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Donald O Mutti, Shane P Mulvihill; Estimation of Time Spent Outdoors Using Personal UV and Visible Light Dosimetry. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3641.
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Myopic children have been shown to spend less time outdoors compared to non-myopic children. In addition, the risk of the onset of myopia is reduced when more time is spent outdoors. Two factors that might be responsible for this protective effect are exposure to bright visible light or exposure to ultraviolet (UV) light when outdoors. Personal dosimetry may be a useful technique for measuring these two exposures.
Eighteen young adult subjects (24.9±1.9 years) wore two electronic personal dosimeters side by side on the upper arm for one week (7.1±0.4 days) during July-October 2013. One was sensitive to visible light (Daysimeter, Lighting Research Center, Rensselaer Polytechnic Institute) and the other to UV (National Institute of Water and Atmospheric Research, New Zealand). Each reading was accompanied by a date and time stamp. The Daysimeter recorded average lux exposure each minute. The UV dosimeter recorded UV exposure every 5 seconds. Subjects were instructed to go about their normal daily activities. Subjects also completed a custom activity frequency survey at the end of the week. The UV dosimeters were calibrated against a Kipp & Zonen UV-SBT radiometer to emphasize exposure to UVB. Being outdoors was defined as any reading >1000 lux on the Daysimeter or any positive reading on the UV dosimeter.
The Daysimeter estimate of time spent outdoors was the highest at 790 minutes per week (range = 207-1593) followed by the survey estimate of 538 minutes per week (range = 140-1680). The UV dosimeter provided the lowest estimate at 272 minutes per week (range = 92-768). Time outdoors by each method was significantly different from the other two (repeated measures ANOVA p<0.026 with Bonferroni correction). Each method was significantly correlated with the other two (r = 0.58 to 0.76, p<0.012). Time indoors from the survey was uncorrelated with time outdoors by any method (p<0.69).
Each method provides a significantly different estimate of time outdoors. The highest estimate using visible light is likely to be the most accurate. Surveys may be subject to poor recall of brief and non-specific outdoor activities. UV exposure may not always occur when outdoors due to car windows, low solar angle, cloud cover, or shade. Given that visible and UV light exposures differ, double monitoring of each type of exposure may be useful in studies of time outdoors and refractive error.
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