BACKGROUND: Indoor air quality (IAQ) is an important determinant of human health. In Japan, IAQ guidelines have been established for 13 chemicals since 1997. Regarding ethylbenzene (EB), a previous guideline value of 3800 µg/m METHODS: As it was defined that the IAQ guidelines would not exert adverse health effects on humans even if they inhaled the chemicals from indoor air over a lifetime, we investigated the general toxicity, developmental and reproductive toxicity, genotoxicity, and carcinogenicity of EB based on reliable hazard information cited in published assessment documents by domestic, foreign, or international risk assessment organizations. All the collected hazard information was examined, and we originally judged the no-observed adverse effect level and the lowest observed adverse effect level of each toxicity study. We then selected the most appropriate key study, an endpoint, and a point of departure and derived the hazard assessment values for each toxicity category. Finally, we selected a representative hazard assessment value for EB from the minimum hazard assessment value among general toxicity, developmental and reproductive toxicity, and carcinogenicity. RESULTS: Among the three toxicity categories, the minimum hazard assessment value was obtained from general toxicity, which was 0.0858 ppm (370 µg/m CONCLUSIONS: It would be appropriate to adopt 0.0858 ppm (370 µg/m