Horizontal axis is number of voyage for benzene transfer and vertical axis is carcinogenic risk calculated by equation (1). Unit risk of benzene is indicated as 2.2x10-6〜7.8x10-6 (m3/μg). The result of minimum value, 2.2x10-6 (m3/μg), maximum value, 7.8x10-6 (m3/μg) and middle of these values, 5.0x10-6 (m3/μg) are represented in this figure. It is found that increase of number of navigation increases carcinogenic risk. As the number of benzene transfer operation reaches to 1000 times, carcinogenic risk is about 0.0009 for the maximum unit risk, 0.0003 for the minimum unit risk. This numbers represent that nine or three crews of ten thousand develop cancer caused from only benzene transfer operation.
Fig.9 represents the effectiveness of wearing personal protection equipment. In this figure, countermeasure A is to put on air purifying mask when working in cargo pump room. In addition to the countermeasure A, countermeasure B means to put on mask when working out of the accommodation during tank cleaning operation. It is assumed that mask can protect crews from inhalations perfectly. These countermeasures are expected to reduce the average exposure concentration of crews significantly. As shown in Fig.10, averaged concentration is reduced to 44% for countermeasure A and 15% for countermeasure B respectively and both of them are reduced to lower than 1(ppm) recommended by IMO consequently.
The result of carcinogenic risk is represented in Fig.10, taking into account the effect of countermeasure. In this figure, the value of unit risk is 5.0x10-6 (m3/μg). The risk of cancer is reduced proportional to the reduction of averaged concentration by countermeasure.
We investigated the working environment level on chemical tanker engaged in benzene transfer operation and conducted onboard measurement on concentration of benzene gas. Furthermore the risk of carcinogen is calculated. It is found effective for crew to put on personal protection equipment to reduce the potential of inhalation of benzene gas when working in cargo pump room during all operation and when working on weather deck during tank cleaning operation. However, chemical tankers transport many kinds of chemical and their amount are very large. Some of them are recognized as carcinogen. In this paper, we have focused on benzene and its responsibility to the carcinogenicity. Further investigation and research are needed to evaluate the actual health condition of crews exposed to another chemicals.
We would like to express sincere appreciation to the captains, crews, workers in marine terminal and other cooperators for conducting onboard measurement.
 K. Yamaguchi S. Fujii, et al, Exposure of Crews to Benzene Vapor during Marine Transfer Operations, Journal of Japan Institute of Navigation, Mar. 1999, Vol.102, pp.211-217
 e.g. Rao V. Kolluru et al, Risk Assessment and Management Handbook, McGraw-Hill
 EPA IRIS(Integrated Risk Information System) Substance file-Benzene, CASRN 71-43-2