Independent and joint associations of loneliness and household fuel use with cognitive function and functional dependency among middle-aged and older Chinese adults: a cross-sectional study using CHARLS 2015

Loneliness and household air pollution from solid fuel combustion have each been linked to unhealthy ageing, yet their joint relevance for cognitive and functional outcomes remains unclear. We conducted a cross-sectional analysis of the 2015 wave of the China Health and Retirement Longitudinal Study (CHARLS), including 13,879 community-dwelling adults aged ≥ 45 years. Loneliness and primary household cooking fuel were treated as binary exposures. Outcomes were total cognitive score and functional dependency. Multivariable linear regression and logistic regression were fitted, incorporating a product term to assess multiplicative interaction. Additive interaction for functional dependency was evaluated using the relative excess risk due to interaction (RERI), attributable proportion (AP), and synergy index (S). Overall, 24.0% of participants were lonely, 63.0% used clean fuels, and 25.3% had functional dependency. The lonely–solid fuel group had the lowest mean cognitive score and the highest prevalence of dependency, whereas the not lonely–clean fuel group showed the most favorable profile. In fully adjusted models, loneliness and solid fuel use were independently associated with lower cognitive scores, with no evidence of interaction. For functional dependency, loneliness and solid fuel use were each associated with higher odds, and a significant loneliness × fuel interaction was observed on the multiplicative scale (OR_interaction = 0.723, 95% CI 0.603–0.867). Loneliness and household fuel use were independently associated with cognitive function, while fuel use modified the association between loneliness and functional dependency on the multiplicative scale. These findings highlight heterogeneity in vulnerability and support integrated psychosocial and household-environment approaches for healthy ageing.