Objective To determine the association between solid fuel exposure and cancer risk among middle-aged and elderly adults in China, to investigate the underlying biological pathways through selected serological markers, and to examine whether adequate physical activity can mitigate this risk by modulating these pathways.

Methods Based on baseline characteristics, health status indicators and hematological data from the China Health and Retirement Longitudinal Study (CHARLS, 2011‒2018), multivariate logistic regression analyses were conducted to assess the association between solid fuel use and cancer risk, with stratified analyses conducted by physical activity levels. In addition, mediation analyses were performed to evaluate the role of serological markers including hemoglobin concentration and hematocrit in the association between solid fuel use and cancer incidence.

Results Solid fuel use was significantly associated with an increased cancer risk (OR=1.344, 95%CI: 1.113‒1.615). This association remained significant among individuals with low levels of physical activity ( OR=1.344, 95%CI: 1.067‒1.673 ), but not statistically significant among those with adequate physical activity. Hemoglobin concentration and hematocrit showed a negative mediating effect between solid fuel use and cancer incidence, and this effect was stronger among those with low levels of physical activity.

Conclusion Indoor solid fuel use represents an important environmental risk factor for cancer incidence in China’s middle-aged and elderly population, while regular physical activity may reduce carcinogenic risk through modulation of inflammatory levels and hematological indicators such as hemoglobin and hematocrit. Public health strategies should integrate clean energy promotion with exercise interventions to mitigate the cancer burden associated with solid fuel pollution.