People hospitalized are too often harmed by preventable medical errors during their in-patient stay. A patient gets the wrong dose or type of medication, contracts a new infection while in the hospital, gets a pressure sore while immobilized in bed, has surgery on the wrong part of their body, or falls in their room — these all are preventable events!

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The exact toll of these errors on patient lives and hospital budgets is very difficult to quantify. However, conservative studies report that 33 out of every 100 patients experience some type of medical error during their hospitalization. Of these, 18% will get a new infection from their hospital experience.

This means that at least 9% of patients acquire a new infection, referred to as a health care-associated infection (HAI), while in the hospital. The full cost of HAIs on the American society, not just the incremental cost to hospitals, is estimated to be $96-$147 billion in the United States alone. Globally, HAIs kill more people than AIDS, breast cancer, and auto accidents combined.

This is a horrible situation. The surgeon and patient safety champion Dr. Atul Gawande describes victims of HAIs as, “the easiest 100,000 lives we can save,” because no new cure is needed. Instead, hospitals need systems in place that will help to solve this costly and preventable problem.

This ongoing problem requires us to ask if current infection control protocols are missing an important component in their prevention.

Thankfully, we now have new data that reinforces the influence of the hospital physical environment. With this new data, we have an opportunity to use a new tool to combat this epidemic of HAIs.


Dynamics of indoor microbes

Thousands of years ago when humans started building shelters, we also unknowingly began to influence the communities of microscopic viruses, bacteria, and fungi that cohabitate with us. As construction technology progressed and energy conservation was prioritized, indoor building climates became tightly controlled and sealed from the outdoor air. Mechanical systems regulating temperatures and humidity created indoor micro-climates with evolutionary pressures (“survival of the fittest”) not found in the natural outdoor environment. Consequently, microbial communities in buildings have become distinct from those outdoors.

Now, centuries after constructing the first shelters, we are experiencing an unanticipated consequence of our sealed indoor environments — daily exposure to microorganisms that have adapted to indoor conditions and become more infectious and allergenic to humans.

Each time a person enters a building, they shed approximately 37 million bacteria per hour into the surrounding air or onto surfaces touched, spreading their microorganisms all over the room. The resulting community of microbes, referred to as the microbiome, is unique in each building depending on how it is constructed and operated, and on the activities of the occupants.

For example, one hospital study showed that patient rooms with mechanical ventilation had much higher numbers of circulating human-related bacteria compared to patient rooms with open windows that brought in outdoor air.

Hospitalized patients are exposed to infectious HAI microbes from two main sources: people and building reservoirs. A wide array of pathogens carried into the hospital by sick patients, visitors, and staff are expelled into the building through common activities such as talking, coughing, vomiting, skin shedding, and toilet-flushing. A single sneeze injects approximately 40,000 infectious aerosols into the room air, so clearly the indoor microbial load can become huge.


An imbalance of power

In addition to the sheer number of pathogens in hospital buildings, there also exists an unfortunate imbalance of power which favors the development of HAIs. In confined hospital spaces, biological extremes of virility and defenselessness coexist. On one side are patients who may be especially vulnerable to infections because of decreased immune defenses from illness and medications, breached skin barriers from surgery, or indwelling catheters and impaired respiratory system defenses from dry indoor air. On the other extreme are microbes that have survived the powerful selection pressures from anti-microbial medications, housekeeping disinfectants, and indoor building climates.

The availability of protected building reservoirs and an abundance of secondary hosts (patients) help microbial communities become well established. Not surprisingly, hospitals have unwittingly become reservoirs and vectors for ubiquitous HAI pathogens.

Today’s hospital infection control protocols focus largely on hand, instrument, and surface hygiene, as well as on cough etiquette and facial masks. While these strategies target the interruption of transmission through contact and short-distance, large-droplet spray, they do not immobilize tiny, aerosolized droplets which can spread infectious microorganisms over significant distances and for extended periods through the air.


Indoor air management downplayed

This lack of attention on managing long distance transmission of infectious aerosols occurs for several reasons. One reason is that epidemiologists continue to debate about the importance of the airborne route because there is a lack of easily-compared data on aerosolized microbes and infectivity.

While the magnitude of airborne droplet transmission continues to generate disagreement, epidemiologists do concur that despite robust surface hygiene interventions to control HAIs, the number of recorded cases has increased by 36% in the last 20 years and continues to grow every year.

