Introduction

Sarcopenia is a progressive, age-related systemic disease characterized by a significant loss of skeletal muscle mass, strength, and function1. Globally, the prevalence of sarcopenia ranges from about 6–22%, making it a significant public health concern in aging populations2. As a core indicator of clinical assessment, the quantitative measurement of muscle mass plays an essential role in diagnosing and managing sarcopenia3. The muscle mass index is widely used to assess skeletal muscle health and has been validated in numerous studies4,5. Loss of muscle mass–a hallmark of the aging process, is strongly associated with disability, reduced mobility, falls, fractures, and increased mortality6. However, despite the increasing risks of sarcopenia, specific pharmacological treatments for low muscle mass (LMM) are still under investigation. Currently, muscle mass management still relies mainly on nutritional and exercise interventions, with neuromuscular electrical stimulation therapies are increasingly being applied in clinical practice7,8,9 .

Notably, in addition to traditional factors such as age, exercise, and nutrition10,11, the influence of environmental exposures on muscle homeostasis is gaining increasing attention. Several studies have shown that the decline in muscle mass is related to air pollutants, including PM2.5 (Particulate Matter 2.5), CO (Carbon Monoxide) and NO2 (Nitrogen Dioxide)12,13,14. Notably, a study has shown that modern people spend around 90% of their time in indoor environments, making indoor air quality a key factor in health15. A significant body of research has examined the adverse health outcomes (e.g., cardiovascular disease, respiratory diseases, and mortality) caused by indoor air pollution16,17, highlighting the importance of focusing on indoor air quality. Previous studies have also indicated that, in addition to the impact of outdoor air pollution on muscle mass loss, indoor air pollution may also contribute to muscle mass loss, especially for older Chinese adults who spend extended periods at home18,19. Using cooking fuels can seriously affect indoor air quality and increase the concentration of airborne pollutants such as PM2.520,21. A national study revealed that the incidence of sarcopenia was 4.5% lower among individuals using clean fuel for cooking, 2.9% lower among those using it for heating, and 3.0% and 1.9% higher among solid fuel users for cooking and heating, respectively22.

Indoor ventilation, a key method for improving indoor air quality, effectively reduces pollutant concentrations by introducing fresh air into the indoor spaces, either naturally (e.g., through open windows) or mechanically (e.g., using air conditioning), thereby mitigating their adverse effects on health23,24. For example, a European study of nursing homes found that enhanced indoor ventilation was effective in mitigating the adverse effects of indoor air quality on the health of older residents and significantly improved overall quality of life25. Additionally, epidemiological studies have shown that higher indoor ventilation frequency (IVF) is associated with reduced risk of allergic symptoms, respiratory diseases and cognitive function in young people in developed regions26,27,28. Currently, there are no reports examining the impact of quantifiable IVF on muscle mass. Most environmental health studies focus primarily on cardiorespiratory or metabolic outcomes, with limited exploration of muscle mass. To date, no studies have quantified the effect of IVF on LMM.

To fill this research gap, this study, based on data from the 2017–2018 China Longitudinal Healthy Longevity Survey (CLHLS), aimed to explore the association between IVF and LMM, providing new ideas for the development of individualised health management strategies.

Materials and methods

Data source and sample

This study utilized data from the CLHLS, conducted by Peking University’s Research Center for Aging Health and Development. CLHLS utilized a targeted random sample design to ensure the representativeness and reliability of the sample, providing comprehensive and extensive data on the healthcare status, caregiving, and medical requirements of older individuals in China. The detailed information regarding the CLHLS has been previously published29,30.

The 2018 CLHLS conducted interviews with a total of 15,874 participants. Excluding those participants with missing information on IVF, LMM and covariates, 9,708 participants were finally included in the analysis (Fig. 1). The project has obtained approval from the Biomedical Ethics Committee of Peking University, China (IRB00001052-13074).

Fig. 1
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Flowchart for the study design and participants.

