NOTES AND CORRESPONDENCE. Operational Nowcasting of the Sun Protection Factor for the Urban Area of Vienna

Transcription

1 836 JOURNAL OF APPLIED METEOROLOGY NOTES AND CORRESPONDENCE Operational Nowcasting of the Sun Protection Factor for the Urban Area of Vienna ALOIS W. SCHMALWIESER AND GÜNTHER SCHAUBERGER Institute of Medical Physics and Biostatistics, University of Veterinary Medicine, Vienna, Austria 25 January 2000 and 6 September 2000 ABSTRACT A method was developed to produce hourly updated nowcasts of recommended sun protection factors (SPF) for different photobiological skin types. Actual UV ( nm) measurements and a forecast of the UV index provide the basis of the method. The method, used operationally for the urban area of Vienna, provides a useful tool to inform people about the use of sunscreens, thereby lowering the risk of health damage from overexposure to solar ultraviolet radiation. The validation of the nowcast shows that deviations from the measured daily dose depend on the time of the nowcast. Forecasts made 2 h before solar noon produce deviations of the daily dose of less than 0.5 SPF (for skin type I) in 68% of all cases; forecasts made two or more hours after noon were, in every case, within 0.5 SPF of the observed value. 1. Introduction Although there are many positive effects of outdoor activities, there is a risk of health damage from overexposure to solar ultraviolet (UV) radiation. Immediate UV-induced reactions of the skin are erythema and photodermatoses. Long-term effects are skin cancer and photoaging along with damaging effects to the immune system and DNA (WHO 1994). Several UV forecast systems have been established all over the globe (Vanicek at al. 2000), as well as UV monitoring networks (WMO 1997). Following the recommendations of the World Meteorological Organization (WMO) and World Health Organization (WHO), the erythemally effective solar ultraviolet radiation is expressed in units of UV index. The UV index is a dimensionless quantity for the erythemally weighted irradiance divided by 25 mw m 2. Because biological reactions are related to the dose, the UV index as a measure for the intensity is not a directly applicable recommendation and therefore is difficult for people to use. Another more descriptive quantity for the exposure to erythemally effective UV radiation is the sunburn time (SBT). The SBT gives the duration of exposure that causes a just noticeable reddening of the skin depending on the photobiological skin type. The SBT has two essential disadvantages. First, the SBT is Corresponding author address: A. W. Schmalwieser, Institute of Medical Physics and Biostatistics, University of Veterinary Medicine, Veterinärplatz 1, A-1210 Vienna, Austria. alois.schmalwieser@vu-wien.ac.at very short (in the range of 20 min) because the just noticeable reddening is hard for people to perceive. Second, there is a negative association with the term sunburn. So, these two units find little acceptance with the public. In contrast to the terms UV index and SBT, the sun protection factor (SPF) has high acceptance because it is related to cosmetics, beauty, and so on. The SPF is given on every sunscreen product and therefore is familiar to the general public. In general, the UV index forecast models deliver high accuracy for clear-sky situations (e.g., Schmalwieser and Schauberger 2000). The dominating factor for the intensity of downwelling UV radiation, however, is cloud coverage, a quantity that is difficult to forecast. As shown by several authors (e.g., Kuchinke and Nunez 1999), the cloud modification factor for a cloud coverage of, for example, 4/8 can vary between 0.4 and 1.1 depending on cloud type, height, and so on. If the actual cloud cover is taken into account, the forecasted daily course of the UV index can be used to nowcast the daily dose. To avoid overexposure, a threshold SPF can be calculated. 2. Method To reduce risk of health damage to the public caused by overexposure to solar UV radiation, we suggest a forecast of the SPF provided as a function of photobiological skin type. Such a forecast must be valid until sunset and must be updated continuously to include the actual weather situation. To fulfil these requirements, 2001 American Meteorological Society

