Continuous long-term monitoring of UV radiation in professional mountain guides reveals extremely high exposure



Ultraviolet radiation (UVR) is estimated to be one of the most important risk factors for nonmelanoma and melanoma skin cancers. High occupational UV exposure is assumed to be associated with skin cancer. Mountain guides receive considerable UV doses due to altitude-related increase of UVR and reflection from snow- and ice-covered surfaces. The aim of our study was to assess the annual occupational UV exposure of mountain guides. Spore film test chambers containing spores of Bacillus subtilis (VioSpor) were used as UV dosimeters with a spectral sensitivity profile similar to erythema-weighted data calculated from spectroradiometric measurements. Nine mountain guide instructors carried dosimeters on the sides of their heads on a total of 1,406 working days during one year (July 1999–June 2000). Dosimeters were changed monthly. Measurements of 92 months could be evaluated (4–12 months/mountain guide). The mean individual monthly UV exposure was 107 standard erythema doses (SED) (median 71 SED; range 10–505 SED). The mean annual cumulative UV exposure was 1,097 SED (median 1,273 SED; range 312–1,770 SED) per mountain guide. The mean UV dose per day (4–10 hr) was 6.6 SED (median 5.7 SED; range 0.6–24.2 SED). This is the second study of continuous annual UV dosimetry in a cohort of outdoor workers. Our study showed that it is not sufficient to interpolate annual UV exposure from a few days' measurements. Only long-term dosimetry can give reliable yearly information of UVR load. Median daily UV exposure exceeded limits for UV radiation (e.g., ACGIH effective dose 30 J/m2 per 8 hr period corresponding to 1.08 SED/day) 6-fold; maximal exposure exceeded these limits 23-fold. These extremely high exposure values are suggestive for an increased risk of skin cancer and thorough epidemiologic studies in the collectives of professional and recreational mountaineering are required. © 2002 Wiley-Liss, Inc.

During the last decades an epidemic increase in skin cancer has been observed in the Caucasian populations of industrial countries. A rise in nonmelanoma skin cancer has been noted particularly in Australia, the United States and Central as well as Northern Europe.1, 2, 3 UV exposure is considered to be the most important risk factor for development of nonmelanoma skin cancer and melanoma.4, 5 High occupational UV exposure is assumed to be associated with skin cancer.4, 6, 7, 8, 9

Numerous epidemiologic studies explored the relationship between skin cancer development and UV exposure, which were assessed by questionnaires in a retrospective manner.5 However, the association between estimated exposure and dosimeter readings has been shown to be poor.10

Sensitivity of the human skin to UVR is dependent on the wavelength of UV radiation from 250–400 nm.11, 12 Biologically weighted dosimeters integrate the UVR effect over the whole spectrum and simulate the relative sensitivity curve critical for human skin for each individual wavelength within this range.

In various studies, polysulphone dosimeters as small portable badges have been used to determine the anatomical distribution of sunlight on dummies and living subjects during various occupational and recreational activities.10, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 The optical absorbency of the film material increases in a dose-dependent manner upon exposure to UVR especially in the UV-B range (280–315 nm).16, 17, 23

Recently a new approach to UV dosimetry has been developed. The biologic effects of UV light are measured by determination of the harmful effects on spores of Bacillus subtilis. After UV irradiation, the spore film is allowed to germinate in culture. The proteins synthesized by the bacteria exposed to UVR are photometrically quantified and compared to controls with defined radiation exposures. Applying this new method, standardized personal dosimeters have already been produced and tested under different stationary conditions24, 25, 26, 27, 28 and in personal dosimetry.29, 30, 31, 32, 33, 34, 35, 36

The highest UV exposures have been measured in mountain guides.31 During 23 different occasions of mountaineering activity in the Swiss Alps, Alaska, Bolivia and Tibet, we previously found a mean personal daily exposure of 29.8 SED (standard erythema doses). Personal UV doses ranged from about 11 SED/day to more than 42.5 SED/day.31

The aim of our present study was to measure the annual occupational UV exposure of mountain guides, a professional group that is highly exposed to UVR from the sun.



