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Microbes Among Us

As researchers continue to study the bacterial and fungal microbiome in the indoor environment, the Department of Defense can learn from research of facilities comparable to its property inventory while accounting for mission and locational differences that could introduce dissimilar microbiomes from those researched to date.


By Maj. Andrew Hoisington, Ph.D., P.E., M.SAME, USAF


 Air Quality in Theater

Americans, on average, spend more than 90 percent of their life indoors, exposed to microbial communities (microbiome) on surfaces and in the air.

Microbial populations in the indoor environment historically were viewed as an indoor air contaminate, with facilities designed to increase ventilation to provide a healthy environment. Beginning 30 years ago researchers started to identify patterns in which occupants of select facilities were becoming ill at a higher rate than other facilities. They termed this phenomenon Sick Building Syndrome.

One possible link of indoor air quality and Sick Building Syndrome was the pres­ence of mold. It was the first major issue to gather attention in indoor air quality, which led to the studying of bacteria and fungi in the built environment. A second advance­ment in indoor air microbial research came about 10 years ago, aided by the technology of molecular biological tools and computa­tional software capable of analyzing trends and comparing large datasets.

To date, molecular research has been conducted on the indoor microbiome of residences, hospitals, retail stores and university buildings among other facility-types. Numerous findings have been published characterizing the indoor microbiome, what influences that microbi­ome, and the impact on occupants. And yet despite the Department of Defense (DOD) being the largest property owner in the world with more than 500,000 buildings and structures at 5,400 sites in over 30 countries, its facilities have not been exten­sively sampled for the indoor microbiome.



Researchers have discovered the bacteria identified in the built environment closely related to the human occupants. This is not entirely surprising because the human microbiome is one of the most studied biome. Human occupants are prevalent in the facilities and bacterial cells outnum­ber human cells 10:1 on any single person. The human microbiome is diverse, with only 10 percent average shared bacterial communities between people. In contrast, humans share 99.9 percent of DNA with each other. Simply identifying an influ­ence from human occupants to the indoor microbiome may not be that informative.

A study of residential units, for instance, observed that the microbial communities were similar in different locations of the homes but changed dramatically in differ­ent homes. Any one person microbiome is most similar to their significant others or children. Interestingly, it took as little as three days of occupancy change to signifi­cantly alter the observed indoor microbi­ome. Individual rooms also can change the indoor microbiome. For instance, a rest­room has a different bacterial microbiome given than any other room in a facility.



To date, molecular research has been conducted on the indoor microbiome of residences, hospitals, retail stores and university buildings among other facility-types. Numerous findings have been published characterizing the indoor microbiome, what influences that microbi­ome, and the impact on occupants.



Another major research area of the built environment is the study of how a facil­ity’s design, operation and environmental factors influence the indoor microbiome. One university study detected a rela­tionship between microbial diversity in areas with a high degree of human occu­pant traffic (such as central hallways) and correspondingly less diversity in those areas of the building not frequently visited (mechanical spaces, for instance). A few studies have attempted to determine if air exchange rate contributes to the diversity of the indoor microbiome—but no conclu­sive link has yet been observed. Seasonality influences also have been inconclusive across multiple studies, although outdoor bacterial studies have shown seasonality in the bacterial microbiome.

While the bacterial microbiome detected indoor ties to the human occupant in multi­ple avenues, the indoor fungal microbiome is influenced primarily by the outdoor environment. In a worldwide indoor dust study sampling for fungi, latitude was the best predictor of the fungal communities. The fungal microbiome outdoors is influ­enced by environmental conditions. Due to outdoor influences, fungal communi­ties indoors are also influenced by seasonal changes. A study of retail buildings in the northeast detected natural occurring fungal spore releases in indoor air samples. Despite the relationship between the indoor and outdoor fungal microbiome, a connec­tion with the indoor communities and air exchange rate has not been determined.



DOD has many facilities that are closely related to buildings that have already been studied and the relationships are likely the same. Therefore, when a family moves into a base residential home they can expect that they will “seed” the home in approximately the first three days (and probably the office as well). That idea may influence a change of occupancy cleaning— reducing the time, expense and use of environmentally degrading cleaners that may increase the amount of antibiotic resistant microbes. However, there are two other issues that could influence the indoor microbiome in DOD facilities: high ventilation temporary facilities and operational cost limits.

Tents and trailers in the Middle East operate with high air exchange rates, primarily due to infiltra­tion. Particles of air spend on average 20 minutes or less in most tents or trail­ers. It might be expected that the influence from the outdoor environ­ment would be magnified in this type of facility for fungi, in particular, and less so for bacteria. However, outdoor air from the Middle East region has not been as exten­sively studied as other areas in the world using molecular methods, so this could introduce unknown additional bacterial and fungal microbes. Compounding the situa­tion, dust has been shown to be an effective transport mechanism for microbes over a long distance. Around-the-clock operations and close quarters also might increase the diversity of the bacterial microbiome in some temporary facilities where the bacte­rial communities never become stable.

Budget reductions may influence the bacterial and fungal microbiome as DOD seeks to balance risks across all mission sets. These budget reductions could impact the maintenance of the facilities to include simple HVAC filter replacements. Dirty HVAC filters cause large pressure drops, reduced performance, and may cause sloughing of a considerable concentra­tions of bacteria and fungi. The influence of soiled filters has not been investigated for the indoor microbiome.

Another impact of budget limitations could be a reduction in HVAC usage. HVAC usage in commercial buildings can account for over 40 percent of the total energy usage. The amount of outdoor air that requires conditioning has a direct influence on the energy used in the facility HVAC units. To reduce energy, some facilities can enter a no heat/no cool period at certain times of the year. Larger systems can maintain ventilation systems during that time, but smaller systems are completely shut down, allowing ventilation only through infiltra­tion. If that occurs, the bacterial and fungal communities may resemble more of the human microbiome. It also may increase the microbes in the indoor environment and change the types of microbes in the environment. The lack of a connection between the indoor bacterial and fungal microbiome with the air exchange rate may indicate that type of operation does not degrade the indoor air quality. That comes from a traditional view of the bacterial and fungal microbiome being a contaminate from the outdoor environment.



DOD is providing some investigations into the indoor microbiome. The Defense Advanced Research Projects Agency over­sees the Biological Technologies Office and a unique combination of microbiome and building research is occurring in a military hospital in Chicago (hospital microbiome). The risk to DOD personnel from either microbes in tents in the Middle East or budget limitations is probably unlikely—but our knowledge on the subjects is limited for those two cases.

Energy influences on the indoor micro­biome could provide a significant impact to operating expenses. That is a future item of interest to many building professionals. Research likely will be led by industry and academia, which DOD can incorporate into future operating protocols.

The indoor bacterial and fungal micro­biome is a topic that should be considered by engineers and medical personnel and will likely influence building design and operations in DOD facilities in the future.

*The views expressed in this article are those of the author and do not reflect the official policy or position of the U.S. Air Force, Department of Defense, or U.S. government. 



Maj. Andrew Hoisington, Ph.D., P.E., M.SAME, USAF, is Assistant Professor, and Environmental Engineering Division Chief, Department of Civil & Environmental Engineering, U.S. Air Force Academy, Colo.; 719-333-8975, or This email address is being protected from spambots. You need JavaScript enabled to view it..