Modification of low temperature-related hospital admissions for cardiovascular diseases by multiple green space indicators at multiple spatial scales: Evidence from Guangzhou, China

Cardiovascular diseases (CVDs) are a serious threat to global public health (Jagannathan et al., 2019). In China, CVDs are the leading cause of death and premature death (Ma et al., 2020). According to statistics, over 40% of China's population deaths are attributable to CVDs. Existing studies have indicated that abnormal temperature is a key factor in inducing CVDs. Various abnormal temperature factors, such as extreme temperature, cold spells, and heat waves, impact the occurrence of CVDs, among which low temperature has a significant adverse effect on the occurrence of various CVDs subtypes, whereas the adverse effect of high temperature may insignificant in some CVDs subtypes and regions (Phung et al., 2016). Low temperature can induce CVDs through a variety of pathophysiological mechanisms (Freund and Sawka, 1996; Keatinge et al., 1984). Continued attention to the relationship between low temperature and CVDs is crucial (Chen et al., 2019).

The urban built environment modifies the relationship between air temperature and health, the reason is that various built environment factors affect temperature distribution within the city, this disparity results in different health risks to the public in different parts of the city (Schinasi et al., 2018). Urban “green space” is any vegetation in the urban landscape––a place that provides leisure and recreation for residents and a habitat for nature (Bolund and Hunhammar, 1999; Kabisch and Haase, 2013). Modifying temperature is an important way of green space influencing the human health. It is well known that under high temperature weather, green space can produce cooling effects through shading and evapotranspiration (Davtalab et al., 2020; Yu et al., 2018). Several studies have focused on the modification of high temperature related disease by green space, especially in the context of global warming, for example, Burkart et al. (2016) selected Lisbon, Portugal as the study area to explore the impact of green spaces on heat-related mortality; Bao et al. (2021) studied modification of heat-related stroke by green space in Shenzhen, China. The results of the above studies all show that the high temperature related disease risk decreases with the increase of green space rate. However, as an important climate modification factor, the impact of green space on air temperature is not limited to high temperatures. In low temperatures, green space can protect against cold wind and release sensible heat (Hong et al., 2012; Song and Wang, 2015; Vogel, 1989), this may have a protective effect on human health. To date, little is known about the modification of low temperature-related diseases, including CVDs, by green space. A study conducted in Wuhan, China (a subtropical city) has found that the low temperature related hospital admission risk of ischemic stroke decrease with the increase of green space rate (Li et al., 2021). The modification of low temperature related CVDs by green space is a question worthy of further consideration (Meili et al., 2021; Zhang et al., 2018).

Most of previous studies involving modification of temperature-related CVDs by green space only construct the vegetation scale indicator (Bao et al., 2021; Li et al., 2021), and rarely discuss the spatial distribution pattern of green space. The vegetation density and its arrangement can affect urban microclimate (Hami et al., 2019; Pérez and Perini, 2018), and may affect the relationship between temperature and the occurrence of CVDs. Shen and Lung (2016) selected the Taipei metropolitan area as their study area, and chose the annual mean temperature as the mediating variable. The researchers found that the temperature is positively related to the green patch distance and the green patch fragmentation, and the increase in temperature would increase the mortality of CVDs. However, in this study, the abnormal temperature interval is not set, so the modification by the green space distribution pattern on the low temperature is not analyzed. In addition, the accessibility of green space has been used to study CVDs (Coutts et al., 2010; Ngom et al., 2016). However, there is still a lack of attention to accessibility when discussing the modification of temperature-related CVDs by green space. In terms of spatial scale, previous studies have involved large scales to explore the modification by green space, for example, Shen and Lung (2016) et al. used blocks as research units, and the unit area exceeded 4.3 km2; Li et al. (2021) took the block divided by the main road network as the research unit, and the unit area could reach 3 km2. For the elderly, who are most prone to CVDs, the scope may exceed the daily activity space of this group (World Health Organization, 2016). In addition, Previous studies focused on a single spatial scale of green space (Bao et al., 2021; Burkart et al., 2016; Li et al., 2021), the modification by green space may change if the scale is altered. In conclusion, exploring the modification of temperature-related CVDs by green space with different spatial attributes and scales is necessary.

Based on the previous literature (Coutts et al., 2010; Lee et al., 2021; Liu et al., 2022; Ngom et al., 2016; Rahnama and Shaddel, 2019), this study constructed six green space indicators from three aspects: green space scale, distribution pattern, and accessibility. The time-stratified case-crossover design (TSCC) is used to explore the modification of the occurrence of low temperature-related CVDs by each indicator in circular buffers with radii ranging between 100 and 1000 m with the patient's home address as the center, at intervals of 100 m (Liu et al., 2022; World Health Organization, 2016; Yeager et al., 2020). We chose Guangzhou, China as the study area. Guangzhou is a subtropical city with a developed economy in China. Previous studies have shown that low temperature has adverse effects on CVDs in Guangzhou (Yang et al., 2015). The purpose of this study is to solve three important questions: 1) Whether green space attributes affect the occurrence of CVDs under low temperature conditions? 2) What are the characteristics of this effect with the change of spatial scales, and which scale has the strongest modification effect? and, 3) How does the modification change as the temperature further decreases? In the context of rapid urbanization, the answers to the above questions are significant to urban planning and related policy formulation.

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