Three main goals: Keep outside contaminants from entering the building, incorporate hvac elements in the design that will enhance air quality, and handle contaminants generated in the hospital's interior.

In a waiting room a patient coughs, unknowingly spraying tiny Mycobacterium tuberculosis-laden particles into the air. The droplets catch a faint breeze flowing through the room. A nearby patient, already weakened by the human immunodeficiency virus (HIV), breathes deeply and sighs. Highly infectious bacteria spewed so forcefully by a single cough settle into the HIV patient's lungs and ultimately cause another of the 30 million annual tuberculosis deaths worldwide.

A Third-World scenario? Or one in a United States clinic poorly equipped to handle the rising incidence of tuberculosis?

The Johns Hopkins University Division of Infectious Diseases estimates that approximately one-third of the global population is infected with Mycobacterium tuberculosis and that seven to eight million new cases of tuberculosis occur each year. Annual tuberculosis mortality is between two and three million people, making this disease the most common infectious cause of death in the world.

Most tuberculosis cases and deaths occur in developing countries, notably in Asia and Africa. In the United States, between 1953 and 1983, the annual incidence of tuberculosis declined an average of 8% per year. From 1985 to 1992, however, the United States' incidence of tuberculosis increased by 20%.

While doctors and nurses tend to the illnesses, hvac design engineers are now at the center of designing systems to improve indoor air quality (IAQ) at hospitals, thereby greatly enhancing the hospital's cleanliness. A good hvac system improves patients' recovery rates and increases worker productivity by maintaining a healthful work environment.

To stop the possible spread of tuberculosis, for instance, Portage Medical Systems in Hancock, MI, is actively building more isolation rooms. While a multi-drug regimen is the preferred treatment for tuberculosis, the prevalence of drug-resistant tuberculosis is increasing in the United States and internationally, exacerbated by the HIV epidemic. The patient must be isolated during treatment to prevent the spread of the disease, which explains why the new, $20 million facility is building more isolation rooms. The move is also designed to protect hospital workers and other patients.

In truth, from an hvac engineer's perspective, there are many IAQ concerns in the health care market.

Innovative HVAC Throughout

A wide variety of hospital facilities operate in the United States today, ranging from simple outpatient clinics with same-day surgery suites to traditional municipal hospitals to teaching and research institutions. Many facilities, such as the University of Missouri-Columbia Hospitals and Clinics (for which HDR Engineering is also designing extensive renovations), contain elements of each.

Over the past 10 years, according to U.S. Census Bureau data, the value of annual construction of hospitals and related institutions has averaged approximately $15 billion per year, or approximately 3% of the overall construction market.

New hvac demands are being placed on every renovation, expansion, and new hospital facility. Engineers are developing sophisticated hvac designs tailored to the varied and multiple uses throughout hospital space, not only isolation rooms, but operating rooms, hazardous materials handling rooms, laboratories, and other non-traditional hospital spaces.

A good hospital hvac system design satisfies three main goals.

1. Outside contaminants are prevented from entering the building.

2. Hvac elements that will enhance air quality, or, at a

minimum avoid compromising air quality, should be included in the design; and

3. Contaminants generated in the hospital's interior should be handled properly.

Inhaling and Exhaling

The optimum hospital hvac system breathes in clean air and expels spent or dirty air. Proper location of intake and exhaust vents is critical to prevent outdoor contaminants from being drawn into the hospital. A teaching hospital, for example, demands high-quality air intake for medical air systems, lab air, and general-purpose ventilation. The same building can have infectious isolation exhaust, laundry exhaust, emergency generator exhaust, and toilet exhaust all in the vicinity of the air intakes.

In general, exhaust discharge for fume hood systems, chemical exhaust, isolation room exhaust, or similar systems should be vertical. The vertical height should be designed to avoid recirculating contaminants back into the building, taking into account location of intakes, operable windows and doors, prevailing winds, and building elements. Exhaust ductwork and fans should be labeled as hazardous, if appropriate.

Either a utility fan with upblast discharge or a mass airflow fan is optimal for rooftop, contaminated-air exhaust. The fans should be arranged or designed to provide a minimum 3,000-fpm discharge velocity. The advantage mass airflow fans have over conventional utility exhaust fans is that they induce additional airflow from the outside, thus increasing dilution. Also, air is discharged with greater momentum, increasing the distance above the roof at which the plume disperses, thereby reducing the possibility that discharge will be drawn back into the system.

