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Due to the severe outdoor PM2.5 pollution in China, many people have installed air‐cleaning systems in homes. To make the systems run automatically and intelligently, we developed a recurrent neural network (RNN) that uses historical data to predict the future indoor PM2.5 concentration. The RNN architecture includes an autoencoder and a recurrent part. We used data measured in an apartment over the course of an entire year to train and test the RNN. The data include indoor/outdoor PM2.5 concentration, environmental parameters and time of day. By comparing three different input strategies, we found that a strategy employing historical PM2.5 and time of day as inputs performed best. With this strategy, the model can be applied to predict the relatively stable trend of indoor PM2.5 concentration in advance. When the input length is 2 h and the prediction horizon is 30 min, the median prediction error is 8.3 µg/m3 for the whole test set. For times with indoor PM2.5 concentrations between (20,50] µg/m3 and (50,100] µg/m3, the median prediction error is 8.3 and 9.2 µg/m3, respectively. The low prediction error between the ground‐truth and predicted values shows that the RNN can predict indoor PM2.5 concentrations with satisfactory performance. [ABSTRACT FROM AUTHOR]

The indoor environment influences occupants' health. From March 1, 2018, to February 28, 2019, we continuously monitored indoor temperature (T), relative humidity (RH), and CO2 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 CO2 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 CO2 concentration level. [ABSTRACT FROM AUTHOR]

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 CO2 decay method, we found the median infiltration rate for 294 residences to be 0.34 h−1. Using occupant‐generated CO2 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. [ABSTRACT FROM AUTHOR]

Abstract In order to investigate the current indoor air quality (IAQ) in China, we installed IAQ monitoring sensors in 117 homes. According to the results, only about 2% of homes were able to maintain an indoor PM2.5 concentration below 75 μ g / m 3 during the monitoring period, and 10% of homes were able to keep the nighttime carbon dioxide (CO 2) concentration below 1000 ppm. In regard to the relationships between pollutant concentrations and outdoor environmental parameters, we found that when nighttime outdoor temperatures were between 10°C and 22°C, the CO 2 concentrations essentially decreased as the temperature increased. In regard to the indoor/outdoor (I/O) ratios of PM2.5, the naturally ventilated homes had a median I/O ratio of around 0.88–0.97 when the outdoor PM2.5 concentration was lower than 75 μ g / m 3. The homes with portable air cleaners were able to maintain 89%, 71% and 56% of days with indoor PM2.5 concentrations lower than 25 μ g / m 3 in severe cold, cold and hot summer and cold winter zones, respectively. If we want to maintain both indoor CO 2 and PM2.5 concentrations below guidelines (CO 2 : 1000 ppm, PM2.5: 25 μ g / m 3 ) for more than 95% of days during the whole year, the mechanical ventilation may be a good choice. But the efficiency of the particle filters in mechanical ventilation systems should be improved, with minimize efficiency of 86%, 85%, 74%, 58%, and 62% for severe cold, cold, hot summer and cold winter, moderate and hot summer and warm winter zones, respectively. Highlights • About 2% of homes were able to maintain an indoor PM2.5 concentration below 75 μg/m3. • About 10% of homes were able to keep the nighttime CO 2 concentration below 1000 ppm. • The median PM2.5 I/O ratios for NV homes were between 0.88 – 0.97 when outdoor PM2.5 concentration was lower than 75 μg/m3. [ABSTRACT FROM AUTHOR]

Abstract A mechanical ventilation system in a residential building provides a more reliable, controllable, and comfortable means of ventilation than natural ventilation through an open window. However, the operation of a mechanical ventilation system consumes energy and may generate noise and secondary air pollutants. Thus, it is worthwhile to study residents' actual ventilation patterns. This study investigated the usage of natural and mechanical ventilation in 46 apartments across five different climate zones in China by on-site monitoring and a questionnaire survey. On average, the daily natural and mechanical ventilation durations were 11 h and 7.2 h, respectively. Large differences existed among climate regions and seasons. From north to south, as the climate became warmer, the usage of natural ventilation increased. From a seasonal perspective, the natural ventilation duration was the longest in summer and the shortest in winter. The trend of mechanical ventilation usage was opposite to that of natural ventilation. Generally, as the outdoor air temperature increased, the duration of natural ventilation increased and the duration of mechanical ventilation decreased. This study found that occupants prioritized their thermal comfort needs over healthy indoor air quality (IAQ). However, occupants were willing to spend money on energy when the health requirement could be met by mechanical ventilation. This study provides further understanding of occupants' ventilation behavior, and the results can serve as more accurate boundary conditions for analyses of IAQ, energy consumption, and thermal comfort in Chinese residential buildings. Highlights • The usage of natural and mechanical ventilation in 46 Chinese apartments was monitored for a year. • From north to south, as climate became warmer, natural ventilation increased and mechanical ventilation decreased. • Residents prioritized their thermal comfort needs over healthy indoor air quality (IAQ). • Occupants were willing to spend on energy when the health requirement could be met by mechanical ventilation. [ABSTRACT FROM AUTHOR]

