Controlled Mechanical Ventilation and Indoor Air Quality in Classrooms: Evidence From a Quasi-Experimental Study

Abstract

Introduction: Indoor air quality control in school environments is a relevant public health issue. Effective ventilation contributes to reducing health risks associated with pollutants and infectious aerosols. Controlled mechanical ventilation (CMV) systems, designed for air exchange in confined spaces, may be a strategy to improve microclimatic parameters.

Objectives: To evaluate the effect of CMV systems specifically designed for air exchange on indoor carbon dioxide (CO₂) concentrations (as an indirect indicator of ventilation and system effectiveness) and indoor air temperature (a parameter related to comfort and energy efficiency) in classrooms of a primary school in Bolzano.

Methods: In the necessARIA project, coordinated by the Autonomous Province of Bolzano and funded by the Ministry of Health (PREV-A-2022-12377013 project), 18 classrooms were monitored using Aranet PRO sensors. Measurements of CO₂ (ppm) and temperature (°C) were recorded every minute during October and November 2024. Among these, 11 classrooms had no CMV systems, 5 were equipped with CMV designed for radon mitigation, and 2 were equipped with CMV designed to guarantee ventilation rates in accordance with current building regulations. The activation of the CMV systems in the latter two classrooms on November 1st provided a quasi-experimental setting, allowing for controlled pre-post comparison between classrooms with air-exchange CMV activation and classrooms without such activation, serving as control groups. Analyses were limited to actual occupancy hours (8:00 AM – 1:00 PM on school days). To account for classroom-specific temporal patterns and to deal with autocorrelation, the analyses were conducted separately for each classroom, using generalized least-squares models with autoregressive moving-average error structures: ARMA 2,1 for log CO₂ and ARMA 1,1 for indoor air temperature. Predictors for both CO₂ and indoor air temperature included month, day of the week, and a spline-modeled time function. Indoor air temperature and external air temperature were also considered in the analyses of CO₂ and air temperature, respectively. For each outcome, a meta-analysis stratified by CMV type was conducted on the coefficient for month (November vs October) derived from the models.

Results: Descriptive analysis showed that, between October and November, the median CO₂ concentration rose from 1097 ppm to 1306 ppm (+19.1%) in classrooms without CMV, and from 1042 ppm to 1279 ppm (+22.7%) in classrooms with radon CMV. In contrast, in classrooms with CMV designed for air exchange, the median CO₂ concentration decreased from 1072 ppm to 979 ppm (-8.7%) after activation on 1 November. Indoor air temperature decreased slightly across all groups. The metanalysis of coefficients confirmed these results: the activation of CMV designed to meet ventilation standards was associated with a reduction in indoor CO₂ levels in the post-intervention phase compared to the pre-intervention phase (−7% in geometric mean), while during the same period, an increase was observed in classrooms without CMV (+17%) and in those with radon CMV (+31%). No relevant differences between groups were observed for indoor air temperature.

Conclusions: These findings suggest that systems specifically designed for air exchange contribute to reducing indoor CO₂ levels in educational settings without compromising thermal comfort.