Aerosols and Bacteria From Hand Washing and Drying in Indoor Air

Abstract

Effective hand drying is an important part of hand hygiene that can reduce the risk of infectious disease transmission through cross-contamination of surfaces by wet hands. However, hand drying methods may also cause aerosolisation of pathogenic microorganisms if they are present in washed hands. This study investigated experimentally the impact of washing hands and different hand drying methods on the concentration and size distribution of aerosols and bacteria in indoor air. In this experiment, aerosol and bacteria concentrations were measured in indoor air while volunteers rinsed their hands with water or washed with soap and water prior to drying them with paper towels or jet air dryers. Results showed that the concentration of aerosols and bacteria in air increased with people walking in the room and washing hands, with a further increase during the hand drying process. The concentration of aerosols decreased with particle size, with maximum concentrations after drying hands of 6.63 × 10⁶ ± 6.49 × 10⁵ and 2.28 × 10⁴ ± 9.72 × 10³ particles m^-3 for sizes 0.3 to <0.5 and ≥5.0 μm, respectively. The concentration of bacteria in indoor air after drying hands increased to a maximum of 3.81 × 10² ± 1.48 × 10² CFU m^-3 (jet air dryers) and 4.50 × 10² ± 4.35 × 10¹ CFU m^-3 (paper towels). This study indicates that the increase of aerosols and bacteria in air after drying hands with jet air dryers or paper towels are comparable and not statistically different from concentrations associated with walking and washing hands in the same environment. This work can support the development of hand hygiene practices and guidelines for public washrooms.

Keywords: aerosol; bacteria; hand hygiene; jet air dryer; paper towel; washroom.

Figure 1

Diagrams representing (A) experimental chamber and (B) different models of jet hand dryers used in this study and hands position for drying. Sampling locations for (C) jet air dryer model A, B and C and paper towels and for (D) jet air dryer model D. Samplers are represented by the gray circle, with aerosol/particles counts samplers at locations 1, 2, and 3 and samplers for bacteria counts locations at 1', 2, and 3.

Figure 2

Diagram summarizing the experimental steps. Each experimental condition (from purge to post testing) were done in triplicate.

Figure 3

Normalized concentrations for model D for aerosol bins (A) 0.3, (B) 0.5, (C) 1.0, and (D) 5.0 for location 2 (near the hand dryer). Each curve represents () control experiment 1 (walking only), () control experiment 2 (walking and hand washing), and drying hands with jet air dryer model D after () rinsing and () washing, and drying with paper towels after () rinsing and () washing. Vertical bars represent standard deviation of triplicates.

Figure 4

Increases in bacteria concentrations at each sampling location: (A) 1', side of the dryer, (B) 2, near the dryer, and (C) 3, opposite the dryer, for controls and for each hand drying method. Bars with patterns represent controls experiment 1 and 2: walking only (bars with dots) and walking and washing (bars with stripes); bars with solid fill represent tests, with drying after rinsing hands (white bars) and after washing with soap (gray bars). Asterisks indicate comparisons in which differences are statistically significant (p < 0.05). Vertical bars represent the standard deviation of triplicates.