The article discusses relationships between indoor and outdoor levels of various air contaminants (nitrogen monoxide, nitrous oxides, sulphur dioxide and ozone), in ten non-smoking residential buildings in Hong Kong. The experiment used passive samplers. Estimated the source and sink strengths of the contaminants, applying an integral approach to the mass balance equation. The results indicated that the buildings acted as sinks for the contaminants. When compared to the newly developed indoor air quality guidance notes in Hong Kong, the nitrogen oxides level gave some cause for concern. Other levels were low.