In this study, a multivariate regression model was applied to identify the greatest home environmental risks for children’s infections. Perceived dry air had the strongest association with pneumonia and common colds. The reported rate of “dry air perception” is high in Tianjin, 53.1% in our study. We found that dryness was significantly associated with dampness indicators, consistent with findings from previous studies   . “Perceived dry air” has been shown to not necessarily be due to physically dry indoor air  but rather to polluted air. Building dampness promotes the growth of, mould , as well as increases VOC emissions from building material , which may result in “dry air perception”.
Condensation on the child’s room window panes in winter was a strong risk for respiratory infections among children. Condensation on windowpanes in winter was reported to be an indicate of insufficient ventilation in homes [40, 41]. Chinese homes do not have a mechanical ventilation system. Most of residential buildings in China still depend on “window ventilation” or infiltration to passively introduce outdoor air to indoors. In Tianjin winter, ~ 90% of homes closed window at night. Ventilation is mainly through infiltration, with a median value of 0.30 h− 1 . In such tight buildings poor ventilation rate could not efficiently dilute airborne-transmitted pathogens and thus result in their accumulation in our occupied spaces. A study conducted in army trainees found that rates of febrile acute respiratory diseases were significantly higher among trainees in modern (energy efficient design) barracks (adjusted relative risk 1.51) . Mendell estimated that in offices with lower ventilation, relative risk for short-term sick leave was 1.53 (95%CI 1.22–1.92) . A study conducted in Peru indicated that after simple modifications to existing hospital infrastructure, the rate of natural ventilation increased considerably and the estimated reduction in risk of TB infection was 72% (inter-quartile range 51–82%) .
In addition to the modern practice of tight buildings, more and more modern materials are being used for indoor furnishings and decoration . China is currently the largest producer of wood-based panels, coating and furniture in the world. Massive new building furnishing materials have emerged in people’s daily life. In our study, children who live in urban apartment decorated with modern floor and wall covering were reported to have more respiratory infections. Renovation during the first year of children’s life was also a risk. Considerable evidence has demonstrated that the chemical emissions of modern indoor materials (new wall coverings, new furniture, new synthetic carpets) are associated with increases in respiratory infections among children or infants . Home renovation and modern decoration materials are positively associated with high concentrations of formaldehyde and volatile organic compounds (VOCs) . These indoor pollutants may affect the human health by developmental delays in children, reduced activity of the immune system and direct toxicity. As a comparison, suburban or rural children who lived in Pingfang with less “modern” redecoration and materials had fewer respiratory infections. Pingfang children mostly spend more time outdoors and thus are less exposed to indoor pollution, which may be a reasonable explanation for lower incidence of respiratory infections in suburban and rural areas .
Among life style factors, we found associations of respiratory infections with Cesarean delivery and daycare attendance. In particular, cesarean delivery was a significant risk factor with a high PAF for childhood croup, pneumonia and ear infection. Bosch et al  pointed out that cesarean delivery affected early respiratory microbiota development, thereby possibly increasing the morbidity of respiratory infections later in life. In our study, the Cesarean delivery rate was 62% in Tianjin, higher than the national level of 46%  and the recommended value by WHO of 15% .
Consistent with studies in America  and Sweden  our analysis demonstrated that daycare attendance was a significant risk factor for common cold among children in Tianjin. Daycare occupancy level and weekly exposure time are measures of contact with other children. We found that children spending more time in daycare or in higher occupancy daycare centers were susceptible to have pneumonia, ear infections and common colds. Daycare attendance was a contributor for common cold infections (See Table 9, PAF = 12.3%), second only to dry air perception. More frequent contacts with persons may be the reasonable explanations for this observation. For respiratory infection viruses such as influenza, adenovirus, rhinovirus, and coronavirus, which transmitted by small particle aerosols, airborne transmission is considered the predominant transmission pathway [53, 54], however contact (direct or indirect) can also spread these pathogens. A study on the transmission of common cold in sharing offices indicated that workers sharing offices have a significant risk of common cold than those working in single rooms (AOR 1.35) . Overcrowding in a large urban jail was reported to be a significant risk factor (p = 0.03) for an outbreak of pneumococcal disease among inmates . Likewise, an early case-control study in American showed that inmates infected with TB were less likely than “healthy” inmates to have a TV in their individual cells (42% vs. 87%; p = 0.003) and spent more time in the communal day room (median, 28 h/week vs. 3 h/week; p = 0.01), thus more contacts with infected inmates .
Currently in China, approximately 250 million households (accounting for 55% of population in China) live in cities. Rapid urbanization and modernization has led China to experience a dramatic change in both indoor environment and lifestyles in the past two decades. Modern high-rise apartments are constructed tightly and decorated with modern materials. The result is insufficient ventilation that results in increased concentrations of indoor-generated pollutants, including chemical compounds and airborne transmitted pathogens. It is almost impossible to sun-cure bedding frequently in modern urban buildings. The Cesarean delivery rate is higher due in part to hospitals’ financial motivation . It is common that children attend daycare centers around three years old since generally, both parents work. Meanwhile more health issues related to indoor environment are noticed.
Parallel CCHH studies performed in other cities have found similarly high rates of respiratory infections in children, especially in urban areas [19, 31]. The incidence of lifetime-ever pneumonia among children in modern cities, like Tianjin, 28.7%, are substantially higher than those in USA and European cities . Pneumonia is an infectious disease caused by bacteria or virus. The high infection rate of pneumonia in China may be partially due to insufficient vaccine coverage with Pneumococcal Conjugate Vaccine (PCV) and Haemophilus Influenzae Type b (Hib) vaccine. However, the incidence and severity or duration of pneumonia could be influenced by environmental factors . Our analysis in the present study was hierarchical so as to identify the important factors among a variety of possible risk factors. The most prominent risk factors from multivariate analysis were related to modern home environment and life styles. The higher rates of childhood respiratory infections are linked with the combination of nutritional, environmental and behavioral influences, which affect the development of immune system and the breeding and transmission of pathogen.