The synthesis of vitamin D in skin is a two-stage process that begins with the production of previtamin D after irradiation of 7-dehydrocholesterol by ultraviolet (UV) radiation. A number of personal ...and environmental factors control the probability of a suitable UV photon reaching a molecule of 7-dehydrocholesterol in the skin. These are astronomical factors that govern the solar zenith angle (SZA), and the local state of the atmosphere, determining the available solar UV radiation; skin pigmentation and age, determining competing absorbers of UV radiation and available 7-dehydrocholesterol; individual behaviour in the local surroundings, determining exposure of unprotected skin to available UV radiation. The only one of these influences that can be determined unequivocally for any situation is the SZA. The other influences must be considered either as individual case studies, or be represented by “typical” and “idealised” situations for the weather, skin and behaviour. At large SZAs there is insufficient solar UV radiation to initiate significant vitamin D synthesis. At smaller SZAs assessment of solar exposure necessary for vitamin D synthesis can only be indicative and application of any such assessment necessarily requires awareness of both self- and the local environment.
Twilight is characterised by changes in both quantity ("irradiance") and quality ("colour") of light. Animals use the variation in irradiance to adjust their internal circadian clocks, aligning their ...behaviour and physiology with the solar cycle. However, it is currently unknown whether changes in colour also contribute to this entrainment process. Using environmental measurements, we show here that mammalian blue-yellow colour discrimination provides a more reliable method of tracking twilight progression than simply measuring irradiance. We next use electrophysiological recordings to demonstrate that neurons in the mouse suprachiasmatic circadian clock display the cone-dependent spectral opponency required to make use of this information. Thus, our data show that some clock neurons are highly sensitive to changes in spectral composition occurring over twilight and that this input dictates their response to changes in irradiance. Finally, using mice housed under photoperiods with simulated dawn/dusk transitions, we confirm that spectral changes occurring during twilight are required for appropriate circadian alignment under natural conditions. Together, these data reveal a new sensory mechanism for telling time of day that would be available to any mammalian species capable of chromatic vision.
Sunlight exposure, with resulting cutaneous synthesis, is a major source of vitamin D for many, while dietary intake is low in modern diets. The constitutive pigment in skin determines skin type, ...observed as white, brown, or black skin. The melanin pigment absorbs ultraviolet radiation (UVR) and protects underlying skin from damage caused by UVR. It also reduces the UVR available for vitamin D synthesis in the skin. It has been shown that the white-skinned population of the UK are able to meet their vitamin D needs with short, daily lunchtime exposures to sunlight. We have followed the same methodology, based on a 10-year UK all-weather UVR climatology, observation (sun exposure, diet, vitamin D status), and UVR intervention studies with Fitzpatrick skin type V (brown) adults, to determine whether sunlight at UK latitudes could provide an adequate source of vitamin D for this section of the population. Results show that to meet vitamin D requirements, skin type V individuals in the UK need ~25 min daily sunlight at lunchtime, from March to September. This makes several assumptions, including that forearms and lower legs are exposed June-August; only exposing hands and face at this time is inadequate. For practical and cultural reasons, enhanced oral intake of vitamin D should be considered for this population.
A paucity of vitamin D is a common deficiency globally, with implications for many aspects of health besides the well-known impact on musculoskeletal health. The two sources of vitamin D are through ...oral intake, or through endogenous synthesis in the skin when exposed to ultraviolet radiation in sunlight. Assessing nutritional needs, whether by food, food fortification or supplementation, is aided by an understanding of local potential for cutaneous synthesis of the vitamin, dependent on latitude and climate, personal skin type and local culture. To aid these discussions we provide indicative exposure times for the maintenance of vitamin D status as a function of latitude, month and skin type, for the clear-sky case and all-sky conditions, for an ambulatory person wearing modest skirt/shorts and T-shirt. At latitudes greater than ±40 degrees, lack of available sunlight limits vitamin D synthesis in some months for all, while at the equator exposure times range from 3 to 15 min at noontime, for white and black skin, respectively. Rather than a sun exposure prescription, the data are intended to show where nutritional vitamin D intake is necessary, advisable, or can be mitigated by sun exposure, and allows for such advice to be personalized to account for different sub-groups in a multicultural population.
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.
•Google Street View images can be used to obtain urban canyon geometries worldwide.•Hugin software was used to reconstruct fisheye polar projections of urban canyons.•Rayman software was used to ...model total shortwave irradiances at reconstructed canyon.•Models combined with full-sky measurements returned good approximations to measured canyon irradiances.•Proposed procedures approximate urban sky irradiances for all sky conditions.
This paper presents an implementation of digital image processing tools, in order to reconstruct building geometries and urban sky view factors for individual street canyons around the world. By using the Google Street View image database, it is possible to reconstruct canyon fisheye images at any point where a Google panorama is available. The Hugin software allows the geometrically correct merging of independent images in order to create a fisheye projection, covering the upper hemispheric view of the sky. Sky view factor calculations may then be performed by specialised software on the reconstructed fisheye image. In order to analyse the fitness of reconstructed panoramas, control images taken with a fisheye lens camera were used for comparison. The Rayman model was then used to calculate total shortwave global irradiances from the restricted canyon view and from a full hemispheric view. Finally, total shortwave global irradiances were measured for the same position in the street canyon and compared against observatory measurements under a full sky view, this in order to evaluate the performance and usefulness of idealised calculations. The reconstructed urban geometries returned acceptable fine-scale descriptions, and their potential utility for scientific purposes in the atmospheric sciences was demonstrated with the examples provided.
