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Indoor Air Quality in New California Homes with Mechanical Ventilation Wanyu Chan1,*, Yang-Seon Kim1, Brett Singer1, Iain Walker1 1 Lawrence Berkeley National Laboratory, Berkeley, USA *Corresponding email:wrchan@lbl.gov SUMMARY The Healthy Efficient New Gas Homes (HENGH) study measured indoor air quality and mechanical ventilation use in 70 new California homes. This paper summarizes preliminary results collected from 42 homes. In addition to measurements of formaldehyde, nitroge n dioxide (NO2), and PM2.5 that are discussed here, HENGH also monitored other indoor environmental parameters (e.g., CO 2) and indoor activities (e.g., cooking, fan use) using sensors and occupant logs. Each home was monitored for one week. Diagnostic tests were performed to characterize building envelope and duct leakage, and mechanical system airflow. Comparisons of indoor formaldehyde, NO 2, and PM2.5 with a prior California New Home Study (CNHS) (Offermann, 2009) suggest that contaminant levels are lower than measured from about 10 years ago. The role of mechanical ventilation on indoor contaminant levels will be evaluated. KEYWORDS Formaldehyde; nitrogen dioxide; particles; home performance; field study 1 INTRODUCTION The HENGH field study (2016–2018) aimed to measure indoor air quality in 70 new California homes that have mechanical ventilation. Eligible houses were built in 2011 or later; had an operable whole-dwelling mechanical ventilation system; used natural gas for space heating, water heating, and/or cooking ; and had no smoking in the home. Study participants were asked to rely on mechanical ventilation and avoid window use during the one -week monitoring period. All homes had a venting kitchen range hood or over the range microwave and bathroom exhaust fans. This paper presents summary results of formaldehyde, NO2, and PM2.5 measurements in 42 homes. The full dataset is expected to be available in summer 2018. 2 METHODS Integrated one-week concentrations of formaldehyde and NOx were measured using SKC UMEx-100 and Ogawa passive samplers. Formaldehyde samplers were deployed in the main living space, master bedroom, and outdoors. PM2.5 were measured using a pair of photometers (ES-642/BT-645, MetOne Instruments) indoor in the main living space and outdoors. PM2.5 filter samples were collected using a co-located pDR-1500 (ThermoFisher) in a subset of the homes and time-resolved photometer data were adjusted using the gravimetric measurements. Results are compared with a prior field study CNHS (2007–2008) (Offermann, 2009) that monitored for contaminant concentrations over a 24-hour period in 108 homes built between 2002 and 2004, including a subset of 26 homes with whole-dwelling mechanical ventilation. 3 RESULTS Figure 1 compares the indoor concentrations of formaldehyde, NO 2, and PM2.5 measured by the two studies. Results of HENGH are one-week averaged concentrations, whereas CHNS are 24-hour averages. HENGH measured lower indoor concentrations of formaldehyde and PM2.5, compared to CNHS. For NO2, the indoor concentrations measured by the two studies