Automatic Hand Dryers Can Be a Source of Microbial Contamination

Research presented at ASM Microbe Online found that automated restroom hand dryers can harbor and spread bacteria, including Staphylococcus and coliforms. The study showed that after handwashing, hands can be re-inoculated by microorganisms present inside the hand dryer.

Automatic electric high-speed hand dryers are considered an environmentally friendly alternative as they reduce paper waste and are thought to be more sanitary than paper towel dispensers since they eliminate direct contact with the dispenser and towels. Increasingly, these hand dryers are the only option in public


“Our purpose for doing this research was to determine if high-speed automatic electric hand dryers in public restrooms are antiseptic or if they can serve as a source of contamination to hands during drying,” said Craig Oberg, Ph.D., Brady Distinguished Professor of Microbiology at Weber State University in Ogden, Utah, who supervised 2 undergraduate students on the research.

Results showed the bottom of dryers in both the men’s and women’s restrooms had the most contamination, an average of about 300 organisms per 5 cm2. The middle section had about half as many organisms averaging 140 organisms per 5 cm2, while the top of the dryer contained 75 organisms per 5 cm2. Selective plating showed Staphylococcus and coliforms (fecal bacteria) were present inside the drying chamber. Overall there was no difference between the two brands of dryers tested (Dyson Airblade vs Mediclinics Dualflow Plus) and no difference between dryers in men’s and women’s restrooms.

The deeper hands are placed in the dryer, the greater the likelihood of contamination. “As a preventative measure, the inside of dryers should be cleaned on a regular basis to prevent people from contaminating their hands immediately after washing them,” said Oberg.

University building restrooms with variable degrees of foot traffic were selected for sampling. The researchers sampled men’s and women’s restrooms in 3 buildings (library, student union building and science laboratory building) with 4 bathrooms tested in each building. Testing was conducted by swabbing a 5 cm2 area of the top, middle and bottom of each hand dryer using 3M Quickswabs. Pour plates containing TSA, mannitol salts agar and violet red bile agar were used to enumerate swab samples with plates counted at 48 hours after incubation at 37° C.

Since the use of swabs for sampling only recovers between 1 and 10% of the total organisms present, the actual level of microbial contamination in restroom hand dryers was at least 10 times greater than actual plate counts. Differences were observed based on sampling location inside the dryer and for restrooms in higher traffic areas, which had a higher level of microbial contamination, probably based on increased use.

This research was performed by 2 undergraduate researchers, Hyrum Packard and Riley Nichols, in the Department of Microbiology under the direction of microbiology faculty, Craig Oberg and Matthew Domek, at Weber State University in Ogden, Utah. This project was funded by a grant from the WSU Office of Undergraduate Research. This research is presented as an eposter at ASM Microbe Online.

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