Objective The aim of the study is to study functional impairment due to indoor air–related symptoms and associated characteristics. Methods A questionnaire survey of a random sample of Finnish people aged 25 to 64 years. Analyses were done using multivariate multinomial logistic regression. Results A total of 23.1% reported indoor air–related symptoms, 1.8% severe functional impairment due to symptoms, 5.3% moderate, 11.1% mild, and 4.9% reported no impairment. Those with severe functional impairment showed the strongest associations with comorbid diseases, for example, asthma and irritable bowel syndrome, perceived sensitivities to several environmental factors, like chemicals, and likelihood of having symptoms in multiple organs, while those with no or little functional impairment showed weak or even inverse associations. Similar results emerged with severity of indoor air–related symptoms. Conclusions Individuals with indoor air–related symptoms are a very heterogeneous group. This should be better considered in future research and clinical practice.