Outdoor heat stress and ultraviolet-induced erythema by months in Florence: useful information for the local population and tourists

Solar radiation affects human beings in different ways with direct effects occurring when people are exposed outdoors during working or leisure/sport activities. Erythema is the most familiar short-term symptom of human skin associated with overexposure to unperceivable ultraviolet radiation (UV). In this study the newly developed Universal Thermal Climate Index (UTCI) will be used for the first time to investigate whether the perceived outdoor heat stress might represent a valuable proxy for the unperceivable effect of UV-induced risk of erythema in a Mediterranean city, Florence, Tuscany (Central Italy). The aim of this study is therefore to propose a real approach to link solar UV radiation and thermal factors by means of the application of two internationally recognized indices, in order to estimate their reciprocal interaction and the potential impact on human health. Meteorological data and UVB (280-320 nm) measurements were obtained for the 2004-2012 period by a weather station located in the municipality of Florence. The UTCI was assessed and continuous measurements of erythemally effective UV (UVEry) were performed by means of broadband temperature-corrected radiometers with the spectral response close to the erythemal action spectrum. Hourly UVEry doses were expressed as Standard Erythemal Doses (SEDs). Descriptive analyses of the hourly distribution per month of the frequencies of days with heat stress and UVEry exceeding 2.0, 3.0, 4.5 and 6.0 SEDs were carried out based on the general skin-type characteristics. The association between UVEry and UTCI was analyzed by a two-way contingency table approach. The probability of UVEry exceeding specific SED thresholds when heat stress occurs was often significantly higher than the same probability when no heat stress is perceived. However, during several months, too many days occur without any signs of heat discomfort, even when people may be exposed to relevant doses of harmful UVEry for the skin of various phototypes. These findings underlie the need for public health authorities to provide differentiated advice per month in relation to potential UV skin damage in the city of Florence. Such specific UV information is fundamental for educating the local population and tourists and making them aware of sun exposure.