Indoor air measurements were conducted in one unoccupied apartment of a 'near-zero-energy' residential building with a unique, wooden construction. Ozone, NO2, fine particles and volatile organic compounds, formaldehyde, acetaldehyde and peroxyacetyl nitrate (PAN) were measured under 'as is' conditions and after intentional intervention by adding ozone to simulate an ambient air ozone episode. Undisturbed concentrations were: O 3 5-10 ppb, NO2 5-8 ppb, fine particles 2000-5000 cm -3, formaldehyde 35 ± 5 μg/m3, PAN 0.3 ppb. During intervention, O3 was 50-60 ppb, NO2 15-20 ppb, fine particles 20,000-25,000 cm-3, formaldehyde 44 ± 2 μg/m3 and PAN 0.7-1 ppb. It was shown that chemical reactions had taken place in the indoor air. Ozone-initiated chemistry produced various aldehydes, PAN and, as a direct response to increased ozone concentrations, also fine particles. Calculations made by a simple model of PAN formation showed that an air change rate of around 0.2 h-1 would provide optimum conditions for PAN formation in a setting comparable to that of the investigated apartment. This air change rate is well below the Swedish national minimum of 0.5 h-1. Further, the calculations show that the non-photochemical PAN formation could be a consequence of mixing ozone and nitrogen dioxide with terpenes and acetaldehyde.