Just because a topical antiseptic is swabbed on the skin doesn’t mean it stays on the skin.

In a new study, Northwestern Engineering 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 April 2 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 the 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 percent 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.”