Journal of Photochemistry and Photobiology B: Biology 141 (2014) 106–112

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The enhancement of biological ocular UV radiation on beaches compared to the radiation on grass Guang-Cong Liu, Fang Wang, Yan-Yan Gao, Zheng Yang, Li-Wen Hu, Qian Gao, Jun-Chol Ri, Yang Liu ⇑ School of Public Health, China Medical University, No. 92 Bei’er Road, Heping District, Shenyang 110001, Liaoning, PR China

a r t i c l e

i n f o

Article history: Received 17 March 2014 Received in revised form 7 September 2014 Accepted 8 September 2014 Available online 13 October 2014

a b s t r a c t Background: The influence of albedo on ocular UV exposure has seldom been reported. This paper aimed to explore the enhancement effect on measured ocular UV radiation due to a sand surface compared to measured ocular UV radiation due to a grass surface. Methods: We measured ambient and ocular UV radiation over the beach and grass surface in Sanya City of China (18.4°N, 109.7°E). The experimental apparatus was composed of a manikin and a dual-detector spectrometer. Integration of both UVA and UVB radiation was used to denote UV radiation. Then biologically effective ocular UVB radiation (UVBE) and the ratios of UVBE of two surfaces were calculated. Result: Maximum of ocular UV radiation versus time over the two surfaces is bimodal. UVBE on the beach is significantly larger than UVBE on the sand, and UVBE peaked at different solar elevation angle (SEA) over the two surfaces (about 53° and 40° on the beach and grass, respectively, according to Bayesian regression). The maximum of ocular UVBE ratios is greater than two, which peaked SEA was about 50°. One hour’s cumulative radiation under sunny weather exceeds thresholds for photokeratitis, conjunctivitis and lens damage. Conclusions: Higher albedo significantly increased biological ocular UV radiation. Tourists on tropical beaches should take protective measures and avoid facing the sun directly, especially when SEA is around 50°. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The ocular hazard of solar UV radiation gained increasing importance since the depletion of the ozone was discovered. It is well-established that solar UV exposure, especially exposure to UVB and UVC, causes various ocular disorders, such as photokeratitis, conjunctives, cataracts and pterygium [1]. Some studies demonstrate that sunlight exposure is a risk factor for age-related macular degeneration [2–4]. Evidence also shows that UV exposure could be associated with uveal melanoma [5]. As a result, solar UV radiation has been increasingly measured in the past decades [6–11]. Hu et al. [11] discovered the bimodal distribution of the diurnal ocular UV radiation (UVR), and Gao et al. [12] later confirmed this phenomenon and found that both diurnal ocular UVA or UVB radiation are bimodal. The albedo of the surface might play an important role in ocular UV exposure because photons directly reflected at the surface contribute to the ocular exposure. For example, it is well known that prolonged exposure to UV on snow surfaces leads to photokeratitis. Grifoni et al. [13] reported a significant contribution of ground albedo and SEA in determining the erythemal UVBE irradiance. ⇑ Corresponding author. Tel.: +86 2423256666x5404. E-mail address: [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.jphotobiol.2014.09.009 1011-1344/Ó 2014 Elsevier B.V. All rights reserved.

Turner et al. [14,15] indicated that high albedo of either the horizontal or vertical zinc aluminum trapezoidal sheeting could enhance the erythemal effect. Sliney et al. [16] noted that terrain reflectance was much more important than solar elevations; and that daily UV exposure on the snow surface exceeds the ICNIRP guidelines (ICNIRP 2004) [17]. Several studies reported similar albedos for UVB radiation: approximately 2% for grass and approximately 9% for beach [18–20]. On the other hand, compared to UVB, the use of UVBE can help better interpret the biological ocular UV hazard because the biological effect of different wavelengths of UV on the eyes was proved to be significantly different [21–23]. For example, the photokeratitis action spectrum peaks at 288 nm and the photo conjunctivitis action spectrum peaks at 260 nm. We performed a UV measurement to explore the difference between ocular UV radiation on grass and beach surfaces using a manikin head with spectral detectors. Both ocular UV and UVBE were used to assess ocular exposure. 2. Study location The study was performed in Sanya City (18.4°N, 109.7°E, average altitude 18 m) in the Hai Nan Province of China. Sanya is the southernmost city on Hai Nan Island. The experiment was

G.-C. Liu et al. / Journal of Photochemistry and Photobiology B: Biology 141 (2014) 106–112

conducted on May 11th (on the grass) and 14th (on the beach), 2013. The maximum solar elevation of the two days during which we performed our measurements were both nearly 90°. The air pollution index (API) was under 30 during the whole month. The ozone measured by the Ozone Monitoring Instrument (OMI) was 270 and 275 Dobson Units for the 11th and 14th of May, 2013, respectively. We selected open grassland in a park in Sanya City as the grass surface. Ocular UV radiation on the beach was measured on a public beach (the Haitang Beach) (Fig. 1).