2 APRIL 2001 NOTES AND CORRESPONDENCE 837 the developed nowcasting method consists of the following basics. The SPF is derived from the daily dose determined by measurements of solar irradiance. Therefore, in general, the exact SPF can be calculated only from the time course of measured irradiance. By using a UV forecast model, a first approach for an assessment can be given by a prediction of the daily course of the clear-sky irradiance. This prediction can be refined by correcting the predicted clear-sky irradiance with measurements of the biologically effective UV irradiance. Last, the nowcast procedure has to be validated to test the quality of the resulting forecast. a. Model calculations The erythemal effective irradiance is calculated using the Austrian UV forecast model. The model calculates the UV clear-sky irradiance using date, solar elevation, total ozone content, and altitude. A detailed description of the model can be found in Schmalwieser and Schauberger (2000). The model has been validated by comparison with measurements (Schmalwieser and Schauberger 2000) and with other models (Schmalwieser and Schauberger 2000; Köpke et al. 1998; Schauberger et al. 1997). For Vienna, the forecast shows high agreement ( 0.5 UV index) with measurements (Schmalwieser and Schauberger 2000) in 63% and an underestimation in only 12% of all cases. The remaining part (overestimation) of 25% is caused by the attenuation by clouds. b. Measurements Actual measurements with a temporal resolution of 1 min form the basis for the inclusion of the actual weather situation as well as for the final validation. The measurements are made with a UV biometer (Model 501, Solar Light Co.) that was developed based on the experience gained with the Robertson Berger meter design (Berger 1976). The spectral response ranges from 250 to 400 nm, corresponding to the Erythema Action Spectrum of McKinlay and Diffey (1987) and CIE (1987), which describes the spectral sensitivity of the biological effect. The device is part of the meteorological station at the University of Veterinary Medicine (48.3 N, 16.4 E, 153 m above sea level) and is part of the Austrian UVB-Monitoring Network (Schmalwieser and Schauberger 2001; Silbernagl and Blumthaler 1998). The calibration factors for the devices (Bodhaine et al. 1998) are derived two times per year corresponding to total ozone column and solar zenith angle as described by Silbernagl and Blumthaler (1998). For this derivation, dependence of solar zenith angle is recalculated continuously, and the ozone value is taken from the last measurements of the National Aeronautics and Space Administration Earth Probe Total Ozone Mapping Spectrometer. These data are available via file transfer protocol with a temporal delay of 1 day. As shown by Schmalwieser and Schauberger (2000), the usage of ozone values over Vienna with a temporal delay of 1 or 2 days does not effect UV index calculations significant. The station is situated very close to the Danube River and Danube Island. The river and island are two of the most important areas for outdoor activities for Viennese inhabitants during the entire year and are used frequently for leisure-time activities during the summer. c. Sun protection factor and the sensitivity of human skin The goal of our approach is to make recommendations for avoiding health damage caused by exposure to solar ultraviolet radiation. For describing the subjective erythemal potential of exposure to UV, the minimal erythemal dose (MED) is commonly used for observational studies in humans and other animals. The MED is a subjective measure based on the reddening of the skin; it depends on many variables, such as individual sensitivity to UV radiation, skin pigmentation, anatomic site, radiometric characteristics of the source, and so on. One MED is defined as an effective UV dose that causes noticeable reddening of previously unexposed human skin. Because individual humans are not equally sensitive to UV, values of 1 MED vary in human populations roughly within the range of J m 2 for white-skinned people. The standard erythema dose (SED) is a standardized measure of the erythemogenic UV radiation. One SED is equivalent to an erythemal effective radiant exposure of 100 J m 2. An individual SPF value SPF i for a commercial sunscreen product is defined as the ratio of the MED on the protected skin D MEDp to the MED on unprotected skin D MEDu of the same subject (COLIPA 1994), D MEDp SPF. (1) i D MEDu For a whole day out-of-doors, the SPF of a sunscreen has to ensure that the received erythemal effective daily dose D daily is less than or equal to the MED D MEDu, such that D daily D. (2) MEDu SPF The erythemal effective daily dose D daily is the sum of the irradiance that occurs over the whole day. For a vertically oriented receiving surface above snow-free ground, the irradiance is reduced by about ⅔ with respect to a horizontal surface (Schauberger 1990; Schauberger 1992). This reduction is used to assess the skin dose. Under the assumption of a whole-day exposure, the recommended SPF is calculated for a maximum UV skin dose of 1 MED on a vertical receiving surface, which is equivalent to 3 MED on a horizontal surface. To account for the variable photobiological skin

