Bioaerosol Control in Urban Elementary School Classrooms with In-room HEPA Supplements: Distribution of Effectiveness, Contribution to Room-Air Mixing and Acoustic Envelope Assessment

Odessa M. Gomez, Halle Sago, Anna Segur, Sylvia Akol, MARK HERNANDEZ, University of Colorado, Boulder

     Abstract Number: 194
     Working Group: Indoor Aerosols

Abstract
The need to reduce airborne (biological) particle exposures in urban public schools is evident. In response, we report here a systematic ventilation and air-mixing assessment of classrooms distributed across more than 20 buildings in a large US school district.  We report the effect of deploying a new generation of HEPA air purifiers on the fluorescent fractions of airborne respirable particulate matter (PM_BIO) as well as acoustic impacts in 75 elementary school classrooms before and during their occupation.

Food grade carbon dioxide gas was used to determine the mixing regime and effective air exchange provided by the ventilation system classrooms under the following conditions: doors closed and open, as well as with and without in-room air purifying filters engaged (RMAP, Carrier Global, Indianapolis, IN). Real-time, multiple-channel fluorescent aerosol cytometers (InstaScope, DetectionTek, Boulder, CO), positioned in each classroom monitored the total and biological fractions of airborne PM, through at least three consecutive air exchanges, before, during and after students’ occupation.  Using accepted non-parametric tests (Kruskal-Wallace), the airborne (bio)aerosol loads were compared in occupied classrooms with operating air purifiers engaged during instructional periods, to otherwise similar classrooms without air purification equipment installed and to those loads immediately outdoors. The effects of HEPA air purifier operations on room air mixing were assessed by deploying arrays of carbon dioxide monitors symmetrically in the classrooms at childrens’ breathing height.  The acoustic projection of the HEPA air purifiers as deployed were compared to the cumulative background sound pressure in the classrooms using radial meters. These metrics were juxtaposed to teachers’ surveys and student absenteeism.

As judged by in-situ sound pressure, the acoustic envelope projected by the air purifiers was less than 10% of the background; all sound emissions fell within a range that does not interfere with classroom communications under World Health Organization (WHO) guidelines.  Tracer gas patterns showed that a substantial fraction of classrooms surveyed were not operating under well-mixed conditions, and that air purifiers could have a measurable effect on air mixing where air exchange was less than 5 hr^-1.  As judged by real-time fluorescent cytometry of occupied classroom air, HEPA air purifiers processing 500 ft³/min of room air were able to sustain significantly reduced bioaerosol loads in occupied classrooms. Teacher surveys indicated that acoustic impacts and air purifier placement drive faculty acceptance of HEPA air purifier operations. Results suggest that distributed networks of smaller air purifiers are preferred over larger units in practice.