Researchers have discovered that traces of chlorhexidine, a powerful antiseptic commonly used in hospitals, linger on surfaces much longer than previously known — long enough to help microbes build tolerance.
Image Credit: Scientific Frontline
Scientific Frontline: Extended "At a Glance" Summary: Microbial Tolerance to Environmental Disinfectants
The Core Concept: Chlorhexidine, a heavily utilized clinical antiseptic, persists on environmental surfaces at sub-lethal concentrations long after its initial application, enabling local bacteria to survive and develop chemical tolerance.
Key Distinction/Mechanism: Unlike primary sterilization, where high doses of disinfectants eradicate pathogens on contact, the secondary "lingering" phase creates a low-dose exposure environment. Rather than being destroyed, surviving microbes adapt to the chemical residue and subsequently spread throughout the environment via direct physical contact and by hitchhiking on airborne particles, such as shed skin cells.
Origin/History: While chlorhexidine has been a staple in healthcare infection prevention since the 1950s, its prolonged environmental impact was detailed in an April 2026 study published in Environmental Science & Technology. Northwestern University researchers simulated hospital cleaning and surveyed medical intensive care units (MICUs) to map the persistence and transport of these tolerant microbes.
Major Frameworks/Components:
- Sub-Lethal Residue: Trace amounts of chlorhexidine remain on common hospital materials (plastics, metals, laminates) for more than 24 hours after cleaning, continuously exposing microbes to the chemical.
- Aqueous Reservoirs (Hotspots): Sink drains and P-traps serve as primary breeding grounds for highly tolerant bacteria due to constant moisture and the aerosolization of pathogens when water is running.
- Airborne Translocation: The discovery of tolerant bacteria on undisturbed, high-altitude surfaces (like doorsills) confirms that microbes utilize room airflow pathways to migrate via suspended dust and biological particulate.
Branch of Science: Environmental Microbiology, Civil and Environmental Engineering, and Infectious Disease Epidemiology.
Future Application: These findings will drive the development of modernized hospital sterilization protocols and advanced architectural ventilation systems designed to mitigate the airborne transport of pathogens. Furthermore, it provides a framework for revising public health guidelines to prioritize traditional soap and water over chemical disinfectants in non-critical domestic and commercial environments.
Why It Matters: Understanding the complex interactions between chemical disinfectants and microbes within built environments is critical for combating the global threat of antimicrobial resistance. This research exposes the unintended consequences of aggressive sterilization, highlighting the necessity of targeted hygiene strategies that protect vulnerable patients without inadvertently cultivating resilient, drug-tolerant pathogens.
Just because a topical antiseptic is swabbed on the skin doesn’t mean it stays on the skin.
In a new study, Northwestern University scientists studied how a powerful antiseptic, called chlorhexidine, affects bacteria in hospital environments. To prevent infections, hospitals heavily rely on chlorhexidine wipes to sterilize patients’ skin before procedures.
Through laboratory experiments, the researchers discovered that traces of chlorhexidine linger on surfaces much longer than previously known — long enough to help microbes build tolerance. By analyzing samples from a medical intensive care unit (MICU), the team also found chlorhexidine-tolerant bacteria spread throughout the hospital environment through touch — and, surprisingly, through the air.
The findings offer new insights into how disinfectants interact with microbes in indoor environments and could help inform strategies for preventing infection and antimicrobial resistance. The study was published in the journal Environmental Science & Technology.
“Even though chlorhexidine is applied to patients’ skin, we saw evidence that it affects the microbes in the room all around the patients,” said Northwestern’s Erica M. Hartmann, who led the study. “Microbes and chemicals do not stay where we put them, and they can influence antimicrobial resistance. Our results suggest this is true for hospitals, but I have no reason to think there’s anything special about hospitals. I expect we would see the exact same thing if we looked at personal care products and microbes in homes, schools or anywhere else.”
An indoor microbiologist, Hartmann is an associate professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering.
‘Keeping high-risk patients safe’
Widely used in healthcare since the 1950s, chlorhexidine is an important chemical for preventing infections in hospitals. Healthcare workers use products containing chlorhexidine in routine medical care, including the daily bathing of MICU patients, preparing skin before surgery or catheter insertion, sterilizing equipment and washing hands. It’s also commonly used in prescription mouthwashes for dental care and in veterinary clinics.
