Indoor environments are the dominant setting for human exposure to respiratory viruses, yet routine molecular surveillance of indoor air remains rare outside single-site studies. Within the Horizon Europe TwinAIR project, a single harmonised workflow was applied to 102 indoor air samples (2000 L each) collected between October 2024 and February 2026 at ten pilot settings in Germany, the United Kingdom, Ireland, Spain, Greece and Sweden, covering university teaching rooms, school classrooms and staff rooms, healthcare-associated offices, workplace offices, an elderly care facility and public transport vehicles. Air was collected onto gelatine membrane filters with a Sartorius MD8 Airport sampler, total nucleic acid was extracted with the QIAamp Viral RNA Mini kit, and five targets—SARS-CoV-2, human adenovirus, respiratory syncytial virus A/B, influenza A and rhinovirus—were quantified against target-specific standard curves as genome copies per cubic metre (GC/m³). At least one target was detected in 84 of 102 samples (82.4%; 95% CI 73.8–88.5). SARS-CoV-2 was the most frequent and most abundant target (64.7%; up to 8.36 × 10⁶ GC/m³) and adenovirus was the second most frequent though consistently low in concentration (33.3%); both were recovered from all six countries. Rhinovirus (14.7%, five countries), RSV A/B (8.8%, four) and influenza A (6.9%, four) were detected less often yet reached high concentrations when present, showing that detection frequency alone substantially understates airborne viral burden. Detection of viral genome does not establish infectivity.