Human exposure to solar UV mainly depends on: the ambient UV levels, the protection employed and the anatomical distribution of individual exposure for different activities. To estimate the long term human UV exposure, the following equation (Wong et al., 1996) for UV exposure to a selected body site was used:

(1)

where for a selected period of exposure time, M months, the day is broken up into hourly intervals labeled by i, and the time interval, M, into monthly intervals labeled by h. AE is the ambient erythemal UV exposure (the total amount of UV falling on a horizontal plane, which was measured by ambient UV monitoring stations), FO is the activity index (the proportion of time spent outside) which is assumed to be the same pattern all year, ER is the fraction of ambient UV radiation falling on a particular anatomical location (in this study this is ignored), PF is the protection factor (reduction of UV exposure by a protection device) and N(h) is the number of days in the month (Wong et al., 1996). The number of working days used in this study is 5 days per week and the values of PF is equal to 1, so it is regarded as the ‘worst case scenario’. However this is sufficient to provide an indication of the effect of varying the time of the day spent outdoors.

The ambient erythemal UV levels in Toowoomba (27.5° S, 151.9° E, elv. 693m) were monitored continuously by a UV monitoring station (Monitor Sensors, 7-9 Industry Drive, Caboolture, Qld, Australia). The monitoring station was calibrated seasonally against the erythemal spectral irradiance by employing the erythemal action spectrum (CIE, 1987) and the spectral irradiance recorded by a spectroradiometer as described previously in (Wong et al., 1995). The average ambient erythemal UV exposure in Toowoomba at each six minute interval of the day was recorded by the monitoring station and averaged for each hour every month. These values were then used as the average hourly exposure for each hour of the day to produce the average hourly exposure for each month.

The activity index (FO) is the probability of exposure of a population group as a fraction of the time of day split into hourly intervals. The activity index for summer, winter and spring was obtained for this group of home workers via a questionnaire that subjects’ completed over five consecutive days starting from the 13 February 1997, 21 July 97 and 10 October 97 respectively. The subjects were asked through a questionnaire to indicate on a bar graph the times they were indoors or undercover in the shade. The time spent outdoors but in the shade was weighted by a factor of 0.5 in a similar manner to Airey et al., (1997).

The five day activity index collected through the course of this experiment does not take into account weekend activities. The calculations performed are in order to calculate occupational exposure of home workers at a sub tropical location of Toowoomba in south east Queensland. The number of days, N(h), in equation (1) was taken as the number of working days in each month. The activity index for each hour was averaged over the five days of each questionnaire period. The activity index was linearly interpolated between the questionnaire periods to provide an activity index at intermediate points. The exposure ratio, ER, is defined as the fraction of ambient UV radiation reaching a particular body site. For this study, the value of ER was taken to be 1, which represents the vertex of the head in an upright body position.

The ratio of the contributions to risk of developing basal cell carcinomas (BCC) and squamous cell carcinomas (SCC) is governed by a power law relationship, as shown in equation 2 (Wong et al., 1996):

where UV_i is the personal erythemal annual UV exposure after protective strategies have been used, UV_j, is the personal annual erythemal UV exposure without any protective strategies and k is the factor for SCC (2.5) and BCC (1.4) (Diffey, 1992). As the home workers do not adhere to a strict timetable for outdoor activities, such as gardening, various scenarios for the time spent indoors were investigated, namely, (1) 12:00 to 13:00 Australian Eastern Standard Time (EST), (2) 11:00 to 12:00 EST, (3) 08:00 EST to 10:00 EST, (4) 14:00 to 16:00 EST, (5) 07:00 EST to 12:00 EST and (6) 12:00 to 17:00 EST. By investigating the effect of time of day when outdoor activities are undertaken by home workers, the change in UV exposure can be investigated. It was assumed in this study that during periods of no UV exposure (as indicated by the activity index), the subjects were indoors or in a location where they were not exposed to solar radiation. No other UV protective devices such as hats and sunscreen were assumed to be used in this study.

Figure 1 shows, as an example, the average activity index for the home workers during 1997. The home workers indicated that for a large proportion of the day, they were exposed to small, infrequent levels of solar UV. This is to be expected, as the home workers were not outside in the sun for defined periods of the day when compared to say, a group of outdoor workers.

Figure 1 - Activity Index for home workers in Toowoomba during 1997

Figure 2 shows the estimated year long occupational exposure to the vertex of the head (in MED) for the Toowoomba home workers for each month in hourly intervals. The highest exposure period for this group was in the summer months (November through to February), but they still received a high exposure, in excess of 0.25 MED in one hour in winter. The average estimated exposure to the vertex of the head between 12:00 and 13:00 EST during summer (February) was 4.9 times higher when compared to that in winter (July). The effect of any protective devices, such as hats and sunscreens has been neglected in this study. Consequently, the presented results are regarded as the ‘worst case scenario’.