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Indoor ventilation frequency

IVF was assessed based on the frequency of window opening per week in each season over the past year, as self-reported by the participants. Items were scored as follows: 0 points for 0 occurrences per week, 1 point for 1–5 occurrences per week, and 2 points for more than 5 occurrences per week. The IVF score, ranging from 0 to 8, was calculated by summing the individual IVF scores for each season. Scores of 0 to 3, 4 to 5, and 6 to 8 represent low, intermediate, and high IVF levels, respectively31,32.

Low muscle mass

According to the criteria established by the Asian Working Group for Sarcopenia, muscle mass was primarily assessed using appendicular skeletal muscle mass (ASM), measured by bioelectrical impedance analysis and dual energy X-ray absorptiometry33. However, the instruments required for these methods are expensive, bulky, and necessitate rigorous training, which limits their application in community settings. Kawakami, R et al. used anthropometric and physical function parameters to develop equations for estimating muscle mass34, providing a simple method for assessing muscle mass, and the method has been widely used in Chinese populations35. Therefore, our study evaluated LMM using an ASM prediction equation derived from weight, height, waist circumference, and calf circumference.The ASM prediction equation was: ASM (kg) = 2.955 * sex (male = 1, female = 0) + 0.255 * weight (kg) + 0.081 * height (cm) − 0.130 * waist circumference (cm) + 0.308 * calf circumference (cm)-11.89734. LMM was defined as < 7.0 kg*m-2 in males and < 5.7 kg*m-2 in females, calculated by multiplying ASM with height squared35.

Covariates

To ensure the authenticity and reliability of the research results and minimize the potential influence of confounding variables, we implemented control measures for several covariates associated with IVF and muscle mass, based on a review of relevant literature. Specifically, these covariates included age (65–79, ≥ 80), gender (male, female), residence (rural, urban), living arrangement (living with family, other), education (0, 1–6, > 6 years), marital status (married, widowed, single, divorced), economic situation (good, average, poor), work (manual labor, other), smoking (yes, no), drinking (yes, no), exercise (yes, no), cooking fuel (renewable clean energy, other), cooking ventilation (yes, no), life satisfaction (good, average, poor), self-rated health (good, average, poor), hypertension (yes, no), diabetes (yes, no), heart disease (yes, no), dementia (yes, no), insurance (yes, no), PM2.5 (quartiles), NO2 (quartiles). Detailed assignment and classification criteria are provided in Supplementary Table 1.

Statistical analysis

The basic characteristics of the study sample across LMM and IVF were delineated by descriptive analysis. Categorical variables that fit a normal distribution are described as percentages (n(%)). Categorical variables were compared between subjects with different demographic characteristics using chi-square tests. Binary logistic regression model was used to evaluate the association between IVF and LMM. Three models were fitted to this study: Model 1 was a crude model that did not account for any covariates; Model 2 adjusted for gender, age, residence, living arrangement, education, marital status, economic situation, work, smoking, drinking, exercise, cooking fuel, cooking ventilation; Model 3 further adjusted for life satisfaction, self-rated health, hypertension, diabetes, heart disease, dementia, insurance, PM2.5, NO2. The study also conducted subgroup analyses of gender, age, residence, living arrangement, education, marital status, economic situation, work, smoking, drinking, exercise, cooking fuel, cooking ventilation, life satisfaction, self-rated health, hypertension, diabetes, heart disease, dementia, insurance, PM2.5, NO2 based on Model 3 and calculated interaction effects of subgroup variables. We performed two sensitivity analyses to assess the robustness of the results. First, we used chained equations to interpolate missing values to ensure a thorough examination of the dataset. Second, recognizing the unreliability of self-reported data from older people with dementia, we excluded participants with dementia from the analysis. The statistical analysis was conducted using the SPSS 27.0 software package. The criterion for statistical significance was set at a level of P < 0.05.

Results

Basic characteristics of the participant

Table 1 describes the presence or absence of LMM in different basic characteristics of the included participants. Of the 9,708 participants in this study, 6,048 (62.30%) suffered from LMM. 61.26% of participants were aged older than or equal to 80 years of age, 54.30% were male, and 43.16% lived in rural areas.The chi-square test showed that there was a significant difference (p < 0.05) between those with LMM and without LMM in the gender, age, residence, education, marital status, economic situation, work, drinking, exercise, cooking fuels, cooking ventilation, life satisfaction, self-rated health, hypertension, diabetes, and heart disease groups.

