The Hidden Dangers of Hospital Ventilation: Are We Spreading Viruses Further?

Hospital Ventilation

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Infection preventionists (IPs) are taught that proper ventilation and air purification are critical in reducing airborne pathogen transmission. However, a new study from University College London (UCL) and University College London Hospitals (UCLH) raises serious concerns about whether these strategies could increase the spread of viral particles in hospitals. The findings, “The Influence of Mechanical Ventilation and Portable Air Cleaners Upon Aerosol Spread in a Hospital Outpatients Clinic” published in Aerosol Science & Technology, suggest that ventilation systems and portable air cleaners (PACs) may, in some cases, redistribute viral aerosols instead of eliminating them, potentially escalating the risk of nosocomial infections.

Ventilation May Be Moving Viral Particles—Not Removing Them

The study took place in a modern outpatient clinic, using an aerosol generator to mimic viral particles exhaled by an infected individual. Researchers tracked the movement of these particles under various conditions, including changes in ventilation settings, PAC placement, and door positioning. Their findings were unsettling:

• Built-in ventilation systems increased aerosol migration across the clinic by up to 5.5 times compared to when ventilation was turned off.
• Large PACs in waiting areas increased the aerosol spread between rooms by up to 29%, a discovery given their widespread use to limit airborne transmission.
• Unpredictable particle flow resulted in the highest concentrations being detected in rooms furthest from the source of infection rather than in adjacent rooms.

In one example, researchers observed 247% more aerosol particles in a waiting room far from the infection source than in the room next to it. The highest concentrations of viral particles were often found at the nurses’ station—a high-traffic area where IPs and health care staff spend significant time.

“The COVID-19 pandemic really highlighted the risk of picking up airborne viral infections in hospitals, which naturally led to efforts to reduce this risk. In many hospitals, the use of ventilation systems and portable air cleaners has increased,” Professor Laurence Lovat, senior author of the UCL Surgery & Interventional Science and UCLH study, said. “While the urgency of the situation demanded a rapid response, since then, we’ve been studying precisely how viral particles move around in real spaces and have been surprised by what we’ve found. Putting air cleaners in rooms led to unexpected increases in the circulation of aerosols in some cases, but it took months to understand what we were seeing. Each scenario produced different, unexpected results, depending on the spaces and airflow sources involved.”

Lovat continued, “Even at UCLH, a modern hospital built less than 20 years ago, airflow patterns were not predictable. In older hospitals, which often have natural draughts, the situation would likely be even more complex.”

Why Is This Happening?

The study highlights a fundamental problem: airflow in hospitals is not predictable. PACs, ventilation systems, and even human movement create complex interactions of air currents, which can unintentionally corral viral particles into specific areas. Larger PACs, designed to purify the air, may introduce their own turbulence, pushing unfiltered viral particles further into hospital spaces.

Closing doors was one of the few interventions that effectively reduced the spread of aerosols, cutting transmission by 97% when both doors to an infected room were shut. However, in a real-world hospital setting, keeping doors closed at all times is not always practical.

“The results of this experiment might seem counterintuitive if you take the view that changing the air in a room more often reduces the spread of viral particles,” Jacob Salmonsmith, PhD, first author of the study and an Honorary Research Fellow from UCL Mechanical Engineering, said. “While it’s true that air cleaners do remove viral particles from the air and can reduce overall spread, they can also have unintended consequences. In particular, this experiment suggests that larger air cleaners, [with] larger exhaust vents that introduce their own air currents, can cause particles that haven’t been filtered out to spread further than they would have if the cleaner wasn’t there. In any given space you have complex interactions between many different air currents, such as ventilation, doors closing and people’s movement. Our findings indicate that the whole picture needs to be considered when choosing when and where to introduce air cleaners.”

Implications for Infection Preventionists

This study presents a wake-up call for infection control professionals. If ventilation and air cleaning systems are not carefully designed and placed, they may increase the risk of airborne transmission in hospitals. Key takeaways include:

Strategic PAC Placement is Crucial: Large PACs should not be placed in high-traffic waiting areas without thoroughly assessing airflow patterns. Smaller, room-specific PACs may be a safer choice in some environments.

Ventilation Design Requires Rethinking: More airflow doesn’t necessarily mean safer conditions. IPs must collaborate with hospital engineers to ensure that air currents move contaminants away from people, not redistributing them unpredictably.

Door Management Can Be a Simple Yet Effective Barrier: Where possible, closing doors can significantly reduce airborne particle movement. Infection control teams should evaluate when and where door closures can be implemented without disrupting patient care.

AI-Driven Airflow Monitoring Could Be the Future: The research team is developing artificial intelligence models to predict hospital airflow patterns. These tools could help health care facilities make real-time PAC and ventilation system placement decisions.

Hospital Design Must Adapt: Modern hospitals should be designed with airflow dynamics in mind, ensuring that infection control measures do not counteract one another. Retrofitting older hospitals may also be necessary to prevent the unintended viral spread.

A Call for Action

As we continue to combat respiratory viruses such as SARS-CoV-2, influenza, and potential future pandemics, IPs must rethink how we use ventilation and air purification systems. The assumption that "more airflow is better" could put health care workers and patients at risk. It is time to move beyond outdated ventilation models and embrace a more data-driven, strategic approach to airborne infection control.

For individuals engaged in the prevention of hospital infections, this study represents not merely an academic discovery, but rather a compelling challenge to reevaluate existing protocols and guarantee that well-meaning interventions do not exacerbate the circumstances. The implications are far too significant to be disregarded.