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This paper presents the field measurement and evaluation of the indoor air quality of 25 naturally ventilated residential buildings in northwestern China. Concentrations of formaldehyde, benzene, toluene, total volatile organic compounds, nitrogen dioxide and PM 2.5 , and air infiltration rate were measured in the bedroom, living room and kitchen of each building in different seasons. The results show that the concentration of formaldehyde can be used as a representative index of indoor gaseous pollutants, and that formaldehyde concentrations in various locations measured were highest in spring and lowest in autumn. PM 2.5 concentration was the most important influencing factor of indoor air quality in winter, and concentration of outdoor PM 2.5 was the key factor under hazy weather conditions. The median air infiltration rate was around 0.35 h À1 in bedrooms and 0.8 h À1 in kitchens, and these should be increased to 0.66 h À1 and 1.6 h À1 , respectively, to provide acceptable indoor air quality in northwestern China. The findings obtained from this study can be used to understand where and how to improve the indoor air quality of naturally ventilated buildings in northwestern China. The data can also contribute to the development of a national database for improving residential ventilation, energy efficiency and indoor air quality.

The ventilation modalities in most Chinese residences are infiltration and opening windows. We measured infiltration rates and air change rates at night, with no attempt to change occupants' behaviors, of urban residences in five climate zones of China during four seasons. Using the CO 2 decay method, we found the median infiltration rate for 294 residences to be 0.34 h −1 . Using occupant-generated CO 2 as tracer gas, we determined air change rates over the course of 1 year in 46 bedrooms at night from mass balance considerations. In 54% of the measurements, windows were closed, so ventilation was only by infiltration. Windows were mainly closed when the outdoor temperature was below 15°C and above 26°C. The median infiltration rates did not differ appreciably among seasons and climate zones and were always less than 0.45 h −1 .

The indoor environment influences occupants' health. From March 1, 2018, to February 28, 2019, we continuously monitored indoor temperature (T), relative humidity (RH), and CO 2 concentration in bedrooms via an online system in 165 residences that covered all five climate zones of China. Meanwhile, we asked one specific occupant in each home to complete questionnaires about perceived air quality and sick building syndrome (SBS) symptoms at the end of each month. Higher CO 2 concentration was significantly associated with a higher percentage of perceived stuffy odor and skin SBS symptoms. Higher relative humidity was associated with higher percentage of perceived moldy odor and humid air, while lower RH was associated with a higher percentage of perceived dry air. Occupants who lived in residences with high RH were less likely to have mucosal and skin SBS symptoms (adjusted odds ratio (AOR): 0.73-0.78). However, the benefit of high humidity for perceived dry air and skin dryness symptoms is weaker if there is a high CO 2 concentration level. K E Y W O R D SCO 2 , home, odors, perceived dry air, relative humidity, sick building syndrome | 1019 HOU et al.