The ageing of the population has become a global phenomenon. According to the World Health Organization, older adults aged 60 and over was 1 billion in 2019. This number will increase to 1.4 billion in 2030 and 2.1 billion in 2050[1]. China, as the country with the largest number of older adults in the world, faces an even greater challenge of population ageing. The 7th National Population Census of China showed that China had 264.2 million older adults people aged 60 years and above, accounting for 18.70% of the total population. As the population of older adults grows and life expectancy increases, the social security system and public health system will face even bigger health challenges. Thus, we need to take more active and effective measures to maintain the health of the older adults and promote healthy ageing.
Depression and poor sleep quality are two issues that are highly prevalent among the older adults and often occur together which pose a huge threat to their health status[2, 3]. In terms of depression, it is a common mental health problem among older people. A large number of older adults worldwide have experienced depression. A systematic review has pointed out that the prevalence of depression among the older adults ranged from 1–16% [4]. Suffering from depression can lead to high rates of disability, high medical expenses, and reduced life quality. Additionally, sleep health is also an important public health issue of concern, especially in an ageing society. Poor sleep quality is more prevalent among older individuals than young people. About half of older people reported that they have sleep problems, including decreased sleep duration and sleep efficiency, irregular sleep schedules, and inability to fall asleep and stay asleep, ultimately leading to a decrease in sleep quality. Studies have found that poor sleep quality is associated with a wide range of adverse health outcomes, including cardiovascular disease[5, 6], stroke[7], cognitive impairment[8] and depression[9]. The high prevalence and the heavy burden of depression and poor sleep quality indicate the importance to find the risk factors that influence depression and sleep quality in the older adults, which have an important role to improve their quality of life and health status.
Depression and poor sleep quality are associated with multiple risk factors, including socio-demographic characteristics, Socioeconomic Status (SES), social support, lifestyle, stress and health status[10–12]. SES has attracted much attention for its impact on health for a long time, including mental health and sleep health. SES refers to the position of an individual or group in a class society and is usually measured by education, occupation and income. In previous studies, two different views about the mechanism between socioeconomic status and health had been put forward. One is the health selective theory which suggests that unhealthy bodies will limit an individual's educational and occupational achievement, leading to low SES. The other is the social causation theory, which suggests that the differences in SES are the underlying cause of health inequalities, the factors associated with low SES, such as stressful life events and poor living conditions, will increase the risk of diseases[13, 14]. Despite inconsistent theories, there were growing evidence proved that poor living conditions, and unaffordable for cleaner and healthier household products associated with low SES have unavoidable effects on people's health[15]. However, the exact mechanism between them is unclear.
Solid fuels used for cooking activities are a major source of indoor air pollutants, including carbon monoxide (CO), nitrogen oxides, inhalable particulate matter and some carcinogenic compounds[16, 17]. According to a report by WHO, around 2.6 billion people worldwide still use kerosene, biomass (wood, animal dung and crop waste) and coal as the main fuels for cooking. Additionally, 7 million people die prematurely each year due to the combined effects of environmental air pollution and household air pollution[18]. As the largest developing country, a large proportion of people in China still rely on solid fuels for cooking and heating. Accumulating evidence has suggested that exposure to indoor air pollution caused by household solid fuels use could be associated with worse sleep quality and depression. However, current researches on the effects of household solid fuels use on sleep health and mental health in the older adults are inadequate and the results were inconsistent.
In terms of sleep quality, increasing studies have shown that exposure to air pollution has detrimental effects on sleep quality[19, 20], but limited studies have investigated the relationship between indoor air pollution from household solid fuels use and sleep quality, and most study subjects are adults or children[21, 22], lack attention to the older adults who are more likely to develop sleeping health problems. To our knowledge, in China, there is only one study that investigated the relationship between sleep quality and household solid fuels use in the older adults, it was conducted on 1,725 older adults aged 80 years and above in Hainan Province, China[23]. Therefore, there is a lack of relevant studies on normal older adults aged 60 years and above. In terms of depression, recently, several studies have investigated the relationship between air pollution and depression[24], but their results were inconsistent. A cross-sectional study in rural India showed that, compared to a cleaner fuel (liquefied petroleum gas), age-matched women who cooked with biomass fuel had a higher prevalence of depression[25]. Another survey of a total of 4,585 older adults in rural China also showed that those who used solid fuels for cooking were nearly 1.2 times more likely to suffer from depression than those who used clean fuels for cooking[26]. In contrast, a study in the Helsinki region, Finland found no convincing evidence of the effect of long-term exposure to PM2.5 from residential wood combustion on depression[27]. Four European general population cohorts of 70,928 individuals also found no clear evidence for the relationship between air pollution and depressive symptoms[28]. Therefore, more studies based on large national data are needed to demonstrate the relationship between household solid fuels use and depression.
To address the limitations of prior studies, the purpose of this study was to examine the association between SES and depression, SES and sleep quality, as well as the mediating role of household solid fuels in a nationally representative sample of Chinese older adults aged 60 and over. This study helped to understand the mechanisms that influence sleep health and mental health in the older adults, and provided a reference for relevant policy development. Research hypotheses formulated based on this research model are as follows:
H1. People with low SES are more likely to use household solid fuels to cook.
H2. Household solid fuels use has a negative effect on sleep quality.
H3. Household solid fuels use has a positive effect on depression.