Estimates of beneficial and harmful sun exposure times during the year for major Australian population centres

Despite the fact that skin cancer is largely preventable, about half of all Australians will be diagnosed with skin cancer during their lifetime.1 There is strong evidence that ultraviolet (UV) radiation exposure increases the risk of developing skin cancer, and UV radiation has been classified as a carcinogen by the International Agency for Research on Cancer.2,3 In Australia, it is estimated that up to 95% of cutaneous melanomas, and 99% of squamous and basal cell carcinomas, are caused by solar UV radiation.4 Solar UV radiation can also cause serious eye damage, including cataracts, pterygium and cancer of the conjunctiva and cornea.5

Whereas overexposure to UV radiation may lead to skin cancer, inadequate exposure (in particular to UVB) results in vitamin D deficiency.6 Exposing the skin to UVB radiation from the sun produces about 90% of the vitamin D3 (cholecalciferol) that is bioavailable in the body.7 Some foods also contain vitamin D; however, for most people, adequate vitamin D intake cannot be achieved from the diet alone.7

Insufficient levels of vitamin D contribute to the development of osteoporosis, which in Australia costs more than $1.9 billion a year in direct medical costs and, for affected individuals, more than 25 700 disability adjusted life-years are lost each year.8 Suboptimal vitamin D levels have also been linked, although not conclusively, with several health conditions unrelated to bone, including multiple sclerosis, type 1 diabetes, bowel cancer, breast cancer, prostate cancer and non-Hodgkin’s lymphoma.6,9

Recent research indicates that a significant number of Australians have at least marginal vitamin D deficiency, with two studies showing rates of 43% in young women and 23% in the general adult population.10,11 Other groups within the Australian population, such as people with dark skin pigmentation, housebound or bedridden elderly people, and those who cover their skin for cultural or religious reasons, are more susceptible to vitamin D deficiency and have even greater levels of both marginal and frank vitamin D deficiency.7

Given the risks and benefits of sun exposure, there is an urgent need for appropriate health messages regarding adequate levels of sun exposure in Australia. Previous public health messages have sought to encourage people to reduce their UV radiation exposure by limiting their time in the sun when UV levels are highest, or by practising basic sun protection behaviours (use of shade, hats, clothing, sunglasses and sunscreen) if they do need to be outside.12 In this context, the pragmatic question is: “How much sun exposure is enough to provide adequate vitamin D levels without compromising skin and eye health?”.

Box 2 shows sun exposure times required for recommended vitamin D production (and erythema) at 12:00 when 15% of the body is exposed for people with type II skin. In all the cities in January, about 2–14 minutes of sun at 12:00 is sufficient to produce 1/6 to 1/3 MED, which would achieve recommended vitamin D production. However, as erythema can occur in as little as 8 minutes, exposure at 12:00 also increases the likelihood that people will be overexposed to UV radiation, thereby increasing their risk of skin cancer.

Gaining sun exposure further from 12:00 allows a greater margin for error between exposure to achieve recommended vitamin D production and exposure leading to erythema. Therefore, a range from the least to most time required at either 10:00 or 15:00 for sufficient sun exposure to produce vitamin D was also calculated (Box 3). The first figure provided is the time required to achieve 1/6 MED at 10:00 and the second figure is the time required to achieve 1/3 MED at 15:00. Throughout the year, there is wide variability between the higher and lower estimates for sun exposure for vitamin D synthesis at these times. The least possible amount of time for erythema to occur at either 10:00 or 15:00 is given in parentheses to highlight the risk of overexposure, especially during summer in northern Australian centres.

In Box 4, we give recommendations for sun exposure times at 12:00, as well as at 10:00 and 15:00, for people between the ages of 19 and 50 with Fitzpatrick type II skin and 15% of the body exposed. These times apply to people who have no other risk factors for skin cancer and are not using sun protection measures. Based on previous recommendations, and given the high UV levels in Australia throughout much of the year, exposure three to four times a week should be more than adequate.