3 838 JOURNAL OF APPLIED METEOROLOGY TABLE 1. Rough definition and the effect of an erythemal effective dose of 250 J m 2 for different skin types and different orientations. The effect is given in units of minimal erythemal dose (MED) and standard erythemal dose (SED) and is calculated using relative sensitivity from DIN 5050 (1992) and the influence of orientation from Schauberger (1992). Skin type Definition Tan Burn Hair MED Horizontal receiving surface SED MED Vertical receiving surface SED I II III IV Never Sometimes Always Always Always Sometimes Rarely Never Red Blond Brown Black types (Table 1) the recommendation is given separately for skin types I to IV. d. Nowcast The nowcast procedure gives an hourly updated recommendation of SPFs for different skin types that is valid for the remainder of the day. The SPF is calculated by modifying the forecasted SPF for clear sky to account for the actual meteorological situation by considering the relation between recent measurements and the forecasted value of the erythemal effective irradiance. To avoid overexposure and resulting sunburns, the SPF has to reduce the remaining dose of the day sufficiently so that the individual MED is not exceeded. Thus, the sun protection factor SPF nc (st, o) depending on skin type st and orientation o, which is valid for the rest of the day, can be calculated by D nc SPF nc (st, o), (3) D minery(st, o) where D minery (st, o) denotes the individual MED, which depends on skin type st and orientation o. The expected remaining dose D nc is the sum of the expected erythemal effective irradiance E erync (t) until sunset, sunset erync t nc D E (t) dt. (4) nc For clear-sky conditions, the daily course of the erythemal effective irradiance can be calculated using the Austrian UV forecast model. In the case of cloud coverage, high aerosol content, and so on, the forecast has to be modified to correspond to the actual meteorological situation. The comparison of a forecast value of erythemal effective irradiance E eryfc (t) to the actual measured value E eryact (t) provides the necessary information for the modification. The necessary modification factor mf is defined as the ratio of the measurement and the forecast of the erythemal effective irradiance at the time of nowcast (t nc), E eryact(t nc) mf(t nc). (5) E (t nc) eryfc Following this, the nowcast of the expected dose until sunset D nc is given by sunset eryfc t nc D mf E (t) dt. (6) nc Last, the nowcast of the SPF depending on skin type and orientation of the receiving surface becomes sunset eryfc t nc 1 SPF nc(st, o) mf E (t) dt. (7) D minery(st, o) e. Validation The validation of the nowcast is done by comparison with measurements. Values for the most sensitive skin type I are used for validation with the deviations rounded to the nearest integer, because the SPF is given as an integer on sunscreens. 3. Results The nowcast procedure gives an hourly updated recommendation of SPF for the different skin types for the urban area of Vienna that is valid until sunset. To include the actual weather situation, the nowcast of the SPF is calculated by modifying the clear-sky forecast using the ratio between actual measurements of UVB ( nm) irradiance and the forecast value for the time of the nowcast. UVB irradiance is calculated every hour of the day using the Austrian UV Forecast Model Corresponding doses for the rest of the day are also calculated. Both calculations are made for clear-sky conditions. To include the actual weather situation, a comparison is made between the forecast of UVB irradiance for the time of the nowcast and the actual measurement at the same time. The ratio of these values is used as a modification factor, which is applied to the clear-sky forecast of the dose for the rest of the day. The measurement is the mean value over the past 60 min to avoid variations in the case of inhomogeneous cloud cover. The result is a dose for the rest of the day considering the actual weather situation. This dose is then converted to SPFs for different types of skin under the

4 APRIL 2001 NOTES AND CORRESPONDENCE 839 FIG. 1. Visualization of the recommendation for the sun protection factors (number inside the circles) at noon 10 Jun 1998 for Vienna for different skin types. The ladies are drawn corresponding to the definitions of Table 1 [red haired (left) for skin type I through black haired (right) for skin type IV] for an easy identification of the user s skin type. assumption of an exposure during the whole day for a maximum UV skin dose of 1 MED on a vertical receiving surface, which is equvivalent to 3 MED on a horizontal surface. The recommendation is given separately for skin types I to IV (Table 1). The nowcast starts daily at 0900 LT and gives the recommendation in graphical form (Fig. 1). The graphical presentation ensures that people can rate their personal skin type and the corresponding SPF in an easy way by visual identification. The nowcast is presented on the Web page of the Institute of Medical Physics and Biostatistics (www-med-physik.vu-wien.ac.at). During the summer, the nowcast is published by the video system of the Viennese subway information service, which is shown in various subway stations. The validation uses measurements of UVB irradiance for the nowcast days. These measurements were integrated up until sunset and compared with the nowcast values. The nowcasts were evaluated for a period of 3 yr, from 1 July 1996 to 1 July 1999, for the most sen- FIG. 2. Frequency of deviation of SPF for skin type I. Bars show the deviation of the nowcast values from the measured doses depending on the time (LT) of the nowcast. Data cover the period from 1 Jan 1997 to 1 Jan 1999.

