Indoor Air Quality Trends: Sustaining Healthy Air

By Lawrence J. Schoen, P.E.
From the November/December 2014 issue

Though Ebola is the infectious disease that holds much attention these days, far more people have historically died and been made ill from other diseases such as influenza. Four worldwide (pandemic) influenza outbreaks occurred in the last 100 years: 1918, 1957, 1968, and 2009. There were also three notable epidemics: 1947, 1976, and 1977. The 1918 Spanish flu was the most serious pandemic in recent history, responsible for the deaths of an estimated 50 million or more people. The most recent H1N1 pandemic in 2009 resulted in thousands of deaths worldwide.

Dirty air vents.
Photo: Thinkstock/

Statistically, it seems like we have had the recent good fortune of avoiding a truly devastating pandemic. Another way of saying this is that such a tragic event is long overdue.

Influenza and in some cases, the common cold, and other diseases can travel by the airborne route. Tuberculosis definitely does. By airborne, we mean particles or droplets that are so small they remain airborne and behave like a gas. Room air currents and the HVAC system can carry such particles throughout a facility.

Many diseases spread by direct contact, which means any surface contact such as touching, kissing, sexual contact, contact with oral secretions, or skin lesions. Some spread by indirect contact, which involves contact with an intermediate inanimate surface (fomite), such as a doorknob or bedrail that is contaminated.

Exposure through the air occurs through (1) droplets, which are released and fall to surfaces about 3′ from the infected and (2) small particles, which stay airborne for hours at a time and can be transported long distances. When droplets become small particles by evaporation, they may be called droplet nuclei. HVAC systems and room air currents have the greatest effect on airborne transmission.

What Preparation Should Facilities Undertake?

In the event of a pandemic or major epidemic (the difference being the extent of spread), people may be quarantined, travel may be disrupted, schools may be closed, and people may be instructed to avoid crowds and contact. However, some facilities, such as prisons, shelters, and those related to healthcare, cannot close or curtail activities. Fast action regarding building operations may be needed.

ASHRAE recommends that designers and operators go beyond existing minimum practice standards to be better prepared for known risks and for an outbreak that might be caused by a new microorganism capable of spreading by the airborne route. These recommendations and other useful information can be found in ASHRAE’s Airborne Infectious Disease Position Document (updated July 2014 and online at: It covers the basics of infectious disease transmission and practical implications for building owners, operators, and engineers—most importantly, HVAC related interventions.

Such interventions include dilution ventilation, pressure differentials, exhaust ventilation, filtration and air cleaning, ultraviolet germicidal irradiation (UVGI) in the upper room, ducts and air handlers, in-room air flow regimes, temperature, and humidity. These techniques have broad applicability to any disease that is airborne. The following sections discuss two of these interventions. Of course, a prerequisite to all of the strategies is a well-designed, installed, commissioned, and maintained HVAC system.

Room Pressure Differentials

Areas of a facility that are likely to contain individuals with infections that can be spread by the airborne route should be kept at negative pressure by exhausting air from them. Furthermore, air from these rooms should be recirculated only within the room and not be mixed with or supplied to other rooms. For example, in healthcare facilities, airborne infection isolation rooms (AIIRs) are kept at negative pressure with respect to the surrounding areas to keep potential infectious agents within those rooms.

Areas of a facility with highly susceptible or vulnerable people should be kept at positive pressure with respect to other areas. This can be done by supplying clean air to such rooms at a greater flow rate than the return. Hospital rooms with immune-compromised people are kept at positive pressure in protective environments (PEs) to keep potential infectious agents (e.g., Aspergillus) out of the rooms.

Ultraviolet Germicidal Irradiation (UVGI)

All UVGI depends on inactivation of viable agents, both in air and on surfaces, depending on strategy. There are two UVGI strategies for general application: installation into air handlers and/or ventilating ducts; and irradiation of the upper air zones of occupied spaces. Shielding is required for lower occupied spaces because UV is harmful to occupants.

Upper air UV fixtures, as seen in this photo, are an option virtually anywhere there may be a threat of infectious and nosocomial diseases.
Upper air UV fixtures, as seen in this photo, are an option virtually anywhere there may be a threat of infectious and nosocomial diseases. (Photo: UV Resources.)

Two strategies used in some but not all healthcare occupancies are in-room irradiation of unoccupied spaces and of occupied spaces (e.g., operating suites) when personnel have appropriate personal protective equipment. In-room UVGI may be performed in patient rooms between successive occupants using elevated levels of irradiation applied in the unoccupied room for a specified length of time. This is primarily a surface disinfectant strategy, though it also disinfects the air that is in the room at the time of irradiation. Because the UV is turned off before the next patient arrives, it has no continuing effect on the air.

In both duct-mounted and unoccupied in-room UVGI, the amount of radiation applied can be much higher compared to what can be used for upper-zone UVGI, resulting in higher aerosol exposure and quicker inactivation. Duct mounted UVGI can be compared to filtration in the central ventilation system, because it inactivates the potentially infectious organisms while filtration removes them. UVGI does not impose a pressure drop burden on the ventilation system.

There is research that shows UVGI in both the upper-room and in-duct configurations can inactivate some disease-transmitting organisms, that it can affect disease transmission rates, and that it can be safely deployed.

Upper-zone UVGI, when effectively applied, inactivates infectious agents locally and can be considered in public access and high-traffic areas such as waiting rooms. The fixtures are typically mounted at least 7′ high, allowing at least 1′ of space above the fixture for decontamination to occur. It is typically recommended when ventilation rates are low.

At air change rates much greater than 6 ach (air changes per hour), there is evidence that upper-room UVGI is less effective relative to particle removal by ventilation (thought to be because the particles have less residence exposure time to UV).

While airborne infectious disease can be transmitted in any facility type, prisons, homeless shelters, and healthcare are considered high-risk due to vulnerable populations and greater potential for the presence of infected individuals. Facilities at risk should incorporate the infrastructure to quickly respond to an outbreak.

Larry Schoen.

Such infrastructure might include HVAC systems that separate high-risk areas, physical space, and HVAC system capacity to upgrade filtration; the ability to increase ventilation even as high as 100% outdoor air; the ability to humidify air; receptacles at the upper room for UVGI; and ceiling heights of at least 8′. Once the building is in operation, filter elements and upper-room UV fixtures should be available for rapid deployment in an emergency.

Schoen is president and principal engineer of Schoen Engineering Inc., located in Columbia, MD. He is an ASHRAE Fellow, member and past chair of its Environmental Health Committee, and chaired the committee responsible for authoring the most recent position document in 2014.


  1. We are developing and deploying UV/ Filtration systems in a very unique set of applications. These applications are something we might want to talk about to see if there are any synergies.
    We are a small start up however we do have a number of contracts.
    In addition I would like permission to quote your article and perhaps include some of the material in a PPT presentation
    Any consideration is appreciated.
    John P Walsh

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