The German Ad-hoc Working Group on Indoor Guidelines of the Indoor Air Hygiene Committee and of the States' Supreme Health Authorities is issuing indoor air guide values to protect public health. Naphthalene is a potentially volatile two-ring hydrocarbon with a mothball-like odor. Indoor air contaminations usually originate from tar-containing building products, sometimes from the use of mothballs. In Germany, indoor air concentrations of naphthalene are usually low, near the detection limit (medians of about 0.001 mg/m3, 95th percentiles up to 0.004 mg/m3). Naphthalene-like volatile compounds have been defined to cover methyland dimethylnaphthalenes and tricyclic aromatic hydrocarbons (e.g., acenaphthene, acenaphthylene, anthracene, fluorene and phenanthrene). Though methylnaphthalenes and dimethylnaphthalenes usually show low indoor air concentrations, they have been suspected to add to the mothball-like odor. Tricyclic aromatic hydrocarbons mostly occur below 0.001 mg/m3 of indoor air. Against this background naphthalene is seen to be the key component of this group of substances in indoor air. No valid human data is available with respect to health effects of inhaled naphthalene. Based on animal data cytotoxic-inflammatory lesions in the rat nasal epithelium are regarded as the critical endpoint. In a subchronic inhalation study in rats (Dodd et al., Inhal Toxicol 24:70–79, 2012), minimal effects were observed following an exposure to 5 mg naphthalene/m3. From this study the Ad-hoc Working Group derived a chronic NAEC of 2.5 mg naphthalene/m3. Time scaling was considered by a factor of 5.6 extrapolating from 6 to 24 h and 5 to 7 days, a factor of 2 applied for the use of F344 rats instead of the more sensitive Sprague-Dawley rats. Incorporating an interspecies factor of 1, an intraspecies factor of 10 and a factor of 2 for insufficient data on the toxicity of naphthalene in children resulted in a precautionary value of 0.01 mg naphthalene/m3 and a hazard-based guide value of 0.03 mg naphthalene/m3. In the European Union, naphthalene has been classified as a suspected human carcinogen. In rats, carcinogenicity (nasal olfactory neuroblastoma) was seen at 53 mg naphthalene/m3. In contrast no valid human data on carcinogenicity of naphthalene is available. The Ad-hoc Working Group holds that the derived guide values sufficiently prevent cytotoxic-inflammatory effects of naphthalene and consequently from its long-term impacts such as potential carcinogenicity. This opinion is supported by a study of Meng et al. (Mutat Res 721:199–205, 2011) initially pointing to a missing primary genotoxicity of naphthalene. Only few data are available for health evaluation of naphthalene-like compounds. Therefore, the indoor air guide values for naphthalene are recommended by the Ad-hoc Working Group to be used as preliminary indoor air guide values for the sum of bicyclic and tricyclic aromatic hydrocarbons, too. Indoor air measurement of tricyclic aromatic hydrocarbons should be restricted to the occurrence of directly emitting building products such as asphalt floor coverings.