Rapid urbanization in Xiamen, China, has led to a surge in demand for low-price rental housing (LPRH) among migrant workers. Government housing plans often fall short of meeting this demand, resulting in the spontaneous development of various LPRH models characterized by fast construction, low costs, and frequently, poor construction quality. This is a global phenomenon, with historical examples including back-to-back houses in the UK, backyard accommodations in South Africa, informal housing in Brazil, squatter settlements in Pakistan and Singapore. These LPRHs often suffer from substandard indoor environmental quality, potentially leading to serious health consequences. The World Health Organization's (WHO) concept of "Sick Building Syndrome" (SBS) highlights the link between indoor environmental factors and a range of non-specific health symptoms. Studies have consistently shown that environmental factors, particularly in low-quality housing, significantly impact SBS prevalence. Xiamen's rapid economic development since 1980 has attracted a large influx of migrant workers, creating a high demand for LPRHs. The informal redevelopment of urban villages has become a primary source of this housing, resulting in the proliferation of partitioned rural rental houses (PRRHs). This study uses Xiamen's PRRHs as a case study to examine the scale of PRRHs, their indoor environmental quality, and the health impacts on renters. The study's theoretical framework is presented in Figure 1, highlighting the interrelation between rapid urbanization, low-price rental housing, indoor environmental conditions, and Sick Building Syndrome (SBS).

Existing literature extensively documents the global issue of substandard housing arising from rapid urbanization. Studies highlight the link between low-quality housing and various health problems, particularly Sick Building Syndrome (SBS). Research emphasizes the role of both environmental (physical environment, air quality) and individual social factors (gender, education, social status) in SBS prevalence, with environmental factors having a more direct impact. Several studies highlight the high prevalence of SBS in low-quality housing in different countries, citing problems like improper temperature and humidity, poor ventilation, and inadequate lighting. The reviewed literature provided a foundation for understanding the potential health risks associated with PRRHs in Xiamen and guided the study's methodology.

The study employed a mixed-methods approach, combining data crawling, environmental measurements, and a questionnaire survey. First, rental housing data were crawled from 58.com, a major online rental platform in Xiamen, focusing on units in four districts with a high concentration of PRRHs. Data were filtered to include only owner-managed units to avoid duplication. Second, indoor air quality and physical environment parameters were measured in 20 one-bedroom PRRH units in Jimei and Tongan districts. Measurements included CO2, TVOC, PM2.5, PM10 (air quality), temperature, relative humidity, wind speed, and illuminance (physical environment). Air quality measurements followed Chinese "Indoor Air Quality Standards", with one measurement point per room. Physical environment measurements were conducted continuously for 48 hours in six units, following Chinese "Standard of Testing Methods of Building Thermal Environment". Third, a cross-sectional questionnaire survey (n=248) was conducted in eight urban villages, two from each of the four selected districts. The questionnaire, based on the MM040EA standard for SBS studies, included sections on renter demographics, PRRH characteristics, subjective indoor environment evaluation, and self-reported SBS symptoms. Data analysis included descriptive statistics, non-parametric correlation analysis (Spearman's correlation coefficient), and binary logistic regression analysis to examine the relationship between indoor environmental factors and SBS symptoms. The reliability and validity of the questionnaire data were assessed using Cronbach’s Alpha and Kaiser-Meyer-Olkin measures.

Data crawling revealed 590 PRRH units, significantly more than urban rental units (467), showing PRRHs' dominance in the rental market across four districts of Xiamen (Figure 2). PRRHs were generally taller than expected, with many having four to six stories. One-bedroom units were the most common (64.7%). Environmental measurements revealed significant excesses of nearly all parameters compared to health standards (Table 1). TVOC, CO2, and PM2.5 exceeded standards significantly. The physical environment parameters revealed high temperatures, high humidity, low illuminance, and very low wind speeds. The questionnaire survey showed that while nearly half of the participants were generally satisfied with their living environment, a deeper analysis revealed significant problems (Figure 3). Over 70% reported problems with high temperature and stuffy air. Figure 4 shows the prevalence of various SBS symptoms among the participants, with psychological symptoms being most common (66.5%), followed by mucosal (55.2%), respiratory (42.7%), skin (40.3%), and general symptoms (37.5%). Correlation analysis (Table 2) revealed significant correlations between most SBS symptoms and several environmental problems, especially poor ventilation, high temperature, and pungent odors. Binary logistic regression analysis (Table 5) confirmed the significant relationship between specific environmental factors (high room temperature, poor air quality) and various SBS symptoms. Specifically, mucosal symptoms were significantly correlated with high room temperature and poor air quality, general symptoms with poor air quality, psychological symptoms with high room temperature, and skin symptoms with poor air quality. The events per variable (EPV) analysis showed sufficient statistical power for the regression results.

The findings demonstrate the significant prevalence of substandard PRRHs in Xiamen and their detrimental impact on renters' health. The high prevalence of SBS-related symptoms highlights the serious public health implications of inadequate housing. This situation mirrors similar challenges faced by other rapidly developing cities globally, including the historical examples of back-to-back housing in the UK and backyard accommodations in South Africa, illustrating the long-term social and economic costs of unchecked, low-quality housing development. While Xiamen has implemented social housing initiatives, these efforts are insufficient to address the massive demand for affordable housing. The study suggests exploring alternative approaches, such as public-private partnerships and policies to incentivize enterprises to provide housing support for their employees or relocate to suburban areas. The city’s recent policy limiting PRRH building height and construction area addresses future construction but fails to address existing health problems. The findings highlight the need for proactive governmental interventions, including financial support for renovations, replacements, and targeted education on health risks. The unexpectedly high satisfaction rate among participants despite severe environmental problems highlights the need for public education on healthy living environments.

This study reveals the widespread prevalence of substandard PRRHs in Xiamen, China, and their significant negative impact on the health of their occupants. The findings underscore the need for proactive governmental intervention in managing rapid urbanization and ensuring access to safe, healthy, and affordable housing. Future research should investigate the differences between PRRHs and publicly funded rental housing to provide more comprehensive guidance for future housing development. Targeted education campaigns should be implemented to raise public awareness of the health risks associated with substandard housing.

The study's focus on the summer season may limit the generalizability of findings to other seasons. The reliance on self-reported health data through a questionnaire might introduce recall bias. The sample size, while substantial, may not fully represent the entire PRRH population in Xiamen. Future research could address these limitations by expanding the study to include other seasons, using objective health indicators, and employing a larger, more diverse sample.