5 840 JOURNAL OF APPLIED METEOROLOGY FIG. 3. Diurnal courses of UV index and modification factor at Vienna on 26 Jun In (a) the solid line shows the measurements, and the dotted line shows the forecast. In (b) the corresponding modification factors are shown. Modification factors are calculated as the ratio of the measurement and forecast values. The modification factor is set to 1 for all times 3 h and more before solar noon. sitive skin type I, which results in the highest SPF and thus the highest absolute deviations. The result is shown in Fig. 2. It is evident that the deviation in daily dose between nowcast and measurements decreases as nowcasts are made later and later in the day. The deviation for nowcasts that are made later than 2 h after solar noon is less than 0.5 SPF in every case. For nowcasts made 1 h after solar noon deviations less than 0.5 SPF occur in 91% of all cases. This percentage decreases depending on the time of the nowcast: 83% at noon, 76% at 1 h before solar noon, 68% at 2 h before solar noon, and 55% at 3 h before solar noon. From this, it can be seen that nowcasts produced 3hormore before solar noon, while helpful in the sense of radiation protection and health care, can be in error by 1 SPF or more. Nevertheless, we want to provide an approximate figure at this time of day. For this purpose, we set a nowcast produced 3 h or more before solar noon equiv-

6 APRIL 2001 NOTES AND CORRESPONDENCE 841 FIG. 4. Sun protection factor for skin type I in Vienna on 26 Jun The dashed line shows the clear-sky forecast of SPF, the solid line shows the measured SPF, and the dots denote the nowcast of the SPF. alent to the clear-sky forecast that is, the modification factor mf is set to 1. Figure 3 shows UVB irradiance during a day with variable cloud coverage. During the morning, the sky was cloudless and measurements were higher than the forecast values. The resulting modification factor is greater than 1 (lower panel of Fig. 3). For these low solar elevations, uncertainties in measurements and forecast are higher than for higher solar elevations so that the modification factors have to be used very carefully. In the late morning, variable cloudiness began. Around midday, cloudiness decreased and UVB irradiance therefore increased. A little later cloudiness increased again and UVB irradiance decreased rapidly. The resulting sharp peak leads to an overestimation of more than 1 SPF (Fig. 4), because the decrease of UVB irradiance was much steeper than it would be under constant cloudiness. As seen in Fig. 2, this case is one of the worst and occurs with a probability of 10%. 4. Discussion and conclusions Past work has shown the high exposure of the population to solar UV radiation during, outdoor activities during leisure time and holidays (Diffey et al. 1982; Kivisäkk 1987; Schauberger et al. 1992). Although information on risks of health damage induced by solar ultraviolet radiation has grown with time, the population s behavior has not changed significantly (George et al. 1996). The reigning beauty ideal and fashion style counter against these information campaigns, and the population enjoys sunbathing and tanning (Fiala et al. 1997). The infrared portion of the solar spectrum is felt as pleasant, and the visible portion induces cheerfulness. Tans draw people close to the reigning beauty ideals and compensate for real or imagined physical imperfections. An important fact for dealing with sun care is that UV-sensitive people expose themselves to the same solar radiation as UV-insensitive people (Stender et al. 1996). Further, Stender et al. (1996) showed that sensitive people in generally use sunscreens with an SPF that is only 1 unit above those used by UV-insensitive people. In addition, people tend to underestimate their personal sensitivity and overrate their personal skin type. Our approach has been to provide a nowcast procedure that gives a recommendation for SPFs valid for the rest of the day. The spatial representativeness of UVmeasuring sites was shown by Schmalwieser and Schauberger (2001). For a correlation coefficient of 0.9, the decorrelation distances are 168 km in the east west direction and 102 km in the north south direction. Therefore, nowcasts from a single measuring site can be spatially valid for the entire urban area of Vienna, where about ¼ of the Austrian population lives. In contrast to the UV index forecast, our nowcast procedure accounts for the actual weather situation by including actual measurements. A recent survey (Vanicek et al. 2000) has shown that most of the UV forecasts (following the recommendation of WHO and WMO) give UVB irradiance in units of a UV index at solar noon. Our procedure extends previous methods by giving recommendations of SPFs that are vaild until sunset for different skin types. The recommendations for the various skin types are provided graphically (Fig. 1) to help people to estimate their skin types by visual identification. For this purpose, the ladies of Fig. 1 are drawn corresponding to the definition of the different skin types (Table 1). The SPF is easy for the public to un-

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