The field-study question
Schools do not operate like controlled test chambers. Occupancy changes throughout the day, students and staff move between rooms, outdoor air affects indoor conditions, and HVAC systems vary by building. Real-world air purifier testing helps determine how an air-cleaning intervention performs under these occupied-space conditions.1,4,5
The seven-week study evaluated plasma-based room air purifiers in four parallel-controlled elementary schools: two treated schools and two untreated control schools. The practical objective was to determine whether purifier deployment was associated with lower airborne bacterial concentrations, lower modeled infection probability, and lower illness-related absenteeism during the same study period.1
Parallel-controlled design
The study included two parallel-controlled comparisons. Each pair consisted of one treated elementary school and one untreated control school. Plasma-based room air purifiers operated in classrooms and common areas at the treated schools, while the control schools operated without the added air purification intervention.1
Each school pair was selected for comparability across relevant building and population factors, including building age, existing air systems, socioeconomic demographics, geography, and morning or afternoon sampling time. Matching cannot account for every difference between occupied schools, but it provides a more meaningful field comparison than a demonstration in a single building.1
Two aerosol scientists sampled each treated and control school pair simultaneously for seven weeks. At each school, they collected samples from five classrooms, one common area, and one outdoor location. Each comparison produced 490 bacterial air samples, providing repeated field measurements rather than a single snapshot.1
What ARE Labs measured
The study combined three layers of evidence. First, ARE Labs measured airborne bacterial concentrations in classrooms and reported the results as colony-forming units per cubic meter of air. Next, the measured bacterial aerosol concentrations were used to estimate infection probability through Wells-Riley and exponential Quantitative Microbial Risk Assessment approaches. Illness-related absenteeism was then analyzed for the same study period.1
The modeled infection probabilities were risk estimates, not clinical infection rates. Likewise, the absenteeism analysis did not assume that the air purifiers caused every difference in absences. The value of the design was the convergence of three findings: measured microbial air quality, modeled exposure interpretation, and attendance-related observations all moved in the same direction at the treated schools.1
| Study pair | School role | Average classroom bacterial concentration | Bacterial concentration comparison | Modeled infection probability | Illness-related absenteeism |
|---|---|---|---|---|---|
| Pair 1 | Treated | 30.2 cfu/m3 | 41.1% lower than control | 4.0% | 5.12% |
| Pair 1 | Control | 51.3 cfu/m3 | Reference | 6.4% | 10.99% |
| Pair 2 | Treated | 56.1 cfu/m3 | 21.9% lower than control | 7.0% | 4.75% |
| Pair 2 | Control | 71.8 cfu/m3 | Reference | 8.6% | 7.28% |
Modeled infection probability values come from report chart labels; absenteeism is an observed study-period outcome.
Lower classroom bacterial concentrations
Across sampled classrooms, the treated school in Pair 1 averaged 30.2 cfu/m3, compared with 51.3 cfu/m3 at its untreated control school. The average classroom bacterial concentration was therefore 41.1% lower at the treated school. In Pair 2, the treated school averaged 56.1 cfu/m3, compared with 71.8 cfu/m3 at the untreated control school, a 21.9% lower concentration.1
Source: 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.
Statistical analysis supported the observed differences. Pair 1 reported p = 0.00002 and Cohen's d = 1.874, while Pair 2 reported p = 0.011 and Cohen's d = 1.844. These measured differences in bacterial concentration provided the technical foundation of the evidence package for this school IAQ field study.1
| Statistical endpoint | Pair 1 | Pair 2 |
|---|---|---|
| t-statistic | -4.996 | -2.716 |
| t-critical value | 2.042 | 2.042 |
| p-value | 0.00002 | 0.011 |
| Cohen's d | 1.874 | 1.844 |
Modeled risk and absenteeism
The study used the measured bacterial concentrations to provide a risk-oriented interpretation. In Pair 1, the average modeled infection probability was 6.4% at the control school and 4.0% at the treated school. In Pair 2, the average modeled probability was 8.6% at the control school and 7.0% at the treated school.1
During the same seven-week period, illness-related absenteeism in Pair 1 was 5.12% at the treated school and 10.99% at the control school. In Pair 2, it was 4.75% at the treated school and 7.28% at the control school. Within the study window, absenteeism was 53.4% lower in treated Pair 1 and 34.8% lower in treated Pair 2.1
Where ASHRAE 241 and CADR fit
ASHRAE Standard 241 provides useful context because it centers infectious aerosol risk reduction on equivalent clean airflow. However, the school study did not directly measure building-level CADR. CADR-style testing, controlled bioaerosol efficacy testing, and field bioaerosol studies address different but complementary performance questions.1,2,3
CADR testing measures how quickly a room air cleaner removes a defined particle challenge under controlled conditions. Room bioaerosol efficacy testing evaluates airborne microbial reduction in a controlled chamber or room-scale setting. Field studies extend that evidence by examining how a deployed intervention performs in occupied spaces subject to real building variability.1,2
What the study shows
For school districts, field studies can connect investments in air cleaning with measurable indicators such as microbial air quality, modeled exposure risk, and illness-related absenteeism. For manufacturers, the results can add occupied-building evidence to a product's claim-support package without extending conclusions beyond what the study measured.1,4,5
The study's defensibility came from its field design. Matched comparison spaces, simultaneous or time-balanced sampling, outdoor and background measurements, repeated sampling during occupied periods, defined endpoints, and a planned analysis helped distinguish measured results from modeled interpretation. This structure supported practical decisions without reducing school IAQ performance to a single device metric.1
Summary
During the seven-week study window, treated schools had lower average classroom bacterial concentrations, lower modeled infection probability, and lower illness-related absenteeism than their paired control schools. The study helped the client and school IAQ decision-makers connect air-cleaning performance with evidence from occupied buildings. ARE Labs kept the protocol, field sampling, modeling, and interpretation aligned with both the findings and the limits of the data.1