“Chlorhexidine is used in environments where patients are incredibly vulnerable, and physicians want to make sure microbial exposures are highly controlled,” Hartmann said. “It’s a well-regulated chemical and really important for keeping high-risk patients safe.”
But after chlorhexidine is applied to the skin, it appears to live a second life.
To track how chlorhexidine affects the environment, Hartmann and her team conducted a two-pronged study. First, the team designed laboratory experiments to simulate hospital cleaning. Then, they conducted an environmental survey inside a MICU.
Residue lingers for longer than 24 hours
In the laboratory, Hartmann’s team applied chlorhexidine to common materials — plastic, metal and laminate — often found in hospitals. Then, they cleaned those surfaces with chlorhexidine-free disinfectants typically used to sterilize hospital environments.
Even after these cleaning treatments, chlorhexidine residue lingered on surfaces after 24 hours. The residue levels were too low to kill bacteria but high enough to expose them to the chemical. In these conditions, surviving microbes can develop tolerance to the disinfectant.
To explore what happens under those sub-lethal conditions, the team exposed several clinically relevant bacteria, including Escherichia coli, to trace concentrations of chlorhexidine. Even after a full day of exposure, the microbes survived.
Sink drains are a hotspot
Next, Hartmann and her team conducted an environmental survey inside a MICU, collecting nearly 200 samples from hospital bed rails, keyboards, doorsills, light switches and sink drains. From those samples, they isolated more than 1,400 bacteria, and about 36% exhibited some level of tolerance to chlorhexidine.
While bacteria showed up all over the MICU, sink drains stood out as the biggest hotspot. Compared to dry surfaces, drains contained far higher levels of bacteria, including strains capable of tolerating much higher concentrations of chlorhexidine. According to Hartmann, hospital workers have long been concerned about sink drains because of the P-trap, the U-shaped pipe beneath the sink that traps a small amount of water to block sewer gas from escaping.
“Wherever there’s water, you will invariably have microbes,” Hartmann said. “Sink drains can be a reservoir for antimicrobial-resistant pathogens in hospitals. And the fear is that every time you run water, it generates aerosols. That has potential for re-exposures.”
Hitching a ride on airborne particles
In perhaps the most surprising finding, Hartmann and her team found bacteria with signs of chlorhexidine tolerance in samples collected from the top of doorsills.
“Our original hypothesis was that we’d find evidence of chlorhexidine in high-touch areas like light switches,” Hartmann said. “We included doorsills as a negative control.”
Because people rarely touch doorsills, the finding suggests bacteria might have hitched a ride on airborne particles, like dead skin cells. According to Hartmann, dust on doorsills can trap these particles circulating in the air.
“The point is not that we need to clean our doorsills,” she said. “The point is that we need to think about airflow pathways as a potential route of exposure or microbe transport within a built environment. Every time we walk around, we shed microbes, skin and chemicals that are on our skin. Some of that potentially floats around and deposits elsewhere in the room.”
Homes, offices do not need to be disinfected
While Hartmann emphasizes that chlorhexidine remains necessary and effective in clinical settings, she said the findings underscore the message that antimicrobial chemicals can have unintended consequences. Unless a person is actively sick or immune compromised, the environment around them does not need to be disinfected. To prevent antimicrobial resistance, Hartmann recommends using plain soap and water to clean our homes and offices.
“The MICU is an incredibly sensitive environment with incredibly vulnerable people,” she said. “But, elsewhere, we rarely need to disinfect. We don’t need to expose ourselves and our environments to these chemicals because those exposures are not necessarily benign.”
Funding: The study was supported by the Searle Leadership Fund.
Published in journal: Environmental Science & Technology
Authors: Jiaxian Shen, Yuhan Weng, Tyler Shimada, Meghana Karan, Andrew Watson, Rachel L. Medernach, Vincent B. Young, Mary K. Hayden, and Erica M. Hartmann
Source/Credit: Northwestern University | Amanda Morris
Reference Number: mcb040226_01
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