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Formaldehyde is concerned as an important indoor carcinogen. While contribution of outdoor formaldehyde to indoor concentration is recognized, long-term measurement about its impact on indoor environments remain missing. We measured both outdoor and indoor formaldehyde concentrations for over one year in Nanjing (east-central China) and calculated the outdoor/indoor (O/I) ratios. 64.8% of the measured outdoor concentration have exceeded the chronic reference exposure criteria of 0.009 mg/m3 set by Office of Environmental Health Hazard Assessment (OEHHA). The outdoor concentration was highest in summer with median value of 0.020 mg/m3 and lowest in spring with median value of 0.009 mg/m3. Diurnally, outdoor formaldehyde concentration was highest at noon with median value of 0.013 mg/m3 and lowest at night with median value of 0.01 mg/m3. Health analysis revealed that cancer risk by exposure to this concentration level is 1.6 × 10−4, higher than threshold limit of 10−6. In addition, the median O/I ratios are 0.18 and 0.27 in two offices, indicating that outdoor formaldehyde contributes to indoor concentrations by about one quarter. The change of O/I ratio also shows a similar seasonal and diurnal trend as outdoor concentrations (highest in the summer in a year and at noon in a day). Outdoor formaldehyde concentration is therefore not negligible as a contributor to indoor concentration, especially as indoor concentration limit gets continuously lowered. This factor should be taken into account in indoor air quality design and maintenance. [ABSTRACT FROM AUTHOR]