Fig.6 shows the average time for each operation. Operation time varies with the kind of operation. Duration of operation except for voyage is determined by the specification of cargo pump and blower. So the duration is same order of magnitude for each operation as indicated in Table. 1. On the other hand, as the duration of voyage operation depends on the distance of navigation, it scatters largely. Fig.7 shows the time-weighted average concentration for typical three working places using the data in Fig.5 and Fig.6. It is necessary to take into account the effect of the duration of each operation to estimate working level accurately. Because, for one vessel, the four operations in transfer cycle is continuous and ratio of duration of operation to total operation time affects to the averaged concentration over whole transfer cycle. The concentration in mess room is about 0.4(ppm) and it is lower than that of cargo pump room and vent post. On the other hand, the concentration in pump room and vent post on weather deck exceeds 1(ppm), recommended worker exposure limit by IMO. Average concentration, 1.1 (ppm) in Fig.7 is calculated from the concentration of three working places and this value is close to the exposure concentration obtained from another analysis.[1]
5. RISK ASSESSMENT
Working environment level of chemical tanker engaged in benzene transfer is clarified in the previous section. Benzene is classified as human carcinogen. The most important problem is whether crews develop cancer or not due to vapor exposure. Human health risk assessment is utilized to estimate this subject quantitatively. Linear and nonthreshold models were employed to calculate carcinogenic risk caused from benzene transfer operation. Carcinogenic risk expressing probability of carcinogenicity can be calculated from following equation [2],
Pc = UR × C (1)
Where,
Pc: Carcinogenic risk
UR: Unit Risk (m3/μg)
C: Average concentration of lifetime exposure
Average concentration of lifetime exposure is obtained from following equation.
C = C0NT/TL (2)
Where,
N: Number of operation for benzene transfer
T: Duration of entire cycle of benzene transfer operation (day)
Co: Averaged concentration of working environment (μg/m3)
TL: Lifetime (day)
In this paper, the lifetime is assumed to 70 × 365 days and T is determined from the sum of the average duration in Fig.7 as 0.82 day. C0 is 1.1(ppm), 3.6x103(μg/m3) obtained from onboard measurement in the previous section. Fig.8 shows the result of calculations.
