status of Asian women has been acknowledged for some time, but the limited seasonal variation in Asian women is a novel finding. The Workshop also considered the dilemma of balancing the risks of vitamin D deficiency (from lack of skin exposure to sunlight in summer) and skin cancer (from excessive exposure to sunlight with concomitant sunburn and erythema). Cancer Research UK advises that individuals should stay below their personal sunburn threshold to minimise their skin cancer risk. The evidence suggests that vitamin D can be produced in summer at the latitude of the UK, with minimal risk of erythema and cell damage, by exposing the skin to sunlight for a short period at midday, when the intensity of UVB is at its daily peak. The implications of the new data were discussed in the context of dietary reference values for vitamin D for the general population aged 4-64 years. Future research suggestions included further analysis of the three FSA-funded studies as well as new research.

Traveling reference spectroradiometer for routine quality assurance of spectral solar ultraviolet irradiance measurements

A transportable reference spectroradiometer for measuring spectral solar ultraviolet irradiance has been developed and validated. The expanded uncertainty of solar irradiance measurements with this reference spectroradiometer, based on the described methodology, is 8.8% to 4.6%, depending on the wavelength and the solar zenith angle. The accuracy of the spectroradiometer was validated by repeated site visits to two European UV monitoring sites as well as by regular comparisons with the reference spectroradiometer of the European Reference Centre for UV radiation measurements in Ispra, Italy. The spectral solar irradiance measurements of the Quality Assurance of Spectral Ultraviolet Measurements in Europe through the Development of a Transportable Unit (QASUME) spectroradiometer and these three spectroradiometers have agreed to better than 6% during the ten intercomparison campaigns held from 2002 to 2004. If the differences in irradiance scales of as much as 2% are taken into account, the agreement is of the order of 4% over the wavelength range of 300-400 nm.

Quality assurance of spectral solar UV measurements: results from 25?UV monitoring sites in Europe, 2002 to 2004

With the transportable reference spectroradiometer QASUME (Quality Assurance of Spectral Ultraviolet Measurements in Europe) routine quality assurance of spectrally resolved solar ultraviolet irradiance measurements were successfully performed at 25 UV monitoring sites in Europe. The absolute scale carried by the QASUME reference spectroradiometer is traceable to the primary irradiance standard of the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, and has proved to represent the average scale in use at 25 independent European laboratories; it can thus be taken as a European irradiance reference. Out of the 27 instruments 13 showed deviations relative to the QASUME reference spectroradiometer of less than 4% in the UVB (15 instruments in the UVA) for solar zenith angles below 75 ◦ . The results so far have shown the unique possibilities offered by this transportable reference spectroradiometer for providing on-site quality assurance of solar ultraviolet irradiance measurements. (Some figures in this article are in colour only in the electronic version)

Accuracy of spectral UV model calculations: 1. Consideration of uncertainties in input parameters

Even in idealized, clear sky conditions, spectral UV models need well-specified input parameters such as solar zenith angle, pressure, ozone and aerosol optical depth to produce realistic results. UV models based on exact analytical methods are widely considered to be absolutely accurate with errors in results widely attributed to inaccurate input parameters. The uncertainties in the standard determinations of each input parameter were determined for the following methods of specifying each parameter: (1) ozone from satellites or ground-based Dobson spectrometer measurements; (2) aerosol optical depth from sunphotometer measurements; (3) single-scattering albedo; three different procedures using ground-based spectrometer measurements and radiative transfer calculations were studied; (4) ground albedo; the determination of the ground albedo is achieved using satellite measurements, which have a resolution of some 100 km; ground-based measurements may give some information about the albedo of the ground, but the albedo needed for radiative transfer calculations is a weighted average of the surrounding albedos. The resulting inaccuracies in UV calculations from errors in input parameters were simulated with the discrete ordinate method model by Stamnes et al. [1988] using the uncertainties specified above for the different input parameters. The simulations of the errors of the spectral UV calculations were carried out for each input parameter at 0°, 50°, and 80° zenith angle and analyzed at 305 and 380 nm. The overall maximum error may be determined by combining the different errors.