It is very important to know the current level of indoor pollution and occupants' ventilation habits. With this aim, this study presents an investigative project that covered five climate zones in China. The project consisted of two parts: long-term monitoring and seasonal measurements. For the long-term monitoring, we installed indoor air quality (IAQ) sensors, window state sensors, and mechanical ventilation sensors inside homes. In regard to IAQ, The median indoor 24-h averaged PM2.5 concentration ranged from 18 to 49 μ g / m 3 . The indoor and outdoor 24-h averaged PM2.5 concentrations were similar for most of the regions. The 24-h averaged indoor CO 2 concentrations were lower than 1000 ppm for all regions in all four seasons. Northern China usually had higher indoor CO 2 concentrations than did southern China. In regard to occupants' behavior, in naturally ventilated homes, both the window-opening probability and open-window duration per day increased from the north to the south of China in winter. According to the example of mechanically ventilated Apartments, where pressure sensors were used to monitor the relative pressure at the air outlet of a mechanical ventilation system, the occupants used the ventilation system mainly in winter and spring. For the seasonal investigation, indoor formaldehyde concentrations in 224 homes were measured in four seasons (sampling time: 20 min). In summer, the indoor formaldehyde concentration reached its peak for the year, with about 35% of homes having concentrations larger than 100 μ g / m 3 under closed conditions (doors and windows have been closed for more than 12 h). [ABSTRACT FROM AUTHOR]

Formaldehyde has substantial adverse impacts on human health, and formaldehyde exposure primarily occurs in indoor environments. This study investigated the infiltration rates and indoor formaldehyde concentrations in 5 climate zones of China. In the winter, apartments in northern China suffer from higher indoor formaldehyde concentrations than apartments in southern China. The median indoor formaldehyde concentrations were 56 μ g / m 3 (25%, 75%: 38, 91 μ g / m 3 ) in northern China and 40 μ g / m 3 (25%, 75%: 30, 61 μ g / m 3 ) in southern China. There is a clear decrease in indoor formaldehyde concentrations in China. We also studied the relationships of the indoor air temperature, years of decoration, infiltration rate and source characteristic with formaldehyde concentrations in closed conditions. A multiple regression model that related these factors to the formaldehyde concentrations in closed conditions was constructed ( R 2 = 0.75 ). The optimal curve for the suitable combination of temperature and infiltration rate to maintain low formaldehyde concentrations with the lowest cost was calculated for northern and southern China. By comparing the optimal curve and the state point of each city, we can infer the suitable tendency of indoor temperature and infiltration rate for each city. In Tianjin and Shenyang, apartments are overheating, thus causing a high percentage of some homes to have formaldehyde concentrations above the Chinese national standard. In Shanxi, Xinjiang, Yunnan and the Yangtze River Delta, the infiltration rate should be increased to some extent to achieve better indoor air quality. In Hunan, Hubei and Chongqing, indoor temperatures could be increased to improve indoor thermal comfort. [ABSTRACT FROM AUTHOR]

