The field-study question
School occupancy, classroom traffic, outdoor air effects, and HVAC conditions vary as students and staff move throughout the day. Real-world air purifier testing examines an air-cleaning intervention amid these changing conditions in occupied spaces.3,4
The seven-week study evaluated plasma-based room air purifiers in four elementary schools: two treated schools and two untreated control schools. It compared airborne bacterial concentrations and attendance observations during the same period.
Parallel-controlled design
The study used two parallel-controlled comparisons, each pairing a treated elementary school with an untreated control school. Treated classrooms and common areas used plasma-based room air purifiers. The control schools operated without the added air purification intervention.
School pairs were selected for comparability in building age, existing air systems, socioeconomic demographics, geography, and morning or afternoon sampling time. Matching makes the field comparison more meaningful than a one-building demonstration, but it cannot remove every difference between schools.
Two aerosol scientists sampled each treated/control pair simultaneously over seven weeks. At each school, they collected samples from five classrooms, one common area, and one outdoor location. Each comparison generated 490 bacterial air samples, providing repeated field measurements rather than a single snapshot.
Repeated sampling gives a fuller description of conditions within these schools. The intervention comparison, however, still involves four schools arranged in two pairs. More air samples do not mean more independently assigned schools. The public summary therefore compares schools within each pair and does not use the sample count as evidence that the findings apply to a larger school population.
What ARE Labs measured
ARE Labs measured classroom airborne bacterial concentration as colony-forming units per cubic meter of air. Illness-related absenteeism was recorded over the same study period. Bacterial concentration is a sampling endpoint; it is not a measurement of clinical infections.
Sampling five classrooms, a common area, and an outdoor location captured several environmental contexts. The displayed outcome is the average classroom bacterial concentration. It is not an indoor-to-outdoor ratio, a whole-building removal rate, or a direct measurement of purifier CADR. Collecting outdoor samples does not establish that every classroom result was adjusted for outdoor conditions.
| Study pair | School role | Average classroom bacterial concentration | Bacterial concentration comparison | Illness-related absenteeism |
|---|---|---|---|---|
| Pair 1 | Treated | 30.2 cfu/m3 | 41.1% lower than control | 5.12% |
| Pair 1 | Control | 51.3 cfu/m3 | Reference | 10.99% |
| Pair 2 | Treated | 56.1 cfu/m3 | 21.9% lower than control | 4.75% |
| Pair 2 | Control | 71.8 cfu/m3 | Reference | 7.28% |
Descriptive values from a non-public ARE Labs 2025 school report. Attendance differences are observational and do not establish causation.
Lower classroom bacterial concentrations
In Pair 1, the treated school averaged 30.2 cfu/m3 across sampled classrooms, compared with 51.3 cfu/m3 in its untreated control school. The treated school's average classroom bacterial concentration was 41.1% lower. In Pair 2, the treated school averaged 56.1 cfu/m3, compared with 71.8 cfu/m3 in its untreated control school, a 21.9% lower concentration.
View figure data as a table
| pair | Treated school | Control school |
|---|---|---|
| Pair 1 | 30.2cfu/m3 | 51.3cfu/m3 |
| Pair 2 | 56.1cfu/m3 | 71.8cfu/m3 |
Source: non-public ARE Labs 2025 school study summary report.
- Values are average classroom bacterial concentrations in cfu per cubic meter.
- The chart does not represent clinical infection incidence.
These are descriptive comparisons of the reported averages from four schools. Repeated samples describe conditions within these schools and do not establish results for other schools.
The percentage differences compare treated and control schools during the study window. They are not before-and-after reductions within the same classroom. The comparison does not establish a device-specific causal effect or separate the device's contribution from ventilation, occupancy, or other building conditions.
Attendance observations and interpretation
Attendance is an observational endpoint. Community illness, reporting practices, school populations, ventilation, and other differences can affect it. Matching schools and collecting samples during the same period do not isolate the air purifiers as the cause of an attendance difference.
During the same seven-week period, illness-related absenteeism in Pair 1 was 5.12% in the treated school versus 10.99% in the control school. In Pair 2, it was 4.75% in the treated school versus 7.28% in the control school. Absenteeism was therefore 53.4% lower in the treated school in Pair 1 and 34.8% lower in the treated school in Pair 2 during the study window.
Bacterial sampling and attendance describe different levels of observation. A classroom air sample cannot identify why an individual was absent, and an attendance percentage cannot identify the organism responsible for an illness. Reporting both endpoints together describes the study design; it does not establish a link from measured bacteria to a particular infection and then to a missed school day.
Where ASHRAE 241 and CADR fit
ASHRAE Standard 241 provides context through its focus on equivalent clean airflow for infectious aerosol risk reduction. The school study did not directly measure building-level CADR. CADR-style testing, controlled bioaerosol efficacy testing, and field bioaerosol studies answer different but complementary questions.1,2
CADR testing can measure how quickly a room air cleaner removes defined particle challenges under controlled conditions. Room bioaerosol efficacy testing can evaluate airborne microbial reduction under controlled chamber or room-scale conditions. A field study examines how a deployed intervention behaves in occupied spaces, where building conditions vary.1
What the study shows
For school districts and manufacturers, field sampling can describe how bacterial concentrations vary under occupied-building conditions. Attendance observations may identify questions for further study, but they do not establish a clinical benefit.3,4
The study paired treated and control spaces, collected simultaneous samples, and included outdoor measurements. Differences in school populations, ventilation, and other building conditions still affect interpretation.
Summary
The seven-week school study reported lower average classroom bacterial concentrations and illness-related absenteeism in the treated schools. These descriptive and observational comparisons do not establish reduced infection or a causal attendance benefit. The work helped the client describe bacterial concentrations and attendance observations in occupied schools while identifying limits on health claims. ARE Labs supported repeated field sampling and a reviewable study record.