Just because you wipe a topical disinfectant on your skin doesn’t mean it will remain on your skin.
In a new study, Northwestern University scientists studied how a powerful disinfectant called chlorhexidine affects bacteria in hospital environments. To prevent infections, hospitals rely heavily on chlorhexidine wipes to disinfect patients’ skin before procedures.
Through laboratory experiments, researchers discovered that trace amounts of chlorhexidine persist on surfaces much longer than previously known – long enough for microorganisms to develop resistance. Analyzing samples from medical intensive care units (MICUs), the research team also found that chlorhexidine-resistant bacteria were spreading throughout the hospital environment through contact and, surprisingly, through the air.
The findings provide new insights into how disinfectants interact with microorganisms in indoor environments and may help inform strategies to prevent infections and antimicrobial resistance.
The study will be published in a journal on Thursday, April 2nd. environmental science and technology.
“We saw evidence that even if chlorhexidine was applied to a patient’s skin, it affected microorganisms in the room around the patient,” said study leader Erica M. Hartman of Northwestern University. “Microorganisms and chemicals don’t stay where we put them, and can influence antimicrobial resistance. Our results suggest that this is also true in hospitals, but there’s no reason to think there’s anything special about hospitals. I think you’d find exactly the same thing if you looked at personal care products and microorganisms in homes, schools, and other places.”
Hartman, an indoor microbiologist, is a professor of civil and environmental engineering at Northwestern University’s McCormick School of Engineering.
“Protecting the safety of high-risk patients”
Chlorhexidine, widely used in medical settings since the 1950s, is an important chemical for preventing infections in hospitals. Healthcare workers use products containing chlorhexidine for routine medical care, including daily bathing of MICU patients, skin preparation before surgery or catheter insertion, instrument sterilization, and hand washing. It is also commonly used in prescription mouthwashes in dental treatments and veterinary clinics.
Chlorhexidine is used in settings where patients are highly vulnerable and physicians want to ensure that exposure to microorganisms is highly controlled. This is a well-regulated chemical and is critical to protecting the safety of high-risk patients. ”
Erica M. Hartman, Professor of Civil and Environmental Engineering, McCormick School of Engineering, Northwestern University
However, when chlorhexidine is applied to the skin, it appears as if it is living a second life.
To track how chlorhexidine affects the environment, Hartman and her team conducted a two-pronged study. First, the team designed a laboratory experiment to simulate hospital cleaning. Next, we conducted an environmental survey inside the MICU.
Residue remains for more than 24 hours
In the lab, Hartman’s team applied chlorhexidine to common materials commonly found in hospitals: plastics, metals, and laminates. Those surfaces were then cleaned using a chlorhexidine-free disinfectant typically used to sterilize hospital environments.
Even after these cleaning treatments, chlorhexidine residue remained on the surface after 24 hours. The residual levels were too low to kill the bacteria, but high enough to expose them to the chemicals. In such situations, surviving microorganisms may develop resistance to disinfectants.
To investigate what happens under such lethal conditions, the research team exposed several clinically relevant bacteria, including E. coli, to trace concentrations of chlorhexidine. Even after a full day of exposure, the microorganisms survived.
SInk outlet is a hot spot
Hartman and her team then conducted an environmental survey inside the MICU, collecting nearly 200 samples from hospital bed rails, keyboards, door sills, light switches, and sink drains. More than 1,400 bacteria were isolated from these samples, and approximately 36% showed some degree of resistance to chlorhexidine.
Bacteria occurred throughout the MICU, but the sink drain stood out as the biggest hotspot. Compared to dry surfaces, drains contained much higher levels of bacteria, including strains that could tolerate much higher concentrations of chlorhexidine. Hartman said hospital staff have long been concerned about sink drains because of P-traps, which trap small amounts of water in a U-shaped pipe under the sink to prevent sewage gas from escaping.
“Wherever there is water, there are microorganisms,” Hartman says. “Hospital sink drains can be a reservoir for antibiotic-resistant pathogens, and there are concerns that every flush creates aerosols, which has the potential for re-exposure.”
Hitchhike on floating particles
Perhaps the most surprising finding was that Hartman and her team found bacteria that showed signs of chlorhexidine resistance in samples taken from above door sills.
“Our initial hypothesis was that we would find evidence of chlorhexidine in high-touch areas like light switches,” Hartman said. “We included the door sill as a negative control.”
Because people rarely touch door thresholds, the findings suggest that the bacteria may have landed on airborne particles such as dead skin cells. Hartmann says dust on door sills can trap these particles as they circulate in the air.
“The point is not that you need to clean your front threshold,” she says. “Importantly, we need to consider airflow pathways as potential routes for exposure and transport of microorganisms within the built environment. Every time we walk around, we expel microorganisms, skin, and skin-attached chemicals, some of which can potentially become airborne and deposited elsewhere in the room.”
No need to disinfect your home or office
While stressing that chlorhexidine remains necessary and effective in clinical practice, Professor Hartman said the findings reinforced the message that antimicrobial chemicals can have unintended consequences. There is no need to disinfect the surrounding environment unless a person is actively sick or has a weakened immune system. To prevent antimicrobial resistance, Hartmann recommends using plain soap and water to clean your home or office.
“The MICU is a very sensitive environment with very vulnerable people,” she said. “However, disinfection is rarely needed elsewhere. Exposure to these chemicals is not necessarily harmless, so there is no need to expose ourselves or the environment to these chemicals.”
“Hospital environments harbor chlorhexidine-resistant bacteria, which may be related to chlorhexidine persistence in the environment,” the study was supported by the Searle Leadership Fund.