The infectivity of airborne pathogens depends on their survival while suspended in air, their ability to revive after landing on a surface or secondary host-patient, and their ability to overcome the defense mechanisms of the secondary host-patient. Until recently, environmental monitoring for infection control has relied on cell-culture tests, which only detect microbes that appear to be alive at the time of collection. This is deceiving. While suspended in tiny airborne aerosols, infectious microbes are often temporarily in “travel mode,” appearing dead and non-infectious when collected during air sampling. But, when re-exposed to physiologic conditions in the next patient, many of these microbes rehydrate and are highly infectious.

We now have exciting new genetic sequencing techniques that can identify both active and dormant microbes through their DNA and RNA “fingerprints.” These powerful tools of metagenomics are broadening our understanding of the vast and dynamic hospital microbiome, the origin of HAIs, and the evolution of antibiotic resistance.

Air sampling that excludes dormant pathogens in tiny aerosols underestimates the infectious load of indoor air, contributing to the infection prevention focus on contact transmission and neglecting the importance of airborne transmission of aerosols.

Authors of current, comprehensive review articles conclude that many pathogens travel through the air at some point between the initial source, the reservoir, and the secondary patient. Until airborne transmission of infectious aerosols is controlled, even excellent adherence to existing contact hygiene protocols will not curtail the HAI epidemic.

To get a better understanding of the relationship between indoor air parameters in patient rooms and the incidence of HAIs, a study was recently done in a newly constructed, approximately 250-bed academic hospital in the north central U.S. Over a 13-month period, hourly measurements of room temperature, absolute and relative humidity, lighting levels (lux), room air changes, outdoor ventilation fractions, carbon dioxide levels, and room traffic were monitored in 10 patient rooms.

During the same period, ICD-9 codes of patients admitted to these rooms were analyzed for the presence of HAIs and multi-variable statistical analysis was run to determine if any indoor conditions independently correlated with these patient infections.

Of all the indoor climate measurements tracked, indoor rh was found to be the most closely related to HAI rates.

Relative humidity was inversely proportional to HAIs (p< .02). In other words, as indoor rh increased, the patient HAI rate decreased.

These surprising findings reinforce the need to understand, monitor, and manage indoor air hydration, or humidification, to decrease patient HAIs (Figure 1).

associated infections

FIGURE 1. Low rh in patient rooms correlates with increased healthcare - associated infections.

What are the mechanisms through which indoor rh relates to infection prevention?

When infected people breathe, talk, or cough, their naturally humid airways expel thousands of droplets containing saliva and mucus with embedded microbes into the ambient air. Thermodynamic equilibria between ambient air and the expelled droplets during condensation and evaporation dictate energy and mass changes. The vapor equilibrium, expressed as rh, of room air determines the resulting droplet size, concentration of salts, and the viability of the infectious micro-organism.

When expelled droplets encounter dry room air with rh less than 40%, they instantly shrink by roughly 90% as rapid equilibration occurs between the moisture levels in room air and in the droplet. The resulting tiny droplet nuclei with diameters less than 0.5 microns can then remain airborne for extended periods of time and be carried over great distances, thereby increasing the chance that they will reach a secondary patient (Figure 2).

infectious droplet nuclei

FIGURE 2. When rh is less than 40%, infectious droplet nuclei travel far and can cause disease in people who have had no direct contact with the original source.

When a patient inhales the desiccated droplet nuclei into their moisturized airways, the droplets rehydrate and pathogens are able to begin a new infectious cycle.

A very different scenario unfolds in a patient room with indoor air rh from 40% to 60%. In this setting, the respiratory droplets maintain diameters around 100 microns. Because the droplet diameter dictates the settling distance and rate, these larger droplets land on surfaces within 4 to 6 ft of their source (a person sneezing, toilet flushing) where they can be efficiently removed with surface cleaning.

Transmission through room air or mechanical systems is decreased, and therefore the possibility of a secondary patient being exposed is proportionately decreased.


Pathogen infectivity

In addition to reduced pathogen transmission in properly humidified air, many aerosolized bacteria and viruses have decreased survival when rh levels are between 40% and 70%. This reduction in pathogen survival decreases subsequent patient infections (Figure 3).

infectious droplet nuclei

FIGURE 3. Influenza A virus infectivity is markedly decreased when rh reaches 40%.

Conversely, while pathogens fare poorly in properly hydrated air, people are much healthier. What are the reasons for this?

Human lung physiology demands provision of 100% saturated air heated to 98.6ºF for their essential function: gas exchange. In the lungs, inhaled oxygen is exchanged for the metabolic waste product carbon dioxide across delicate, one-cell membranes of the alveoli.

