Evaluation and Determinants of Airborne Bacterial Concentrations in School Classrooms

Abstract

A survey of 39 elementary schools was undertaken to determine indoor air concentrations of bioaerosols within a coastal, temperate climatic zone in British Columbia, Canada. This article reports the results for airborne bacteria. Determinants of exposure were grouped into environmental (outdoor temperature, relative humidity, season, weather), ventilation and comfort parameter (indoor relative humidity, temperature, indoor CO₂ concentration, indoor fungal concentration), and occupancy (number of occupants, activity levels, occupancy patterns) variables. Regression modeling was used to evaluate the association between these factors and measured concentrations of indoor mesophilic bacteria. Naturally ventilated rooms (47% of the total rooms surveyed) had higher bacterial counts than mechanically ventilated rooms (geometric mean 325 vs. 166 CFU/m³, respectively, p < 0.001). In univariate analyses, bacterial counts were negatively correlated with supply and exhaust flow rates, air exchange rates, and the percentage of the day that occupants spent quietly sitting at their desks. Analysis of bacterial groups indicated various sources of the bacterial aerosol, with micrococci and staphylococci closely associated with occupancy variables, Bacillus with site and occupancy, and coryneform bacteria with site variables only. Approximately 60% of the variance in the outcome measurement of total bacteria was accounted for by indoor CO₂, lower air exchange rate, the age of the building, signs of current or old moisture stains, room volume, indoor relative humidity, and occupant activity level in a multiple linear regression model, with ventilation factors accounting for 40% of the variance.

Keywords:

• airborne bacteria
• carbon dioxide
• indoor air quality
• occupant activity
• schools
• ventilation

ACKNOWLEDGMENTS

This work was supported in part by British Columbia Health Research Foundation doctoral studentships ST#10(98), ST#13(97), ST#6(96), and ST#23(95).

Notes

^A Defined by measurable air flow on the day of testing.

^B Not significant.

^ANot significant.

^A Geometric mean.

^B Geometric standard deviation.

^C Comparison of means.

^D Not significant.

^E Group significantly different from other groups by Scheffe post hoc procedure.