The standard erythema action spectrum provides an internationally accepted representation of the erythema‐inducing effectiveness of wavelengths in the UV part of the spectrum. The action spectrum ...forms the basis of the UV index used for public health information, defines the standard erythema dose unit and the minimum erythema dose and is the default response spectrum aspired to by a range of UV radiometer manufacturers. However, there are several versions of this erythema action spectrum in use, and only one of them has been endorsed as a standard. While the differences in erythemally weighted radiation incurred by choice of action spectrum will be no more than a few percent, this uncertainty is unnecessary. Here we detail the differences in the different versions of erythema action spectra, illustrate the resulting effects in quantifying UV doses and encourage readers to use only the standard version of the action spectrum in the future.
The body gains vitamin D through both oral intake (diet/supplementation) and synthesis in skin upon exposure to ultraviolet radiation (UVR). Sun exposure is the major source for most people even ...though sun exposure is complex and limited by climate and culture. We aimed to quantify the sun exposure required to meet vitamin D targets year-round and determine whether this can be safely achieved in a simply defined manner in the UK as an alternative to increasing vitamin D oral intake. Data from observation (sun exposure, diet, and vitamin D status) and UVR intervention studies performed with white Caucasian adults were combined with modeled all-weather UVR climatology. Daily vitamin D effective UVR doses (all-weather) were calculated across the UK based on ten-year climatology for pre-defined lunchtime exposure regimes. Calculations then determined the time necessary to spend outdoors for the body to gain sufficient vitamin D levels for year-round needs without being sunburnt under differing exposure scenarios. Results show that, in specified conditions, white Caucasians across the UK need nine minutes of daily sunlight at lunchtime from March to September for 25(OH)D levels to remain ≥25 nmol/L throughout the winter. This assumes forearms and lower legs are exposed June-August, while in the remaining, cooler months only hands and face need be exposed. Exposing only the hands and face throughout the summer does not meet requirements.
In light of the current COVID-19 pandemic, and given the importance of diet to overall health and well-being, nutrients (macro and micro) deserve special attention.10 As a key micronutrient, vitamin ...D should be given particular focus—not as a ‘magic bullet’ to beat COVID-19, as the scientific evidence base is severely lacking at this time—but rather as part of a healthy lifestyle strategy to ensure that populations are nutritionally in the best possible place.11 Vitamin D is unique: it is a prohormone which is produced in the skin during exposure to sunlight (UVB radiation at 290–315 nm) with, usually, smaller amounts obtained from food. ...in Manchester, UK (53.5N) the nadir of seasonal vitamin D status occurs in February, with sunlight exposure once again becoming effective for vitamin D synthesis in the skin only from March onwards.13 Relatively high prevalence of low vitamin D status globally has been reported over recent decades in a wide range of population groups,14 including those in low latitude areas (despite the abundance of sunlight) and not necessarily confined to winter.15 This may be due to environmental factors, such as air pollution, as well as cultural factors that lead to skin being covered and not subject to sunlight exposure.16 Older, housebound individuals are at particularly high risk of vitamin D deficiency.17 Vitamin D status is reflected by the level of the circulating metabolite 25-hydroxyvitamin D (25OHD), which is produced by hepatic hydroxylation of vitamin D coming from either skin or the gut from oral intake.18 If the 25OHD concentration is low (as defined in the UK by a 25OHD concentration of <25 nmol/L7 and in the USA and some other countries by a 25OHD concentration of <30 nmol/L),8 9 such as observed commonly during and towards the end of the winter, this indicates that stores are depleted and vitamin D-requiring functions may be impaired. ...vitamin D, via its active metabolites, regulates more than 200 genes including those genes that are responsible for cellular proliferation, differentiation and apoptosis.19 The discovery of the expression of nuclear vitamin D receptors and vitamin D metabolic enzymes in immune cells provides a scientific rationale for the potential role of vitamin D in maintaining immune homoeostasis and in preventing the development of autoimmune processes.20 The field of vitamin D research has grown exponentially in recent years with a much improved understanding of its biological importance. Epidemiological studies in children have found a strong association between URTI and rickets.28 A large cross-sectional study of the US population reported that URTI infections were higher in those with lower vitamin D status, with the association being stronger in those with respiratory diseases such as asthma and chronic obstructive pulmonary disease.29 There is evidence that lower vitamin D status is associated with acute respiratory tract infections (ARTIs).30 In a recent systematic review and meta-analysis of individual participant data from vitamin D supplementation RCTs, vitamin D supplementation reduced the risk of ARTI, with the greatest benefit in those with vitamin D deficiency at baseline.31 However, it is important to note the limitations to this systematic review/meta-analysis;32 33 there was a high level of heterogeneity in the findings and concomitantly, the overall significant results in the meta-analysis of the 24 included trials was dependent on the inclusion of the two studies undertaken in developing countries:
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