Outdoor formaldehyde matters and substantially impacts indoor formaldehyde concentrations

Formaldehyde is recognized as a significant indoor air pollutant. While its indoor sources have been intensively studied, outdoor formaldehyde and its impact on indoor concentrations have long been neglected. In this study, we compiled outdoor formaldehyde concentration data (154 data points) and calculated the outdoor/indoor (O/I) ratios (164 data points) of five developing and nine developed countries/regions; the majority of this data was published in the past five years. The outdoor formaldehyde concentrations reported from Mainland China, Hong Kong China, and South Korea were often higher than 9 mu g/m(3), which is the inhalation reference limit for chronic exposure to formaldehyde set by the California Office of Environmental Health Hazard Assessment; this indicates that outdoor formaldehyde concentrations pose an overlooked health risk. The median O/I ratios of indoor environments containing > 100 mu g/m(3), 50-100 mu g/m(3), 30-50 mu g/m(3), and < 30 mu g/m(3) of formaldehyde were 0.25, 0.42, 0.37, and 0,37, respectively. Furthermore, the median O/I ratios in developing and developed countries/regions were 0.43 and 0.17, respectively, which implies that outdoor formaldehyde contributes substantially to indoor concentrations in developing countries. The results compiled in this study demonstrate that although outdoor formaldehyde has historically been overlooked, it increases total formaldehyde exposure and thus increases health risks. Exposure analysis, engineering controls, and standard formulations should account for outdoor formaldehyde. We also suggest that the accuracy of various formaldehyde measurement methods should be evaluated.