For isolation room exhaust, if the exhaust discharge has the possibility of being trapped within the building's boundary layer, thus risking re-entry into the building's air intake, HEPA-filtered bag-in, bag-out filtration can be installed upstream of the exhaust fan to trap airborne pathogens.

Maintaining Optimal Humidity Control

The human body feels and functions better when ambient temperature and humidity are within certain optimal ranges: a space temperature of approximately 76 degrees F with relative humidity (rh) between 30% and 60%. These are also defined hospital standards. Hospital IAQ is greatly enhanced by maintaining optimal temperature and humidity, thereby satisfying the second hvac design goal. Conversely, allowing air to fall outside the optimal range adversely impacts air quality.

Below the lower range of optimal, certain organisms thrive. More importantly, however, the efficiency of our natural bodily defenses to stave off infection is lessened. For example, in the winter, dry air steals water from building occupants, leaving them with scratchy throats and dry nasal cavities, thereby reducing their ability to filter airborne contaminants.

Hot summer weather brings the opposite humidity problems for facilities such as the University of Missouri, where summers are hot and humid. Insufficient removal of excess water vapor from the air inhibits the body's ability to release excess heat and water vapor to the surrounding air, causing occupants to feel too hot.

Excessive humidity (greater than 60%) can also cause mold and mildew growth. If left untreated, the mold and mildew can escalate into an IAQ concern sometimes difficult to remediate.

Temperature and humidity control is particularly important in operating rooms. Standard guidelines dictate that the operating room space temperature should not exceed 68 degrees. However, many hospitals push for operating room temperatures closer to 60 degrees due to the intense radiant heat from surgical lighting and the discomfort of hospital operating gowns.

Maintaining the 60% humidity threshold at these low temperatures is generally not achievable with conventional hvac systems. To obtain these levels, low-air-temperature systems or desiccant dehumidification systems are necessary. Such systems, if well thought out and designed, are effective but expensive to install and operate compared to conventional systems.

The Heart of the HVAC System

Air circulates to and from the hospital in vein-like ductwork. This air is processed through the heart of the hvac system, the air-handling unit, generally consisting of an outside air intake, supply fan, return-relief fan, heating-cooling coil, humidifier, and filters. For the hvac engineer, the front line of defense in ensuring proper IAQ is a well- designed air-handling system.

In hospital air-handling systems serving patient treatment areas, the air supply must have a minimum of two filters. The prefilter, typically providing 30% (minimum) filtration, is placed at the upstream end of the air-handling unit, ahead of heating and cooling coils. The final filter, placed downstream of the air-handling unit, should provide 90% (minimum) filtration. The prefilter collects larger particles and extends the service life of the more expensive final filter. Without the prefilter, the coil's tight fins act like a filter and clog in a short period of time.

Historically, the interiors of many air-handling units were lined with exposed insulation, which was difficult to clean and subject to damage, causing maintenance problems and potentially compromising IAQ. Now, double-wall units, with interior insulation covered by a second piece of sheet metal, are more commonly used. The double-wall units allow maintenance staff to wash down the inside of the unit.

A typical air-handling unit mixes outside air and return air, and conditions it by heating, cooling, and humidifying. There are two types of air-handling units: a draw-through and a blow-through unit. In a draw-through unit, the coil is located upstream of the supply fan. In a blow-through unit, the coil is located downstream of the supply fan. For hospitals, a draw-through unit has several advantages over a blow-through unit.

Eliminating Breeding Grounds

In a typical blow-through unit operating in a cooling cycle, the fan blows hot, humid air over cooling coils. As the sensible temperature drops below dewpoint, latent heat is removed from the air, and the rh of the air increases, often to 90% or 95%.

At this point, the highly saturated air hits the filter where water droplets can cling to the filter material, not only reducing its effectiveness and deteriorating the filter, but creating a potential breeding ground for bacteria, mold, and other organisms. When HEPA filters are used and the air is over-saturated, the water droplets can actually clog the filter.

In a draw-through unit, water is pulled across the cooling coils. The rh of the air is similarly increased to around 95%, but heat from the fan then decreases the rh to around 85%. Water carry-over problems are reduced and the downstream filter does not become saturated with water vapor.

In some older hospital hvac systems, ductwork downstream of the air-handling units is lined with insulation rather than wrapped, as is the current standard. Over time, the liners often become friable. Because they cannot be cleaned effectively, they become a prime environment for the growth of undesirable organisms. Therefore, when upgrading an existing hvac system, it is imperative to replace such lined systems with new unlined ductwork including wrapped insulation.