The indoor air quality (IAQ) impacts occupants' health significantly. It is important to know the actual IAQ under different source strengths and occupants' air purifier usage strategies. To achieve this goal, we conducted a Monte Carlo simulation to calculate the indoor pollutant concentrations under different conditions. It is confirmed that our simulation is reliable by comparing the simulation results with actual measurement results. According to the simulation results, we found that indoor nighttime CO 2 , formaldehyde and PM2.5 were lowest in summer. In spring and winter, the nightly indoor CO 2 concentration should be kept below 1000 ppm for F3 homes (formaldehyde semission rate: 0.03–0.06 m g / ( m 2 ⋅ h)) and below 900 ppm for F4 homes (formaldehyde emission rate: 0.06–0.12 m g / ( m 2 ⋅ h)) to ensure that the nighttime formaldehyde concentration is lower than 80 μ g / m 3 for one-occupant room. The mean indoor PM2.5 concentration among all sample rooms decreased from 53.2 μ g / m 3 to 36.8 μ g / m 3 under strategy I (using air purifiers every day) and to 45.8 μ g / m 3 under strategy II (only using when outdoor PM2.5 concentration larger 75 μ g / m 3 ). Under the condition of keeping the nighttime CO 2 concentration below 1000 ppm, the fraction of healthy days increased from 33.1% to 59.4% and 46.9% for one- and two-occupant rooms under strategy I, respectively. The fraction of unhealthy days decreased from 18.2% to about 7% and 11% under strategy I and strategy II, respectively. Additionally, the suitable clean air change rate was 3–4 h−1 under current usage time. • We evaluate the current IAQ with the latest technical standard in China. • The C CO2 , C PM2.5 and C HCHO are lowest in summer for homes in Tianjin. • Keeping the indoor C CO2 lower than 1000 ppm is insufficient in some cases. • The suitable clean air change rate is 3–4 h−1 under current usage habits. [ABSTRACT FROM AUTHOR]

To evaluate the separate impacts on human health and establish effective control strategies, it is crucial to estimate the contribution of outdoor infiltration and indoor emission to indoor PM 2.5 in buildings. This study used an algorithm to automatically estimate the long-term time-resolved indoor PM 2.5 of outdoor and indoor origin in real apartments with natural ventilation. The inputs for the algorithm were only the time-resolved indoor/outdoor PM 2.5 concentrations and occupants' window actions, which were easily obtained from the low-cost sensors. This study first applied the algorithm in an apartment in Tianjin, China. The indoor/outdoor contribution to the gross indoor exposure and time-resolved infiltration factor were automatically estimated using the algorithm. The influence of outdoor PM 2.5 data source and algorithm parameters on the estimated results was analyzed. The algorithm was then applied in four other apartments located in Chongqing, Shenyang, Xi'an, and Urumqi to further demonstrate its feasibility. The results provided indirect evidence, such as the plausible explanations for seasonal and spatial variation, to partially support the success of the algorithm used in real apartments. Through the analysis, this study also identified several further development directions to facilitate the practical applications of the algorithm, such as robust long-term outdoor PM 2.5  monitoring using low-cost light-scattering sensors., (© 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

This study conducted on-site measurements of indoor volatile organic compounds (VOCs) in 251 occupied residences in China, with multiple visits throughout a whole year. Over 1000 samples were collected for measurement of VOCs in 8 cities, covering different climate regions. Overall, the concentrations of total VOCs (TVOCs) in occupied residences are in the range of 104-1151 μg/m 3 , with 20% of the samples over the Chinese standard of 600 μg/m 3 . A higher concentration was evident in the summer (mean = 705 μg/m 3 ) compared to other seasons, especially winter (mean = 289 μg/m 3 ). The TVOCs of residences in areas with central heating (severe cold regions and cold regions) are generally higher than those in areas without central heating. In winter, temperature was the dominant factor, whereas in summer, the building infiltration rate was the key factor influencing the indoor TVOC levels. The TVOCs concentration was also found to be directly proportional to the city economy level. Twenty-nine VOC species with a detection frequency higher than 40% were identified in all samples. Toluene is the most common VOC, with the highest detection rate (90%). The median concentration for a single VOC was between 1 and 14 μg/m 3 . Aldehydes were found to be the largest contributors to total VOCs in the Chinese residential buildings (mass proportion 22%), followed by benzene series (20%), alkenes (18%), and alkanes (15%). Aldehydes, especially long-chain saturated carbonyls, are likely to be the characteristic VOCs in the Chinese dwellings, with Chinese cooking as the major emission source. In addition, n-butane/i-butane showed maximum concentration in some residences (approximately 10 5  μg/m 3 higher than other VOCs) owing to cooking fuel., (Copyright © 2019. Published by Elsevier B.V.)