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3.2. Data calculation We calculated the radiation of each integer wavelength range from 300 to 400 nm by linear interpolation based on the original data. Next, integration of the UVA (320–400 nm), UVB (300–320 nm) and UVR (300–400 nm) radiation was calculated separately based on the interpolated data. The UV irradiance dose (H) for specific time intervals was calculated as follows:

H¼

Z

SðkÞT T

3. Measurement devices The equipment consists of a rotatable base, a 3-layer black shelf and a manikin with realistic facial features. The base was set to turn 1 circle per minute automatically, and black shelves were chosen to prevent their reflection, which may confound the results. The height of the equipment is 1.75 m and its eye level is approximately 1.65 m high. The visual line was approximately 10° below the horizontal line. The height of the nose bridge on the horizontal plane from the eye was approximately 0.6 cm. The horizontal distance between the superciliary arch and eye surface was approximately 0.6 cm. The FOV of the manikin is 139° as shown in Fig. 1. A computer-controlled spectrometer was placed on the middle layer of the shelf. It has two detectors: one was laid at the most anterior point of the manikin’s right eye on a plane tangent to the right cornea, which was used to record ocular UV exposure; the other was placed on the head of the manikin to simultaneously record ambient solar UV exposure. Details about the spectrometer were introduced in our previous papers [12]. 3.1. UV radiation measurement The measurements were performed only under an uncovered sun. Small clouds far from the sun were ignored. Measurements were halted if cloud coverage of the sun occurred and were repeated when the sun was uncovered. The measurements were conducted on the 11th and 14th of May, 2010 from 07:40 A.M. to 18:00 P.M. Central Standard Time (CST; solar noon at approximately 13:00 P.M.). The manikin turned clockwise at a constant speed (1 circle per minute) during data collection. The manikin was set to face the sun initially. The investigation interval was 10 min. During measurement, each spectrometer detector collected data every 2 s (unit l lW/cm2) and the duration of each measurement progression was 1 min.

The ratios of ocular UVB and UVBE radiation were also calculated. Specific wavelengths of UVB (300, 305, 310, and 315 nm) were selected to further explore the influence of albedo on ocular UVBE. According to our previous study [12], the maximum ocular radiation occurred when the SEA was approximately 40°; however, we hypothesised this SEA might vary with changes in the surface. Another study has demonstrated that ocular UV exposure versus time can be well-fitted when divided into three parts: the SEA 6 30° as the first part, which was regressed to an increasing line; 30° < SEA 6 60° as the second part, which was regressed as a quadratic function; and SEA > 60° were treated as the third part, which was regressed to a horizontal line [24]. Thus, we performed a regression on the middle part to find the corresponding SEA in the morning when UV radiation peaks because the weather was the clearest in the morning. Compared to traditional regression, we used the Bayesian approach that can take all variations into account. The regression was performed using WINBUGs software. 3.3. UVBE radiation UVBE radiation is the spectral radiation weighted by the action spectrum for a specific biological process, according to the following equation:

UVBE ¼

Z

SðkÞAðkÞdðkÞ

UV

where S(k) is the measured spectral radiation, A(k) is the particular action spectrum and d(k) is the wavelength increment of the spectral data, 1 nm in this case. In this paper, the action spectra for photokeratitis, conjunctivitis and cataracts from 300 to 320 nm have been employed [21–23]. We also calculated the diurnal cumulative dose of the biological UVBE for photokeratitis, conjunctivitis and cataracts for each hour to assess the true risk for people in tropical area.

Fig. 1. Two backgrounds of our measurements and details of the manikin.

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G.-C. Liu et al. / Journal of Photochemistry and Photobiology B: Biology 141 (2014) 106–112

4. Results The ambient and ocular UVB exposure (containing the average and maximal radiation) at each time point on the beach and grass are shown in Fig. 2. All peak values occurred in the morning. The peak ocular UVB is 244 lW/cm2 (SEA = 59.73°) on the beach and 171 lW/cm2 (SEA = 46.79°) on the grass, not at the highest SEA as the public may expect. The maximal ocular UVB exposures versus time tend to be bimodal. The ocular UVB radiation values versus the SEAs is shown in Fig. 3. Compared to the difference of ambient radiation between the two surfaces, the difference between the ocular exposures and the UVB ratio between the two surfaces, which are shown in Table 1, is much larger. The difference expanded with the increase of ocular UV radiation, and the peak radiation on the beach surface was more than twice as high as that on the grassland surface. The corresponding SEAs that peak ocular radiation occurred at over the two surfaces were significantly different and they differ from the SEA identified by our previous study that was performed on an asphalt surface [12]. Ocular UVBE values versus the SEAs on the two surfaces for photokeratitis, conjunctivitis and cataract are presented in Figs. 4– 6, respectively. As the curve in the morning is smoother than that of the afternoon, we only included data obtained before noon in the regression. The Bayesian regression indicated that the maximum radiation of all three groups of UVBE occurred when the SEA was approximately 53° on the beach and 40° on the grass. Fig. 7 shows the equations and fitted curve of the regressions. Table 1 presents the ratios of the ocular UVB and UVBE (per ocular disorder) of the beach compared to the grass at changing

