Fig. 2: The influence of portable air filtration on respiratory pathogen detection and concentration in ambient air.

For 8 consecutive weeks, samples were taken in three nursery locations on Mondays (n = 8), Wednesdays (n = 8), and Fridays (n = 8). Location 1 = control group (no air filtration); Location 2 = air filtration at 10.7 air changes per hour (ACH) starting Mondays (after sampling) and ending Fridays (after sampling); Location 3 = air filtration at 6.1 ACH starting Mondays (after sampling) and ending Fridays (after sampling). No one was present over the weekends. Panel (a) shows the mean number of pathogens detected in each of the nursery locations in the absence of filtration (not shaded), after 48 h of filtration (Locations 2 and 3, shaded) and after 96 h of filtration (Locations 2 and 3, shaded). * indicates a significant difference. We used a Cochran’s Q to compare the three filtration phases in each location separately, followed by pairwise Cochran’s Q tests when a significant difference (two-sided P-value < 0.05) was found, with Holm correction for multiple testing. In location 1, there was no significant difference between Mondays, Wednesdays, and Fridays (p = 0.6762). In location 2, a significant difference was present across days (0.0006), between Mondays and Wednesdays (pairwise comparison: p = 0.0229), and between Mondays and Fridays (pairwise comparison: p = 0.0009), but not between Wednesdays and Fridays (pairwise comparison: p = 1). In location 3, the difference between days did not reach significance, but there was a trend (p = 0.0701). Panel (b) shows the evolution of Ct values of n positive pathogens (throughout the 8 weeks) in each of the nursery locations in the absence of filtration (not shaded), after 48 h of filtration (shaded) and after 96 h of filtration (shaded). Ct values for a particular pathogen were included only if the pathogen was detected in samples from all three filtration phases during the same week in one location. The horizontal line corresponds to the mean Ct value on Mondays. The central boxplot line corresponds to the median, and the lower and upper boxplot bounds to the 25th and 75th percentiles. The upper/lower whisker extend from the upper/lower boxplot bound to the largest/lowest value no further than 1.5 * IQR. Data beyond the whiskers are outliers, plotted individually. * indicates a significant difference in a linear mixed effects regression model, including week and pathogen as random effects. 95% CI were calculated using the confint command in R, and p-values with the Kenward–Roger approximation of the t-distribution. Ct values did not differ significantly in location 1 (p = 0.9506 between Mondays and Wednesdays; 0.7101 between Mondays and Fridays). In location 2, they were significantly different between Mondays and Wednesdays (p < 0.0001), and Mondays and Fridays (p = 0.0002). In location 3, they were not significantly different between Mondays and Wednesdays (p = 0.3146), but were between Mondays and Fridays (p = 0.0026).