The dosimeter system is enclosed in an aluminum capsule with a diameter of 32 mm, a thickness of 9 mm and a weight of 15 g. Preparation and processing of the spore film detectors (VioSpor, Blue Line Type II, BioSense, Bornheim, Germany) were performed as described previously.26, 27 The dosimeter system (VioSpor) integrates the UVR effect over the whole spectrum in the solar UVB and UVA region (290–380 nm) with a good correlation with the CIE/MED reference spectrum.12, 26, 37 Thus the sensitivity of the spore film dosimeter represents the action spectrum for erythema reaction in human skin. In cutaneous photobiology, radiant exposure is frequently expressed as “exposure dose” in units of J/m2. “Biologically effective dose,” derived from radiant exposure weighted by such an action spectrum, is expressed in units of J/m2 (effective) or as multiples of “standard erythema dose” (SED) or “minimal erythema dose” (MED). One MED has been defined as the lowest radiant exposure to UVR that is sufficient to produce erythema with sharp margins 24 hr after exposure. When the term MED is used as a unit of exposure dose, a representative value is chosen for sun-sensitive individuals.4 The MED will vary according to the wavelength range over which the effective UVR is summed and for radiation protection purposes it is generally taken in the range of 200–300 J/m2 effective.38 The UV doses measured by spore film dosimeters are given in “biologically weighted” standard erythema doses or minimal erythema doses. One SED corresponds to 100 J/m2 normalized to 298 nm; one MED corresponds to 250 J/m2 normalized to 298 nm.

Exposure limits are issued at defined wavelengths but also as effective dose. The threshold limit values (TLV) in terms of effective dose is 30 J/m2 per 8 hr workshift when using the ACGIH action spectrum39 corresponding to 109 J/m2 per 8 hr workshift (1.09 SED; 0.43 MED) using the CIE action spectrum (Jan Laperre, personal communication, 2002).

Study subjects and dosimetry

Nine mountain guide instructors of the German Mountain Guide Association wore dosimeters on all outdoor working days during 1 year (July 1999–June 2000). One dosimeter was used per person per month.

Professional activities were located predominantly in the Alps. All activities were recorded in a standardised diary indicating date, location, time, type of activity, weather and surface conditions.

Four dosimeters were lost. Twelve dosimeters were not used because in these months no professional outdoor activity had been performed by the corresponding mountain guides. Therefore, measurements of 92 months (4–12 months per mountain guide) with a total of 1,406 outdoor working days could be evaluated.

The mountain guides often started working before sunrise. They worked for 4–10 hr/day during the mountaineering activities.

The face is the most exposed body site of an alpinist. During mountaineering in general at least the central (nose, mouth, chin) and lateral (cheeks) parts of the face are not covered by clothing and therefore exposed to UV radiation.

Therefore we chose a vertically oriented dosimeter attached laterally to the head. Dosimeters were attached with a clip to the cap or to the bow of spectacles. Such a vertical orientation registers both incident UVR from the sun at moderately low solar elevation angles and UVR reflected from snow- or ice-covered surfaces.


The dosimeters were well accepted by all mountain guides. The subjects performed their ordinary professional activities, such as rock and ice climbing, hiking, skiing, alpine skiing, cross country skiing or mountain biking.

The mountain guides performed outdoor activities on 2–30 days per month (median 15 days per month).

UV exposure of the mountain guides' lateral faces, registered by dosimeters that were changed every month, were in the range of 10–505 SED per month (mean 107 SED/month; median 71 SED/month). A mean cumulative annual UV exposure of 1,097 SED was measured (median 1,273 SED; range 312–1,770 SED). Averaged over the 12-month period, the individual UV exposure on the side of the face was 6.6 SED/day (median 5.7 SED; range 0.6–24.2 SED/day).

