The wind velocity at HQmix = 1 corresponds to the critical wind speed in Fig. 4(a). Fig. 4(b) represents the probability and Table 5 summarizes the number of hours in a year for the wind velocity to be below the critical wind speed. The whole year corresponds to 8760 h. The C3 is the least inherently occupational healthy process route among the alternatives. It exhibits significant exposure risk (HQ > 1) for around 623 h in a year (7.1% of time) whereas the best process C2/MP has only 2 h/a (0.02% of time) chemical concentrations above the threshold limit value at this location (Table 5). In fact a risk distribution (% of time the process is in small, moderate or significant risk area) can be produced. The information can obviously be used to compare different process concepts.
4.4. Intake-based estimation of exposure risk
Since slope factors are available only for carcinogens, only the intake of carcinogens is estimated here. C2/PA is the only MMA process route containing a carcinogenic substance (formaldehyde). The intake of formaldehyde is calculated using the annual probability distribution of wind speed (Fig. 1), assuming 1632 working hours per year for a worker and the worker is exposed to the process area air for the full working time. The probability of carcinogenic risk is determined by multiplying the daily intake by the slope factor of formaldehyde (0.045 kg-day/mg) (Watts, 1997) (Eq. (7)). Fig. 5(a) shows the cumulative probability of getting cancer from the exposure to formaldehyde in a location with these annual wind distributions. One in 10,000 persons (1 × 10−4) in 45-year worktime is usually used as the benchmark risk for occupational exposures (Chan et al., 2006). The benchmark however depends on the local policy and risk perception. In this case, the carcinogenic risk of formaldehyde exposure is larger than 1 in 10,000 for 98.7% of the year – indicating that the cancer risk exceeds the benchmark (Fig. 5(a)). It is also possible to estimate the total annual intake of formaldehyde in the process (1632 working hours) with these wind conditions – the result is 1.1 mg/kg-per person. This corresponds to an average daily intake of 0.0053 mg/kg-day. The associated average risk of getting cancer as a result of exposure to formaldehyde can now be calculated. The result is 2.4 × 10−4 cancer cases per 10,000 persons per 45-year worktime (Eq. (7)) for the wind distribution in question. This exceeds the benchmark (10−4) implying that the C2/PA route may be an unsuitable process option if build with unsophisticated engineering standards. However using better engineering such as leak proof valves and hermetic pumps, the situation may change. But for this a new analysis is needed based on detailed engineering data such as PID, which presents the real number and type of leak points.