Levels of dioxins in rice, wheat, soybean, and adzuki bean cultivated in 1999 to 2002 in Japan and estimation of their intake

Abstract

A total of 369 samples of rice (n = 311), wheat (n = 10), soybean (n = 44), and adzuki bean (n = 4) collected from various locations in Japan between 1999 and 2002 were analyzed for PCDDs, PCDFs (PCDD/Fs) and coplanar PCBs. Sampling points within about 1 km of operational municipal waste incinerators that were considered sources of dioxins were defined as "near-source" areas, and all other sampling points were defined as "general" areas. The toxic equivalent quantity (TEQ) values of soybean samples collected from near-source areas were significantly higher (p < 0.05) than those from general areas. A significant difference of TEQs among sampling years in rice in general areas was also found. However, the differences could not be explained by the presence or absence of incineration plants in the area surrounding the sampling point or by a temporal decrease of air pollution. The TEQs of the crops varied widely, but the median value of each crop was quite low, at 0.000021, 0.00013, 0.0000095, and 0.00016 pg-TEQ/g wet wt. in rice, wheat, soybean and adzuki bean, respectively. On the basis of these survey results, the daily intake of PCDD/Fs and coplanar PCBs from rice, wheat, soybean, and adzuki bean was calculated. The daily intakes from these crops were estimated to be 0.0056 pg-TEQ/kg B.W./day on the assumption that "not detected" (ND) could be taken as zero, ND = 0, and 0.18 pg-TEQ/kg B.W./day if ND is put equal to 1/2 LOD (half the limit of detection). In comparison with the tolerable daily intake set in Japan for PCDD/Fs and coplanar PCBs (4 pg-TEQ/kg B.W./day), it was considered that the levels of contamination by PCDD/Fs and coplanar PCBs in these crops cultivated in the environment of Japan do not present a problem.