In the course of the year the mountain guides encountered intense UV radiation not only during summer but already in March to May, due to (spring) alpine skiing (Fig. 1). The most extreme individual UV exposure was found during expeditions to the Himalayas in September and October (mountain guides no. 1 and no. 2, Fig. 1). But also rock climbing in low mountain ranges resulted in very high personal UV exposure (mountain guide no. 6 in June).

Figure 1.

Monthly mean UV exposure (SED/day) of 9 mountain guides (MG) during 1 year. Highest exposures were registered in spring/early summer (probably due to alpine skiing and even rock climbing; e.g., in mountain guide no. 6) and in fall (e.g., in mountain guide no. 1 and no. 2 due to an expedition to the Himalayas).


Continuous monitoring of the annual UV exposure performed in the present study showed a mean personal UV exposure of 1,097 SED (median 1,273 SED; range 312–1,770 SED). UV exposure varied depending on the number of days working, season and type of activity.

In the literature the highest personal UV exposures have been registered during mountaineering activities.13, 15, 16, 17, 18, 31 In a previous study we measured UV exposure on the lateral head during selected mountaineering activities and found a mean exposure of 29.8 SED/day. Based on these data for mountain guides, an annual UV exposure of 5,000 SED had been estimated.31 Our previous studies on mountain guides31, 36 were the stimulus to perform this continuous long-term monitoring of UV radiation. Our presented paper showed that it is not possible to estimate a yearly UV exposure from a few days' measurements. Therefore long-term measurements are needed in order to assess the outdoor UV exposure of a given professional group.

In the mountain environment professional outdoor workers such as mountain guides as well as recreational alpinists encounter increased erythemogenic UV radiation due to increased effects of altitude and reflection from snow and ice. An increase in elevation of 1,000 m is associated with an increase in UV irradiation of 9% at 370 nm, of 11% at 320 nm and of 24% at 300 nm.40 The proportion of reflected radiation in relation to incident radiation (albedo) can reach almost 100% in fresh fallen snow.41

In a study with watermen in Maryland/USA, the personal average annual exposure of facial skin was estimated to range from 33–260 MED (1 MED = 350 J/m2) corresponding to about 82.5–650 SED.6 In that study, facial exposure was assumed to be 1–8% of ambient radiation.6 However, based on measurements of facial exposure of both mannequins and human subjects later, it was suggested that the exposure of an unprotected face is probably close to 20% of ambient radiation.4 Using this estimate, the annual facial exposure doses reported in the watermen outdoor worker group would be about 200–1,250 SED.4

In one single study by Vishvarkarman et al., the annual occupational UV exposure had been measured:22 The UV exposure of Australia Post mail delivery personnel and physical education teachers have been monitored with polysuphone dosimeters at various body sites (vertex, hands, shoulders, back, chest, thighs). The annual exposure ranged from 120–440 kJ/m2 for the 2 professional groups (= 1,200–4,400 SED/year). E.g., on the chest the mean annual UV exposure was 1,920 SED for the post mail delivery personnel and 1,400 SED for the physical education teachers. The ambient exposure was also measured and used to compute ambient exposure fractions for the different body sites over an entire year to enable model calculations on human exposure.22

The facial UV exposure of mountain guides seems to be similar to the chest UV exposure of Australian physical education teachers and postmen. However, due to the different dosimeter systems, the different type of activities and different location of dosimeters, our study and the report of Vishvakarman et al. cannot be directly compared. Climbing and skiing mountain guides permanently were changing in altitude and encountered varying surrounding surfaces. So dosimetry of personal UV exposure of mountain guides could not be correlated with “static” measurements of ambient UVR at arbitrarily chosen locations in order to calculate a relative personal dose as a fraction of ambient radiation.

