Assessment of indoor air contaminants is recommended to be based on toxicologically derived guideline values (GV, German: Richtwerte, RW) for single substances, TVOC levels, and statistically derived reference values. This recommendation refers to private and public indoor environments and workplaces without production-related handling of hazardous substances. According to the working group's current assessments, the GV I (RW I) is a concentration below which no adverse health effects are to be expected even at life-long exposure to the respective single substance. Concentrations exceeding GV II (RW II) are likely to represent a threat to health, especially for sensitive people. According to a general scheme, guideline values I and II have been derived for - toluene (0.3 and 3 mg/m(3)), - pentachlorophenol (0.1 and 1 microg/m(3)), - dichloromethane (0.2 and 2 mg/m(3)), - styrene (0.03 and 0.3 mg/m(3)), - tris(2-chloroethyl)phosphate (0.005 and 0.05 mg/m(3)), - bicyclic terpenes (0.2 and 2 mg/m(3), - naphthalene (0.002 and 0.02 mg/m(3)), - aliphatic hydrocarbons (0.2 and 2 mg/m(3)). In case of concentrations exceeding GV II, immediate measures are to be taken, among them restrictions of the time spent in the room and measures to remove or reduce emission sources. At levels between GV I and GV II, increased air exchange and cleaning are adequate first steps. However, if concentrations continue to exceed GV I, more intensive measures are recommended. Based on the TVOC concept (cf. Seifert 1990), 5 stages with specific recommendations are defined provided that the special GVs ("Richtwerte") are not exceeded: Level 1: TVOC < 0. 3 mg/m(3): No hygienic objections, target value. Level 2: TVOC > 0.3-1 mg/m(3): No relevant objections, but increased ventilation recommended. Level 3: TVOC > 1-3 mg/m(3): Concerning hygienic aspects, some objections due to elevated concentration level. Upper range for a maximum of 12 months. Search for sources, increased ventilation recommended. Level 4: TVOC > 3-10 mg/m(3): Major objections. Should not be tolerated for > 1 month. Restricted use only. Search for sources, intensified ventilation necessary. Level 5: TVOC > 10-25 mg/m(3): Situation not acceptable. Use only if unavoidable and then for short periods (hours) only with intensified ventilation. An assessment based on reference values does not imply any health risk assessment. It only provides information on exposure relative to the exposure of the reference group. For comparison, regularly updated and representative reference values are recommended. In case of exceeded reference values, plausibility should be checked. Considering concentration and known toxicological data, health relevance should be estimated in order to decide on the necessity of measures to be taken. Since results of indoor air measurements are strongly influenced by sampling strategy, ventilation and climatic factors, recommendations are given in order to standardise sampling procedures and conditions.