Efficacy of Handwashing Duration and Drying Methods

Introduction: Handwashing is mandated in the FDA model food code, the guidance used by most US states and territories as the basis for regulating restaurants, grocery stores and institutions such as nursing homes. While the factors that influence handwashing effectiveness have been studied, the data are often based on few replicates and methodological differences or statistical flaws complicate comparisons among studies. This research was undertaken to establish the importance of several key handwashing factors using methods that are robust, sufficiently replicated and statistically valid.

Purpose: Our study quantifies the impact of soap or plain water, duration of the wash (5 vs. 20 s), presence of debris and drying method (air vs. paper towel drying) on the removal of microorganisms from hands during handwashing.

Methods: A food-grade strain of nalidixic acid resistant Enterobacter aerogenes was used as a surrogate for transient hand transmitted pathogens like Salmonella. Twenty volunteers were used for each experiment. Samples were collected using the glove-juice method where volunteers’ hands were massaged for ~1 min inside a nitrile glove containing 20 ml of buffer. Aliquots of the buffer were then plated onto MacConkey agar plus nalidixic acid.

Results: Using soap during handwashing resulted in a greater reduction (~1 log CFU/hand) than using plain water. This difference increased to ~1.5 log CFU/hand when subject hands were contaminated with food debris (5 g of 80% lean ground beef). A food code style 20 s hand wash resulted in ~1.5 log CFU/hand greater reductions than a 5 s wash. Average log reductions for towel drying were ~0.5 log CFU/hand greater than with air-drying; greater person-to-person variability was observed with towel drying.

Significance: This study illustrates the superior effectiveness of handwashing that includes the use of soap and which lasts 20 s. This study also demonstrated that towel drying is more effective than air drying.