Table 1 Comparison of muscle mass in different basic characteristics of included participants.
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Table 2 shows different IVFs with different basic characteristics of the participants. More than half of the participants (59.03%) ventilated their homes six to eight times a week, 32.15% 4 to 5 times, and 8.82% only 1 to 3 times. Participants grouped with different IVF showed statistically significant differences (p < 0.05) between gender, age, residence, living arrangement, education, marital status, economic status, work, smoking, exercise, cooking fuel, cooking ventilation, life satisfaction, self-rated health, insurance, and NO2.

Table 2 Comparison of indoor ventilation frequency in different basic characteristics of included participants.
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Association between indoor ventilation frequency and low muscle mass

There are significant differences between seasonal and overall IVF and muscle mass (P < 0.001) (Table 3). Participants with LMM in older adults were observed to have a notably greater prevalence of low IVF and a lower prevalence of high IVF compared to those without LMM in each season.

Table 3 Bivariate association between seasonal indoor ventilation frequency and low muscle mass.
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In model 3, which was adjusted for various factors, participants with intermediate IVF (OR: 0.805; 95% CI: 0.669–0.969) and high IVF (OR: 0.818; 95% CI: 0.684–0.979) were 19.5% and 18.2% less likely to develop LMM, respectively, compared with participants with low IVF (Table 4).

Table 4 Association between indoor ventilation frequency and low muscle mass.
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The association between LMM and IVF by season was investigated in Model 3 (Table 5). Significant associations between IVF and LMM were observed only in spring and winter. Compared with the low IVF population, the probability of LMM in the spring was 25.3% (OR: 0.747; 95% CI: 0.581–0.961) and 23.3% (OR: 0.767; 95% CI: 0.597–0.985) lower in the middle and high IVF elderly populations, respectively, whereas the probability of LMM in the spring was 36.5% (OR: 0.625; 95% CI: 0.474–0.824) and 34.1% (OR: 0.659; 95% CI: 0.501–0.868), respectively.

Table 5 Association between seasonal indoor ventilation frequency and low muscle mass.
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Subgroup analysis

We considered 22 subgroups such as age-sex and further observed consistency in the association of IVF with LMM. The association between IVF and LMM was statistically significant (p < 0.05) for gender, age, residence, living arrangement, marital status, economic situation, work, smoking, drinking, exercise, cooking ventilation, life satisfaction, self-rated health, hypertension, diabetes, heart disease, dementia, and NO2 (Fig. 2). Further interaction analyses showed a significant interaction effect between the drinking subgroup and IVF (P for interaction < 0.05).

Fig. 2
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ORs and 95% CI for low muscle mass in adherence to indoor ventilation frequency, stratified by selected characteristics.

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Sensitivity analyses

Two sensitivity analyses conducted in the fully adjusted model 3 demonstrate the robustness of our findings. The first used chained-equation multiple interpolation to deal with missing data (Supplementary Table 2), and the second excluded participants with dementia, both of which showed that a statistically significant association between IVF and LMM existed (P < 0.001) (Supplementary Table 3).

Discussion

LMM is prevalent in older adults and is associated with many adverse outcomes, such as falls, metabolic disease, and mortality, demonstrating the value of LMM research36,37 In the present day, individuals have allocated over 90% of their time within enclosed spaces, underscoring the significance of upholding optimal indoor air quality and mitigating potential airborne pollutants according to indoor ventilation as pivotal elements in preserving overall well-being15. This nationally representative study found that higher IVF was associated with a lower risk of LMM and observed that the association between IVF and LMM varied across subgroups.

