The German Working Group on Indoor Guidelines of the Federal Environment Agency and the States' Health Authorities is issuing indoor air guide values to protect public health. For health evaluation of 2-furaldehyde in air valid human studies are missing. In a subacute inhalation study well documented and assessed as reliable respiratory epithelial lesions such as squamous metaplasia and atypical hyperplasia were observed. The Working Group assessed a lowest observed adverse effect level of 20 mg 2-furaldehyde/m³ for the endpoint irritation following subacute exposure and extrapolated to a chronic lowest adverse effect level of 1 mg 2-furaldehyde/m³. By applying a factor of 5 for interindividual variability of irritation and a modifying factor of 2 regarding data gaps especially for children a health hazard guide value (RW II) of 0.1 mg 2-furaldehyde/m³ is obtained. A health precaution guide value of 0.01 mg 2-furaldehyde/m³ is recommended.