Highlights
- •Indoor CO2 monitoring reveals to be a practical proxy to Sars-CoV-2 transmission risk.
- •A surveillance activity was conducted inside 11 classrooms during school reopening.
- •All classrooms showed inadequate ventilation conditions (CO2 higher than 700 ppm).
- •Better scenarios were depict when real time visualization of CO2 levels was allowed.
- •A 4 level-risk classification including specific corrective actions was provided.