Our study provides a new finding that high IVF may be a protective factor for LMM. Numerous studies have confirmed the correlation between indoor ventilation and improved home air quality38,39 .For example, DuJ et al. showed that indoor ventilation was effective in improving indoor air quality and reducing pollutants in the absence of outdoor pollution and under favourable climatic conditions32. Through a literature review, we identified several studies reporting the association and intrinsic mechanisms between indoor air quality and sarcopenia and muscle mass12,14. The pathophysiological mechanisms of muscle mass and sarcopenia are intricate and encompass interactions among multiple physiological systems. Possible explanations include alterations in skeletal muscle structure, myofascial dysfunction, muscle balance and resistance to growth, inflammation and impairment of mitochondrial function, as well as neural pathways, which mainly manifested as a decrease in muscle mass leading to the decline of muscle strength and worsens with age1. PM2.5 can modulate inflammation in visceral adipose tissue, lipid metabolism in the liver, and glucose utilisation in skeletal muscle occurs through both CCR2-dependent and CCR2-independent pathway40. The signalling of neurotransmitters and neurotrophic factors, neuronal remodelling, and neurodegeneration are impaired by inflammation and oxidative stress induced by air pollution41. Breathing under normobaric conditions attenuated carbon monoxide-induced decreases in muscle oxygenation, particularly in the intercostal muscles, without affecting endurance42. In conclusion, air pollution can affect muscle through different pathways and comorbidities. Previous articles have highlighted the effects of air pollutants from solid fuels used for indoor cooking and domestic heat production on sarcopenia43,44. It follows that older people who rely on solid fuels for cooking and heating should prioritise ventilation to mitigate the adverse effects of fuel pollutants on muscle health.

Our study showed a significant correlation between an increase in IVF in spring and autumn and a reduced likelihood of developing LMM in older adults. Many studies have reported that air quality is associated with seasonal changes in air pollutants such as PM, CO and airborne microorganisms45,46. Seasonal climate-induced changes in temperature, humidity, etc., and accompanying differences in airborne particulate matter, microorganisms, etc., may be influential factors in the seasonal differences in the incidence of LMM. A survey of window opening behaviour in hospital wards showed that the lower frequency of window opening in summer and winter might be due to higher or lower outdoor temperatures, whereas, in the transition season, outdoor temperatures of 20 to 25 degrees are likely to be the most comfortable, with a correspondingly higher frequency of window opening. The humidity of the outdoor air in each season is also an essential factor influencing the frequency of window openings47. In addition, the results of a study investigating the effects of climate change on the infiltration of outdoor air pollution showed a clear seasonal pattern of particulate matter (PM) infiltration. Specifically, homes measured during transitional seasons, such as spring and autumn, had higher mean PM infiltration rates compared to homes measured during summer and winter48, which may explain the association between IVF and LMM being significant in spring and autumn rather than summer and winter .

