Indoor concentrations and exposure levels of CO, SO₂, NO₂, and O₃ in Chinese residences, schools, and offices (2000–2021): A systematic review

Abstract

This systematic review addressed critical knowledge gaps regarding the spatiotemporal patterns and human exposure to indoor gaseous pollutants (CO, SO₂, NO₂, O₃) in Chinese civil buildings (residences, schools, offices) from 2000 to 2021. A total of 108 field measurement studies revealed divergent temporal trends: a decline in indoor CO, SO₂, and NO₂ concentrations over the past two decades in China, alongside a rise in indoor O₃ concentrations. The highest CO (8.4 mg/m³), SO₂ (1473.6 μg/m³), and NO₂ (217.0 μg/m³) concentrations were observed in northwest China. Time-weighted exposure assessments indicated elevated levels in children compared to adults (office workers) for CO (3.1 vs. 1.7 mg/m³), SO₂ (248.4 vs. 239.5 μg/m³), and NO₂ (50.5 vs. 42.3 μg/m³), while higher O₃ exposure was identified among office workers (25.0 vs. 21.1 μg/m³ in children). These findings underscore the urgency of implementing population-specific interventions, such as solid fuel replacement programs in high-burden rural regions and reducing the use of ozone-releasing appliances in office environments. Future efforts should prioritize formulating more stringent indoor air quality standards and establishing comprehensive long-term monitoring systems across different indoor environments, which would provide a robust foundation for refining population exposure models and developing targeted mitigation strategies aligned with spatiotemporal pollutant dynamics.