Fig. 3. Maximum UVB radiation versus SEA on beach and grass surfaces.

times and SEAs. The maximum ratios occurred when the SEA was 49.59° (2.17, 2, 2.18 and 2.28 for photokeratitis, conjunctivitis, cataracts and UVB). Namely, the beach surface’s higher albedo enhanced the UVBE radiation to nearly 200% of the UVBE radiation on the grass surface. It is obvious that the ratio of ocular UVB is greater than the ratio of UVBE (per ocular disorder), and the ratios increase with the enhancement of the beach’s UVBE (per ocular disorder) radiation (Fig. 8). Fig. 9 shows the ratio of specific wavelengths of UVB radiation on the beach and grass. We can see that the longer wavelengths correspond with higher ratios, and the ratios reach peaks when the SEA is approximately 50°. From Fig. 3, we can see that when the SEA was below approximately 30°, the curves of the two surfaces are almost the same. However, the radiation on the beach continues to increase after the radiation on the grass peaks. UVB radiation peaked earlier on the beach than on the grass in the afternoon. The one-hour cumulative UVBE radiation is shown in Table 2. 5. Discussion

Fig. 2. Maximum and average UVB radiation versus time on beach and grass.

Quite different to the public perception, we found that the ocular UV radiation peaks when the SEA is not the highest over the two surfaces (but when the highest SEA is above 55° for the beach surfaces or 45° for the grass surface). This discovery can remind people to take protective measures when going out at this time period when they might neglect to do so because of the relatively lower ambient solar UV radiation that they can sense. Ocular UVB radiation is significantly larger in the present study [12,25]. From Fig. 2, we found that before 9:00 A.M., the maximum radiations on the beach and grass surfaces have similar increasing trends, but the beach curve continues to increase. This can be explained by the varying proportion of the direct UV, the reflected UV and the scattered UV that compose ocular UV radiation. When the SEA is approximately 30°, direct solar UV rays make up the largest proportion of ocular exposure; as the SEA increases, UV reflection from the surface and scattered UV radiation increases as a consequence of enhancing the ambient UVB. Our manikin’s FOV is 139° as shown in Fig. 1. From this Figure, we can conclude that when the SEA was above 58°, direct UV radiation diminished to zero because the direct UV rays were out of the upper bound of the manikin’s FOV; instead, the ocular UV radiation was composed of scattered UV and reflected UV radiation. The difference between the ocular UV radiation on the two surfaces was obvious: radiation on the grass began to stabilise while the radiation on the sand was still rising. After the SEA reached 81°, the reflection UV also vanished because it exceeded the lower bound

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G.-C. Liu et al. / Journal of Photochemistry and Photobiology B: Biology 141 (2014) 106–112 Table 1 The ratio of UVB and UVBE radiation over the beach and grass versus the SEAs and time. Time

SEA 11th

SEA 14th

Photokeratitis

Conjunctivitis

Cataract

UVB ratio

07:40:00 08:20:00 09:10:00 09:50:00 10:30:00 11:10:00 12:00:00 12:40:00

19.91 29.17 40.86 50.28 59.73 69.2 81.06 89.2

20.09 29.33 40.99 50.38 59.81 69.26 81.1 89.38

1.17 1.07 1.35 1.91 2.22 1.53 1.45 1.44

1.08 0.88 1.3 1.84 2.07 1.32 1.22 1.16

1.17 1.1 1.35 1.91 2.22 1.54 1.46 1.46

1.32 1.39 1.39 1.96 2.32 1.68 1.6 1.65

Fig. 4. Maximum and average ocular UVBE (photokeratitis) on beach and grass.

Fig. 5. Maximum and average ocular UVBE (conjunctivitis) on beach and grass.

of the FOV. During this period, the ocular UV radiation was composed of only scattered UV, which tended to remain stable. Similarly, the biological facial erythemal effect on galvanised corrugated surfaces enhanced approximately 190% of radiation comparing to the radiation on the grass [26]. Ocular UVBE ratios versus time are also bimodal, and they increase with the enhancement of the ocular UVBE radiation on the beach. When the SEA is low (