On July 10, 1976, the most poisonous chemical substance (namely, 2,3,7,8-Tetrachlorodibenzo-p-dioxin) was the cause of the Seveso disaster. The number of possible isotopical isomers of this substance is (2^20*3^2+2^10*3^2+2^10*3+2^10*3)/4, which amounts to 2363136.