For ultraviolet radiation exposure, threshold limit values (TLV) have been issued by the American Conference of Governmental Industrial Hygienists (ACGIH), the International Radiation Protection Association (IRPA) and the International Commission for Nonionizing Radiation Protection (ICNIRP).11, 39, 42

Exposure to solar UVR is cumulative over the whole spectrum. TLVs are given for defined wavelengths but also as biologically effective dose. Theoretically exposure to each wavelength could be measured with spectroradiometers, a procedure that is not practicable in personal dosimetry. TLV in terms of effective dose is 30 J/m2 using the ACGIH action spectrum,39, 42 which differs from the CIE action spectrum,12 to which the spectral sensitivity of the spore film dosimeter used in our study is corresponding. The relation between the 2 action spectra was calculated as follows (for solar radiation only) (Jan Laperre, personal communication, 2002): effective dose CIE = 3.63 × effective dose ACGIH.

With this equation, the effective dose ACGIH (30 J/m2) can be translated towards an effective dose CIE: 109 J/m,2, i.e., 1.09 SED per workshift.

Therefore the averaged median daily UV exposure of mountain guides exceeded the TLV's by a factor of 6, the maximal exposure by a factor of 23.

The annual exposure of indoor workers in Northern Europe is estimated to be 250 SED (100 MED) (20% occupational, 50% recreational, 30% vacational).43 For outdoor workers, it is assumed that occupational exposure adds another 450 SED (180 MED) to a total UV exposure of 700 SED.4, 44

So mountain guides (median 1,273 SED and maximum 1,770 SED per year) received median 2.8 times and maximum 3.9 times the expected occupational UVR load for outdoor workers. The total annual UV exposure of mountain guides (median 1,273 SED and maximum 1,770 SED per year + 250 SED to cover holidays and leisure hours) will be median times 2.2 and maximum times 2.9 the total expected UV dose for outdoor workers.

The risks of solar and ultraviolet radiation to humans are summarized in an IARC monograph4 and in a review by Armstrong and Kricker:5 An association between occupational solar UV exposure and nonmelanocytic skin cancer seems to exist. However, the distribution of basal cell carcinoma is not as closely related to the distribution of exposure to the sun as is that of squamous cell carcinoma. Chronic exposure as assessed through occupational exposure appeared to reduce melanoma risk. Most studies showed positive associations with measure of intermittent exposure such as particular sun-intensive activities, outdoor recreation and vacation.4, 5 In the study with Maryland watermen, the estimates of individual annual and cumulative exposure to UVR were positively associated with the occurrence of squamous cell carcinoma but not with the occurrence of basal cell carcinoma.6, 7 Thus high occupational UV exposure was shown to be associated with an increased risk for squamous cell carcinoma only.

High occupational exposure to solar UV radiation may have an effect on the human immune system as well.45 Contact hypersensitivity as a measure of immune responsiveness in humans was suppressed by UV radiation. Similarly, an overall reduction of T-cell memory responses to microbial antigens resulted from UV exposure. And, recognized by the subjects themselves and also shown experimentally, UV irradiation is one common triggering factor for the reactivation of Herpes simplex virus.45 High incidence rates of seborrhoeic dermatitis have been reported in HIV-infected individuals, indicating immunosuppression to be involved in the pathogenesis. In mountain guides seborrhoeic dermatitis was also found in a clearly higher percentage than in the general population, suggesting UV-induced immunosuppression as a pathogenic principle.46

Since outdoor workers, such as mountain guides, must spend up to 40 hr per week exposed to UV radiation, it is not possible for this population to completely avoid UV exposure. Therefore, the use of sunscreens and protective clothing are reasonable protective strategies.

Observational data on Californian outdoor workers indicate that about 50% of the workers had inadequate protection. In particular the face, which has the highest incidence of skin cancer, was one of the least protected body sites.47

In conclusion, the Bacillus subtilis spore film dosimeter was used as a personal dosimeter to measure the annual occupational UV exposure of mountain guides. Our study demonstrated that only long-term dosimetry can give reliable yearly information of UVR load. It is not sufficient to interpolate from a few days' measurements. UV exposure of mountain guides is far beyond international UV exposure limits. These extremely high exposure values are suggestive for an increased risk of skin cancer. Thorough epidemiologic studies in the collectives of professional and recreational alpinists are required.


We to thank the instructors of the German Mountain Guide Association for their cooperation to perform this study.