Deep in the lung tissue, fragile alveoli sacs are in close proximity to blood vessels. To prevent infectious particles from settling into the alveoli where pneumonia or systemic blood infections could easily result, physiological barriers trap particulate matter in the upper regions of the respiratory system.

Respiratory mucosa from the nose to the small bronchial tubes moistens and heats inhaled air before it reaches the alveoli. Cells lining respiratory passages capture infectious particles on a surface mucus layer, then move the particles away from the lungs by continuous upward movement of numerous hair-like projections, the cilia, which sweep at rates up to several hundred cycles per minute.

The inhaled particles, trapped by mucus and moved upwards into the throat, are swallowed and incapacitated by acidic stomach and alkaline small intestinal tract conditions. Instead of pathogens reaching deep lung tissues where they could cause deadly infections, they became part of the intestinal microbiome where they may well contribute to our health.

When ambient air is dried to 20% rh, patients lose 60 to 80 grams/hour (1.5 to 2 liters/day) of water. The water loss by airways alone is 300 to 500 milliliters per day. In addition to drying the upper respiratory tract mucosa and reducing clearance of infectious droplets, the patient struggles to maintain adequate hydration needed for immune cell functioning and wound healing.

Given the clear health advantages of balanced air hydration with rh of 40%–60%, why are not all buildings humidified to provide this indoor climate? One of the barriers to change is a common misconception about mold.

Many people think that fungal organisms, or mold, can extract moisture from the air and use this water as a necessary factor for growth in building materials. Contrary to this popular belief, mold cannot extract moisture from the atmosphere. The water in buildings that mold uses comes from spills, leaks, or from condensation. If differences between outdoor and indoor temperatures and humidity levels create gradients across the building enclosure, which result in water condensation within the exterior wall, mold growth can start. Prevention of mold growth, reduced to a simple formula, requires keeping the dew point outside of the wall construction with proper insulation. The problem is not the air, but the cold surface. Compensating for insufficient wall insulation or design errors by lowering indoor rh is both an ineffective and unhealthy approach.

We do have the ability to construct buildings with optimal indoor climates, including rh levels of 40% to 60%, without incurring mold problems. Thousands of museums and libraries all over the world, many built more than one hundred years ago, tightly control rh at 50% and do not have mold problems.



The hospital’s physical environment has a significant impact on the health of patients. Unfortunately, too many patients are harmed, and hospitals waste money on avoidable HAIs.

The dry air in most hospitals create habitats for microorganisms that are unprecedented in the natural world and have untold consequences for the selection and transmission of pathogens. By maintaining rh in patient care spaces between 40% to 60%, the transmission and infectivity of airborne pathogens will be reduced, and surface cleaning will be more effective due to less resuspension and re-deposition of pathogens.

Well-hydrated mucus and cilia brush-layer

FIGURE 4. A. rh 40% - 60% Well-hydrated mucus and cilia brush-layer. Cilia movement washes particles upwards to the larynx, where they are then swallowed or coughed out.

In addition to creating a less infectious environment, indoor air hydration will support patients’ physiologic skin and respiratory tract defenses, immune cell functioning, wound healing and total body fluid balance — all natural defenses against HAIs.

Current indoor air guidelines for hospitals do not specify a lower limit rh in patient care areas and are even promoting lowering the minimum acceptable rh level in operating rooms from the current 35% down to 20%. This is a mistake. Management of health care facilities must focus on the number one priority — patient healing. To best protect patient health, optimize clinical outcomes and decrease excess health care costs, we must maintain the indoor rh between 40% to 60%.

Human life is an ongoing struggle against gravity and

FIGURE 5. Human life is an ongoing struggle against gravity and dehydration. Let’s not make this worse by drying out the air!

Proper indoor air hydration with hygienic, low-energy consuming humidification systems provides an underutilized opportunity to improve clinical outcomes by reducing HAIs, thereby shortening in-patient stays, decreasing readmissions, and reducing non-reimbursable hospital expenditures.

Your next steps:

  • Take an open-minded approach, putting patient wellbeing over the “status quo” facility management.

  • Monitor the rh in patient rooms and adjoining support spaces.

  • Accurately record patient outcomes, especially HAIs, in the monitored rooms.

  • Inspect HVAC systems and the building envelope for water infiltration.

  • Choose indoor air hydration (humidification) specialists with both engineering and health care knowledge and expertise to address the unique challenges of hospitals.

  • Install hygienic and low-energy humidification systems to maintain balanced indoor rh record patient outcomes while monitoring and maintaining indoor rh.

  • Communicate changes in patient outcomes, revenue, and ROI from implemented humidification solutions so others can learn from your experience.