Background <p>Air purifiers with high-efficiency particulate air (HEPA) filters aim to reduce particulate matter from the air, including infectious aerosols, potentially lowering airborne disease transmission in classrooms. This pilot study, informing a larger randomized trial (RCT), explored the acceptability and feasibility of air purifiers in classrooms, and estimated their effect on particulate matter (PM_2.5) concentration.</p> Methods <p>This study, conducted in schools in the greater Oslo area, Norway, from January to April 2023, consisted of two parts lasting 6 and 9 weeks, respectively. In part 1, eligible primary school classrooms received either ceiling-mounted or portable purifiers. In part 2, we conducted a three-arm randomized multiple crossover trial of ceiling-mounted versus portable purifiers versus no air purification. Schools with three classrooms on the same floor with adequate space for air purifiers were eligible. We evaluated acceptability and feasibility of installing and operating air purifiers by collecting interview data from students (part 1) and teachers (parts 1 and 2). The feasibility of collecting consent and questionnaires from students’ legal guardians and student absences from municipality records was explored. We evaluated superiority of air purification versus no air purification and noninferiority of portable versus ceiling-mounted air purifiers on PM_2.5 concentration using sensor measured air quality data.</p> Results <p>Five primary schools (five classrooms) and one adult education school (three classrooms) participated in parts 1 and 2 of the study, respectively. Ceiling-mounted and portable air purifiers were feasible to install and operate and were well-accepted by students (<i>n</i> = 11) and teachers (<i>n</i> = 7). No harms were reported. Consent and questionnaires were collected from 23 of 114 students’ legal guardians. Collecting absence data from municipality records was feasible, although inconsistencies in teachers’ recording affected weekly accuracy. Air purification was superior to no air purification in reducing PM_2.5 concentration (<i>p</i> = 0.011), and portable purifiers were noninferior to ceiling-mounted units (<i>p</i> &lt; 0.001) and significantly reduced PM_2.5 compared to no air purification (rate ratio 0.71; 95% CI 0.57–0.89).</p> Conclusions <p>Installing and operating air purifiers in classrooms is feasible and acceptable. Portable purifiers are user-friendly and reduce PM_2.5 concentration. It was challenging to obtain consent from students’ legal guardians to collect absence data; municipality records proved more practical. A larger RCT on air purifier effectiveness in schools will be feasible if data collection challenges can be overcome.</p> Trial registration <p>NCT06374316 (retrospectively registered&#xa0;15 April 2024)&#xa0;</p>