Subgroup analyses showed that adjusted covariates were not significant modifiers of the association between IVF and LMM, but we still observed significant interactions between drinking and the association between IVF and LMM. We observed a significant association of IVF with LMM in men but not women.Previous studies have reported sex differences in muscle homeostasis49,50.Sex steroids promote muscle function and benefit skeletal muscle repair and metabolic function after injury. Androgens may be the main sex steroid regulating muscle homeostasis in men51, which may potentially account for the sex differences in LMM and IVF observed in this study. Previous studies have confirmed that muscle mass decreases with age52, however, our study found that the association between IVF and LMM showed significant age heterogeneity: a significant negative correlation was observed in the 65–79 year old group, whereas no statistically significant correlation was observed in the ≥ 80 year old group. This may be due to the severe loss of muscle mass due to multiple pathophysiological mechanisms in advanced age individuals, when the effects of IVF are masked by the systemic aging process. In our study, we observed that in cities, IVF was significantly associated with LMM, which may be due to denser buildings and poorer air quality in cities, where ventilation can more significantly improve indoor air quality. Yin J et al. also observed a significant difference in the importance of ventilation for public health between urban and rural areas, influenced by factors such as building density and height53 .Smoking impacts cellular-level muscle proteolysis, leading to impaired muscle mass and the increased demand for indoor ventilation to mitigate indoor air pollutants54,55. We observed a significant correlation between IVF and LMM in the non-smoking group, which may be due to the fact that smoke and toxins produced by smoking are more damaging to muscle mass, which is not achieved by improving indoor ventilation. This may suggest that smoking cessation is more effective in protecting muscle mass than ventilation. Previous studies have established physical exercise as a protective factor against sarcopenia, demonstrating its efficacy in preserving muscle mass and strength56,57. Our study further revealed a significant association between increased IVF and LMM specifically within the physically active subgroup, suggesting potential synergistic effects between exercise and optimized ventilation in muscle preservation. This finding warrants further investigation into whether exercise intensity and modality (e.g., resistance training vs. aerobic exercise) differentially modulate the protective benefits of IVF against LMM. Notably, the association between IVF and LMM was exclusively observed in individuals with favorable self-rated health status and absence of chronic comorbidities (hypertension, diabetes, cardiovascular disease, or dementia). Older adults with poor health status or chronic conditions often experience prolonged health deterioration that may lead to advanced muscle atrophy or metabolic dysregulation, potentially obscuring ventilation-related effects. Furthermore, chronic disease patients, particularly elderly or long-term sufferers, frequently exhibit diminished physiological adaptability and recovery capacity. Their muscle mass is typically compromised by disease progression or pharmacological interventions (e.g., corticosteroid use), which may attenuate ventilation-mediated impacts on musculoskeletal health. Our analysis identified a significant protective effect of IVF against LMM under elevated NO2 concentrations. Airborne pollutants including PM2.5 and NO2 have been shown to induce mitochondrial dysfunction through increased reactive oxygen species production and pro-inflammatory cascades58,59. Enhanced ventilation during high NO2 exposure may mitigate these effects through improved air exchange, thereby reducing pollutant concentrations and demonstrating measurable protection - an effect less apparent under low pollution conditions. Additionally, we observed significant IVF-LMM associations and notable interaction effects with alcohol consumption status in drinking populations. Extensive evidence documents alcohol’s detrimental impacts on muscle integrity through multiple pathways, including dysregulated autophagy, mitochondrial impairment, and nutrient malabsorption60,61. Compared to abstainers, alcohol consumers exhibited higher smoking prevalence and prolonged indoor exposure due to social habits or health limitations (e.g., chronic disease-related mobility restrictions), potentially amplifying ventilation sensitivity. Conversely, non-drinkers likely represent a health-conscious cohort (evidenced by balanced nutrition and regular exercise) whose muscle mass appears less susceptible to ventilation variations. However, our analysis was constrained by the database’s lack of alcohol consumption frequency/dosage metrics, precluding dose-response assessment and introducing potential residual confounding. Future investigations should incorporate standardized alcohol biomarkers (e.g., phosphatidylethanol) and detailed consumption patterns to elucidate alcohol-ventilation synergies.

Conclusions

This study examined the relationship between overall and seasonal IVF and LMM in Chinese older adults.The findings suggest that higher IVF is significantly associated with lower occurrence of LMM in Chinese older adults, especially in the Spring and Autumn seasons. This study provides valuable insights to promote successful aging and reduce the burden of LMM in the elderly population.

Limitation

The current study is subject to several limitations. Firstly, as our data is derived from a national cross-sectional study that precludes the establishment of causal relationships, it is imperative to examine the interplay between IVF and LMM. Further extension of follow-up is necessary to assess the longitudinal relationship between IVF and muscle decline in older adults and to determine the causal link between these two factors. Secondly, this study exclusively examines the impact of the window opening on LMM across seasons without considering other ventilation systems, such as air conditioners and air purifiers, and neglects the influence of specific ventilation locations, the area of open windows and the duration of each ventilation session. Third, this study investigated the association between IVF and LMM in different seasons, but the questionnaire content limitations prevented the determination of muscle mass in each season.We plan to refine the seasonal tracking design in future studies to more fully reveal the relationship between LMM and IVF. Fourth, the present study assumes an ideal scenario in which indoor air quality can be enhanced through indoor ventilation; however, pollutants from the outdoor environment and adverse weather conditions may exacerbate indoor air pollution. Hence, additional research is required to explore the underlying mechanisms of the influence of window opening and ventilation on muscle mass in older individuals.