The field-study question
Schools do not behave like controlled test chambers. Occupancy changes, students and staff move throughout the building, outdoor air conditions shift, and HVAC performance varies from room to room. Those conditions can change the environment during sampling, so a single controlled result does not capture the range of conditions encountered in daily school operation. A field study can show how an air-cleaning intervention performs amid that real-world variability, across occupied rooms and over time.3,4
The seven-week study evaluated plasma-based room air purifiers across four parallel-controlled elementary schools: two treated schools and two untreated control schools. The practical question was whether the intervention was associated with lower airborne bacterial concentrations, lower modeled infection probability, and lower illness-related absenteeism during the same study period.
Parallel-controlled design
The study included two parallel-controlled comparisons. Each pair consisted of one treated elementary school and one untreated control school. Treated classrooms and common areas used plasma-based room air purifiers, while the control schools operated without the added air purification intervention.
The school pairs were selected for comparability across practical 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 buildings, but it provides a more meaningful field comparison than a one-building demonstration.
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 produced 490 bacterial air samples, providing repeated field measurements rather than a single point-in-time result.
What ARE Labs measured
ARE Labs measured airborne bacterial concentrations in classrooms and reported the results as colony-forming units per cubic meter of air. Those measurements were used to estimate infection probability with Wells-Riley and exponential Quantitative Microbial Risk Assessment approaches. Illness-related absenteeism was analyzed over the same study period as a third layer of evidence.
The modeled infection values were not clinical infection rates, and the absenteeism results do not prove that differences in absences were caused by the air purifiers. Taken together, the measured bacterial concentrations, modeled risk estimates, and attendance observations were all lower at the treated schools.
| 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 treated-school average was 41.1% lower. In Pair 2, the treated school averaged 56.1 cfu/m3 versus 71.8 cfu/m3 at the 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: 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.
The statistical results supported the observed bacterial concentration differences. Pair 1 reported p = 0.00002 and Cohen's d = 1.874; Pair 2 reported p = 0.011 and Cohen's d = 1.844. These measured concentrations were the technical anchor of the school IAQ evidence package.
| 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 estimate infection probability. In Pair 1, the average modeled probability was 6.4% at the control school and 4.0% at the treated school. In Pair 2, it was 8.6% at the control school and 7.0% at the treated school.
During the same seven-week period, Pair 1 recorded illness-related absenteeism of 5.12% at the treated school and 10.99% at the control school. Pair 2 recorded 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.
Where ASHRAE 241 and CADR fit
ASHRAE Standard 241 centers equivalent clean airflow in its framework for reducing infectious aerosol risk. The school study, however, did not directly measure building-level CADR. CADR-style testing, controlled bioaerosol efficacy testing, and field bioaerosol studies answer separate but complementary questions.1,2
CADR testing can measure how quickly a room air cleaner removes a defined particle challenge under controlled conditions. Room bioaerosol efficacy testing can evaluate airborne microbial reduction in a controlled chamber or room-scale setting. A field study addresses how a deployed intervention behaves in occupied spaces with real building variability.1
What the study shows
A field study can help a school district connect an air-cleaning investment with measured microbial air quality, modeled exposure risk, and attendance-related indicators. Manufacturers can use the resulting data to strengthen the evidence behind product claims while keeping those claims within what the study can support.3,4
Several design choices strengthened the field comparison: matched spaces, simultaneous or time-balanced sampling, outdoor/background measurements, repeated sampling during occupied periods, clear endpoint definitions, and a defined data analysis plan. Together, they helped distinguish measured results from modeled interpretation. This structure supported practical decisions without reducing school IAQ performance to a single device number.
Summary
Across both school pairs, the treated schools had lower average classroom bacterial concentrations, lower modeled infection probability, and lower illness-related absenteeism during the seven-week study window. 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 tied to what the data could and could not show.