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Objectives: High levels of particulate pollution due to secondhand smoke (SHS) have previously been recorded in English prisons. As part of an evaluation to ascertain whether a new comprehensive smoke-free policy introduced in the first four prisons in England was successfully implemented, this study compares indoor air quality on prison wing landing locations three months before and three months after going smoke-free., Design: An indoor air quality monitoring study, comparing SHS levels before and after a comprehensive smoke-free prison policy., Setting: The first four prisons in England to implement a comprehensive smoke-free policy., Primary and Secondary Measures: We compared concentrations of airborne particulate matter <2.5 microns in diameter (PM₂.₅), as a marker for SHS, on wing landing locations three months before and three months after the smoke-free policy was implemented. Static battery operated aerosol monitors were used to sample concentrations of PM₂.₅ on wing landings., Results: After discarding data from monitors that had been tampered with we were able to analyse paired data across four prisons from 74 locations, across 29 wing landing locations, for an average sampling time of five hours and eight minutes. When comparing samples taken three months before with the paired samples taken three months after policy implementation (paired for prison, day of the week, time of day, wing location and position of monitor), there was a 66% reduction in mean PM₂.₅ concentrations across the four prisons sampled, from 39 to 13 µg/m³ (difference 26 µg/m³, 95% CI 25 to 26 µg/m³)., Conclusion: Prison smoke-free policies achieve significant improvements in indoor air quality. A national smoke-free policy would therefore be an effective means of protecting prisoners and staff from harm due to SHS exposure in the prison environment., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2019. Re-use permitted under CC BY. Published by BMJ.)