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are similar. Summary statistics of indoor and outdoor contaminant concentrations (mean and median concentrations; N=number of homes with available data) are presented in Table 1. 0 20 40 60 80 100 120020406080100 Formaldehyde (ppb)Cumulative Frequency (%)HENGH CANH 0 5 10 15020406080100 NO2 (ppb)Cumulative Frequency (%)0 10 20 30020406080100 PM2.5 (ug/m3)Cumulative Frequency (%) Figure 1. Comparisons of indoor contaminant concentrations measured by two studies. Table 1. Summary statistics of indoor and outdoor contaminant concentrations . HENGH - Indoor CNHS - Indoor HENGH - Outdoor CNHS - Outdoor N Median Mean N Median Mean N Median Mean N Median Mean Formaldehyde (ppb) 39 20.0 20.6 104 29.5 36.3 38 2.0 2.0 43 1.8 2.8 NO2 (ppb) 40 3.7 4.4 29 3.2 5.4 40 3.0 3.1 11 3.1 3.5 PM2.5 (ug/m3) 41 4.7 5.8 28 10.4 13.3 42 5.9 7.7 11 8.7 7.9 4 DISCUSSION The lower formaldehyde concentrations measured by HENGH in comparison to CNHS may be attributable to California’s regulation to limit formaldehyde emissions from composite wood products that came into effect between the two studies . Gas cooking is a significant source of indoor NO2 (Mullen et al., 2016). Even though NO2 concentrations measured by HENGH are similar to levels found in CNHS, the two studies differed in that HENGH homes all use gas for cooking, whereas almost all homes (98%) from the prior study used electric ranges. More analysis is needed to determine the ef fectiveness of source control, such as range hood use during cooking, on indoor concentra tions of cooking emissions such as NO2 and PM2.5. Lower PM2.5 indoors measured by HENGH compared to CNHS may be explained from a combination of lower outdoor PM2.5 levels, reduced particle penetration due to tighter building envelopes (Stephens and Siegel, 2012) combined with exhaust ventilation, and use of medium efficiency air filter (MERV 11 or better) in some HENGH homes. Further analysis of the data will evaluate the role of mechanical ventilation, including local exhaust and whole - dwelling ventilation system, on measured indoor contaminant levels. 5 CONCLUSIONS New California homes now have lower indoor formaldehyde levels than previously measured, likely as a result of California’s formaldehyde emission standards. Indoor concentrations of NO2 and PM2.5 measured are also low compared to a prior study of new homes in California. ACKNOWLEDGEMENT LBNL work on the project was supported by the California Energy Commission. Field data collection was performed by the Gas Technology Institute. Support for field teams was provided by Pacific Gas & Electric and the Southern California Gas Company. 6 REFERENCES Mullen NA et al. 2016 Indoor Air 26(2):231–245.