To pull or push? Performance of four passerine species in the lever-pulling task.

Environmental factors are increasingly recognised as important determinants of population health and health-related economic outcomes, with growing evidence linking environmental degradation to higher household healthcare expenditures and widening disparities in health outcomes across populations [–]. Exposure to environmental determinants, including air pollution and natural environments, has been associated with a wide range of chronic diseases as well as both adverse and beneficial health outcomes []. In particular, ambient fine particulate matter (PM₂.₅) is a major environmental risk factor contributing to cardiometabolic diseases, respiratory conditions, and premature mortality []. In contrast, exposure to residential greenspace has been associated with a range of beneficial health outcomes, including improvements in mental health, reduced cardiovascular risk, and decreased all-cause mortality []. These environmental determinants not only affect healthcare costs but also contribute to inequities in health burdens. Exposure to harmful environments, such as air pollution, is associated with higher healthcare utilisation, increased out-of-pocket expenditures, and greater pressure on public health resources [, ]. These burdens are often disproportionately borne by socioeconomically disadvantaged populations, who are more likely to reside in areas with higher pollution levels and limited access to health-promoting environments such as greenspace []. As a result, environmental inequalities can exacerbate existing health disparities, reinforcing cycles of poor health and financial hardship. China has experienced rapid economic development and urbanisation over recent decades, accompanied by substantial environmental changes. The burden of chronic diseases has increased substantially, leading to growing healthcare needs and rising healthcare expenditures []. Household out-of-pocket healthcare expenditure represents a major component of total healthcare expenditure in China and can impose considerable financial pressure on families, particularly among vulnerable populations []. To date, several studies have assessed the relationship between household healthcare expenditure and environmental exposures in China for air pollution and urban greenspace. Zhang et al. constructed an environmental quality index encompassing multiple indicators, including PM₂.₅ concentrations, water quality, and noise levels, to examine the effects of environmental conditions on older adults in China, and found that improvements in environmental quality were associated with a significant reduction in elderly healthcare expenditures []. An analysis of the China Health and Retirement Longitudinal Study indicated that air pollution was associated with increased inpatient care utilisation among individuals aged 60 and older []. Additional studies have reported that higher air pollution significantly elevates household healthcare expenditures, disproportionately affecting low-income households and contributing to health inequalities [, –]. In contrast, empirical evidence on the relationship between greenspace and household healthcare expenditure remains relatively limited. Evidence from Chengdu suggested that a higher degree of urbanisation, was associated with reductions in inpatient medical expenses []. Similarly, another study reported that greater urban greenspace quality, as measured by user satisfaction, was significantly inversely related to healthcare expenditure in Shanghai []. International evidence also supports the link between environmental exposures and healthcare expenditure []. In the United States, exposure to fine particulate matter has been associated with increased hospital admissions and higher Medicare expenditures []. Studies across European countries have demonstrated that elevated air pollution levels are linked to higher healthcare expenditure relating to cardiovascular and respiratory diseases []. In South and Southeast Asia, exposure to air pollution was associated with increased out-of-pocket healthcare expenditure, disproportionately burdening lower-income households []. More importantly, the distributional aspects of household healthcare expenditure across these environmental determinants of health remain poorly understood. Previous studies have assessed environmental exposure inequalities by socioeconomic or income groups, using measures such as the Gini coefficient or concentration index to quantify distributions of PM₂.₅ and urban greenspace. For example, Song et al. applied the Gini index to quantify inequality in greenspace exposure across 303 cities in China, finding that the majority of cities experienced substantial disparities, with 207 cities exhibiting Gini coefficients greater than 0.6 []. Cao et al. used the Palma coefficient, as an alternative inequality measure, to assess disparities in residential greenspace exposure within urban areas, reporting that populations with the highest exposure experi