Clean Air for Everyone (CLAIRE): an Intervention Study to Reduce the Amount of Fine Particulate Matter and Microorganisms in Classroom Air (CLAIRE)

March 12, 2026 updated by: Lidwien Smit

Cluster-randomized Controlled Trial Assessing the Impact of Air Cleaning Technologies on Microbial and Physicochemical Markers in Primary School Classrooms

In this study, the investigators tested whether portable air cleaners can improve the air quality in primary school classrooms. Poor air quality can affect children's respiratory health and learning. Classroom air quality is a key concern, as children spend a large part of their day there, often with many students in one room. The investigators studied air quality in 180 classrooms in 29 Dutch primary schools. Within each school, classrooms were grouped in threes based on similar characteristics, such as size, number of students, and ventilation system. Classrooms in each group were randomly assigned to receive either: an air cleaner with a filter that traps particles from the air, an air cleaner that uses an electric charge to remove particles from the air, or no air cleaner (control). All devices were tested for safety in a laboratory beforehand. Measurements were taken in all classrooms before the devices were switched on, to provide a refence point. The air cleaners were then operated for nine weeks. After that, air cleaners were switched off for two weeks while additional measurements continued. To measure the effect of air cleaners, special cloths were placed in classrooms to collect dust and tiny droplets from the air, which were later analyzed in a laboratory for bacteria and viruses. This included viruses that commonly cause respiratory infections, such as flu and respiratory syncytial virus (RSV), and bacteria originating from the human skin, nose, or mouth that can spread through the air. Small sensors also tracked tiny particles in the air (particulate matter). The investigators collected additional information on classroom attendance and parent-reported respiratory symptoms to explore possible links. This study provides real-world evidence on how portable air cleaners can affect classroom air quality and may help guide decisions to improve indoor air environments in schools.

Study Overview

Status

Completed

Conditions

Intervention / Treatment

Detailed Description

The cluster-randomized trial evaluated the effectiveness of portable air cleaners in 180 classrooms across 29 Dutch primary schools. Classrooms were grouped within schools based on similar characteristics and randomly assigned to receive either HEPA-filter air cleaners, ionization/plasma air cleaners, or no air cleaners. The study contained a three-week baseline, followed by three repeated three-week intervention periods and a two-week post-intervention period, resulting in 14 weeks of measurements per school. Air cleaners were pre-tested for safety and operated at a clean air delivery rate (CADR) of approximately 400-500 m³/h during intervention. Airborne dust was collected using electrostatic dust fall collectors (EDCs), a passive air sampling method, and analyzed for respiratory viruses, human-associated bacteria and a general bacterial marker using quantitative polymerase chain reaction (qPCR). In a subset of eleven classrooms, active air sampling was performed alongside EDCs, to validate and quantify passive measurements. Low-cost indoor air sensors continuously monitored particulate matter, CO₂, temperature, relative humidity, and volatile organic compounds (VOCs). Retrospective parent-reported respiratory symptoms and classroom absenteeism data were gathered by surveys. Outcomes will be analyzed using hierarchical mixed-effects models.

Study Type

Interventional

Enrollment (Actual)

180

Phase

  • Not Applicable

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Locations

  • Netherlands
    • Utrecht
      • Utrecht, Utrecht, Netherlands, 3508 TC
        • Utrecht University

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Ages Eligible for Study

  • Child

Accepts Healthy Volunteers

Yes

Description

Inclusion Criteria:

  1. Primary schools with at least three comparable classrooms between grade 3-8
  2. Regular primary education classrooms
  3. Primary schools located within reasonable travel distance of Utrecht, the Netherlands

Exclusion Criteria:

  1. Schools for special education
  2. Secondary schools
  3. Kindergarten classes (grade 1&2; approximately ages 4-6 years)
  4. Schools unable or unwilling to provide the required study information

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Prevention
  • Allocation: Randomized
  • Interventional Model: Parallel Assignment
  • Masking: Single

Arms and Interventions

Participant Group / Arm
Intervention / Treatment
Experimental: HEPA filter air cleaners
Classrooms received a portable HEPA filter air cleaner, operated for the duration of the study.
Device: HEPA filter air cleaners
Classrooms received a portable HEPA filter air cleaner, operated for the duration of the study. Devices were set to provide an approximate clean air delivery rate (CADR) of 400-500 m³/h. Maintenance was performed according to manufacturer guidelines.
Experimental: Ionization or plasma air cleaners
Classrooms received a portable ionization or plasma air cleaners, operated for the duration of the study.
Device: Ionization or plasma air cleaners
Classrooms received an ionization or plasma air cleaners, operated for the duration of the study. Devices were set to provide an approximate clean air delivery rate (CADR) of 400-500 m³/h. Maintenance was performed according to manufacturer guidelines.
No Intervention: Control
Classrooms received no intervention.

What is the study measuring?

Primary Outcome Measures

Outcome Measure
Time Frame
Presence and/or concentration of airborne bacteria and viruses in classroom air
Time Frame: 12 weeks per school
12 weeks per school
Concentration of particulate matter in classroom air
Time Frame: 11 weeks per school
11 weeks per school

Secondary Outcome Measures

Outcome Measure
Measure Description
Time Frame
Respiratory symptoms
Time Frame: Up to 12 months
Number of participants with reported respiratory symptoms. Assessed using parent-reported questionnaires covering the study period.
Up to 12 months
Student absentee data
Time Frame: 12 weeks per school
Student absence from school (yes or no). Anonymous student absentee data were collected from school records after the intervention period. The outcome reflects absenteeism during the period when air cleaning interventions were applied in the classrooms.
12 weeks per school

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

Lidwien Smit

Collaborators

Utrecht University
Eindhoven University of Technology

Investigators

  • Principal Investigator: Lidwien A. Smit, PhD, Utrecht University

Publications and helpful links

The person responsible for entering information about the study voluntarily provides these publications. These may be about anything related to the study.

General Publications

  • Noss I,Wouters IM,Visser M,Heederik DJ,Thorne PS,Brunekreef B,Doekes G
  • ANSI/AHAM. AC-1-2020: Method for Measuring Performance of Portable Household Electric Room Air Cleaners. 2020.

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

December 6, 2023

Primary Completion (Actual)

May 1, 2025

Study Completion (Actual)

May 1, 2025

Study Registration Dates

First Submitted

March 6, 2026

First Submitted That Met QC Criteria

March 12, 2026

First Posted (Actual)

March 18, 2026

Study Record Updates

Last Update Posted (Actual)

March 18, 2026

Last Update Submitted That Met QC Criteria

March 12, 2026

Last Verified

March 1, 2026

More Information

Terms related to this study

Keywords

Other Study ID Numbers

  • 10031209
  • LSHM22032 (Other Grant/Funding Number: Health~Holland)

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

UNDECIDED

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

No

Studies a U.S. FDA-regulated device product

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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