Our aim was to contribute to developing practical guidelines for sun exposure for different cities throughout Australia using available data on solar UV radiation levels. Our results indicate that fair-skinned people without any underlying medical condition should be able to obtain sufficient vitamin D from short periods of unprotected sun exposure of the face, arms and hands outside of the peak UV period (10:00–15:00) throughout Australia for most of the year.

Across Australia care is required with sun exposure from October to March at 12:00, as the data indicate little margin for error with respect to excess sun exposure. However, in the southern regions, including Melbourne and Hobart from April to September, and Sydney and Adelaide in June and July, some exposure during peak UV times may be warranted to obtain and maintain adequate vitamin D levels. The time taken to achieve 1 MED at 10:00 in some centres is equivalent to the time taken to achieve 1/3 MED at 15:00 in other centres. Thus, sun protection measures are needed when employing the recommendations in Box 4 to ensure that overexposure does not occur.

Care should be taken in extrapolating recommendations to locations other than those we studied because of the variability of UVI readings at differing altitudes and latitudes as well as with cloud variability.13 Moreover, timing of readings was based on local times and so reflect UV exposure as influenced by daylight saving practices in each location.

For those at increased risk of vitamin D deficiency, such as housebound or bedridden elderly, it would be advisable to expose a greater body surface area than to increase the amount of time the skin is exposed. Oral vitamin D supplementation for these groups may be preferable to prolonged sun exposure, to reduce the risk of skin cancer. Moreover, the common practice of “sunning” housebound or bedridden people in front of closed windows will not significantly increase vitamin D synthesis as glass is a very effective UVB filter.

Our study has a number of limitations. Firstly, many researchers have suggested that optimal vitamin D intake has been underestimated and official recommendations are too low.17-19 If optimal serum vitamin D levels are higher than those recommended by Australian and international guidelines, the sun exposure times given (Box 2, Box 3 and Box 4) may be too short for adequate vitamin D production.

Secondly, data reported here are based on UV readings over a single year, which may or may not be typical, and data were averaged for each month. All figures are point estimates and will vary depending on solar UVB levels, as only UVB induces vitamin D synthesis. The intensity of solar UV radiation is influenced by geographical and environmental factors including latitude, cloud cover and ozone levels, which will therefore influence exposure times.

Thirdly, numerous factors affect the ability of the body to synthesise vitamin D from UVB radiation. Many of these are related to disruptions to the UVB–vitamin D pathway, including reduced bioavailability of vitamin D in obese individuals, disorders of the skin, gastrointestinal tract and kidney, and ageing.6 In addition, behaviours and certain characteristics that provide sun protection may compromise the ability of the body to synthesise vitamin D from UVB radiation. These include the use of sun protective clothing and sunscreen, as well as skin type.20-23 Our recommendations only apply to fair-skinned people and do not take into account exposure needed for people with darker skin pigmentation. People with very dark skin (Fitzpatrick type VI) require around six times more exposure to UV radiation to produce as much vitamin D as someone with Fitzpatrick type II skin.23 Based on our results, sunlight exposure during peak times is advisable for people with Fitzpatrick type V or VI skin in certain circumstances, especially in the southern region from April to September, and in the central region in June and July.

Fourthly, the aetiology of squamous cell carcinoma indicates that cumulative, or total, sun exposure is the major contributing factor.2 It therefore seems inevitable that deliberately seeking sun exposure for vitamin D-related benefits may result in some increase in squamous cell carcinoma risk.

Our study illustrates the complexities of calculating UV exposure times and clearly indicates that it is impractical to generate a simple nationally uniform message that prescribes minutes of sun exposure to the general population, given the number of variables that need to be taken into consideration. Even so, across Australia, from Townsville to Hobart, summer UV levels are high enough that short exposure under most circumstances for most people should be enough to produce recommended vitamin D levels, whereas slightly longer exposure times will contribute to skin cancer risk. Therefore, in summer, minimising sun exposure is the key. By contrast, in winter, our individual circumstances including geography, skin type and age all need to be taken into consideration when determining the appropriate level of sun exposure for good health.