Preventing the Spread of Airborne Pathogens

Just as patients' noses and lungs filter the air they breathe, a hospital filtration system traps and removes particles from the airstream. Filters are another element of the hvac that, if designed and installed properly, enhance air quality. If they fail, though, they have the opposite impact.

Common office buildings, including many medical offices, use a single set of 30%-efficient air filters. Although these filters meet minimum code requirements and remove large particles, they are ineffective in removing small but troubling particles, such as pollens.

Particles that pass through low-grade filters fall out and settle on duct system walls and the turning vanes of duct elbows. The remaining particles disperse into an occupied space, where they may collect on surfaces or find their way into occupant's eyes, nose, throat, and lungs. Because most air in a building is recirculated back into the air distribution system, particles that did not find a resting place the first time through have a chance to deposit themselves in the return or supply ductwork on subsequent journeys through the system.

Highest Quality Filtration

In an operating room serving low-immunity patients - such as orthopedics, cardiology, and organ and bone marrow replacement, where infection is a major concern - 90% filtration may not be sufficient. In these spaces, HEPA filtration, which provides 99.97% filtering efficiency, is a better selection.

In an isolation room, all air is typically exhausted; however, if conditions limit exhaust capabilities, air may be returned to the central air system (although this is not the optimal alternative), provided the return air is HEPA filtered. Such filtration presents design challenges because the filters increase system pressure drop. HEPA filter airflow velocities must be kept at about half the rate of traditional filters - 250 feet per minute (fpm), compared to the standard 500 fpm. Also, HEPA filters used for this purpose will likely be considered hazardous waste.

Maintaining Pressure

With the outbreak of drug-resistant tuberculosis, more and more hospitals now need highly specialized hvac systems in isolation rooms to further protect the patients, health care providers, and staff. These systems properly handle contaminated air, the third important goal of a good hvac design.

Negative-air-pressure rooms keep a patient from infecting other patients or staff and are used, for example, for tuberculosis or measles patients. Positive-air-pressure rooms protect the patient from external infection and are used for bone marrow transplant patients, for example. Positive air pressure is only used for these relatively rare conditions and, therefore, is much less common than negative air pressure.

Where airborne pathogens or contaminants exist, dilution, airflow, and exhaust are the keys to maximizing protection of personnel and patients. The more air that can be brought through a room and the greater the distance between discharge and intake (to prevent short-circuiting), the better the design. Isolation room airflow is typically mandated as a minimum of 6 air changes per hour (ac/hr); however, a more effective rate for dilution purposes is a minimum of 12 ac/hr. Many building codes have recently increased the air change requirement to this higher number.

Airflow should always be directed from clean to less-clean areas. In negative-pressure isolation rooms, exhaust airflow intakes should be located near floor level and at the head end of the patient bed. Supply airflow should be located as far away from the head of the patient as possible, so that airborne pathogens emitted by the patient get drawn back toward the patient and to the exhaust.

Typical ceiling diffusers increase turbulence within a room to mix the air and provide uniform temperature and quality. However, in isolation rooms, every effort should be made to limit the amount of air turbulence at the patient's head so that airborne pathogens are not spread unnecessarily. Radial diffusers or laminar flow diffusers, which limit air turbulence, can be used, although a delicate balance should be struck between undesirable turbulence and optimal mix for comfort.

Control Systems

A modern building with microprocessor controls is akin to a body with hundreds to thousands of nerve centers collecting temperature, humidity, pressure, valve, and operating data. Through appropriate sensing points, the control system can tell the engineering maintenance staff, for example:

  • When filters are loaded and need replacement;
  • Whether an air-handling unit is pulling in enough outside air;
  • Whether the building is properly pressurized;
  • Whether humidity and temperature levels are within optimal range;
  • Whether an isolation room is under negative pressure with adequate airflow; and
  • Whether or not a critical exhaust system is operating.

The Bottom Line

As always, good communication between the hospital owner, associated hospital staff, architect, and hvac design engineer is critical to the success of a renovation or new hospital design. All parties must understand the level of IAQ required or desired for each particular area.

The owner needs to recognize the costs associated with high IAQ, including increased energy and capital equipment costs. Alternative systems, approaches, or operating procedures may involve less risk. But the bottom line for hospitals is cleanliness and employee, patient, and visitor health. Hvac engineers lead the way in providing clean, safe, and comfortable air for those who need it most. ES