Three approaches were applied and compared to evaluate additive toxic effects of eight PAHs to aquatic organisms in rivers in the Tianjin area. Although the toxicity of the studied PAH compounds did not significantly increase the risk to aquatic organisms, the results of all three approaches indicated that the additive effect of the eight PAHs was significantly stronger than any individual compound acting alone, which indicated the applicability of the approaches. Further, of the compounds studied, anthracene was the major contributor to the overall toxic effect of the mixture. The calculated geometric means of the hazard quotient for the additive effect varied from 0.00055 to 0.00062, compared to the hazard quotient of individual PAHs which ranged from 5.1 x 10-6 to 0.00053. The hazard quotient distribution geometric mean was 0.00058, with 95% of the quotient between 6.6 x 10-5 and 0.051. Overlapping areas varied from 0.00015 to 0.02 for individual PAHs and was 0.03 for additive toxicity.