Sitting on the floor of a traditional Himba hut in northern Namibia this past summer, I found myself staring at a peculiar hand-sized hole located about halfway up the curved wall thinking to myself, “what could be the purpose of such a small opening?” Turns out, it was the sole portal to the outside world – at least when the tiny three foot door was closed. It was a window – but why so small?
In the absence of available panes of glass to the Himba in this arid region, I reasoned that if they had made it bigger, say two feet by two feet, the rain would trickle in. It would, not to mention, serve as an invite for an enumerable number of crawling and flying pests. The keep-the-rain-out logic started to make the most sense when I realized the gaps and openings surrounding the closed door hardly kept even the largest of nature’s pests – save a goat – from entering the home.
After spending a few hours in this village near the Angola border – we had come to ask permission to take swabs of their homes to characterize the microbes they shared their living quarters with – I started to appreciate why a single, tiny window was all that was needed in any Himba hut. As with most traditional societies and throughout human evolution, people lived outside. Cooking, socializing and a big chunk of sleeping during the warmer and less rainy part of the year, was done outside. Adding more or larger windows to a space that was only used sparingly – and even then mainly at night – and the need to look outside and gain some knowledge of what was going on or to appreciate the “outdoors” was less important. It was likely dark and you got your full of “outside” having spent the majority of your day in it.
Add to this those gaping holes around a door that was more of a suggestion than an obstacle to entry, and the floor and wall plaster made of cow dung, you quickly become conscious of the fact that for 99.99% of human history the outside was always part of the inside, and at no moment during our day were we ever really separated from nature. This ecological reality makes our modern built environment of hermitically sealed lives of ventilated and filtered air and large pane glass windows seem at odds with the natural order of things. Keeping the outside out does have its advantages – protection from the elements and decreasing your chances of being eaten by a predator in some areas among them. But biologists who study the diversity and composition of the built environment microbiome are starting to think maybe we have walled off and filtered out a little too much of the outside – at least from a microbial perspective.
In urban settings, A National Activity Survey found that between enclosed buildings and vehicles, humans spend a whopping 90% of their lives indoors. Researchers who study the ecology of built spaces from the University of Oregon wanted to know how the airborne microbial life differed in spaces that were either naturally vented through open windows or mechanically ventilated (i.e., central heating and air) and how the microbial environment might impact human health. Since hospitals are both a place of healing and where one might acquire a secondary infection as well, they set up special BioSamplers in various patient rooms at the Providence Milwaukie Hospital in Milwaukie, Oregon.
Using the BioSampler, which draws air through two liquid impingers filled with sterile water and thus traps the dust, pollen and attached microbes, they set up measuring stations in rooms 1) vented ‘naturally’ through open windows; 2) mechanically vented rooms and; 3) on the roof of the hospital near the air intake for the ventilating system. They also measured things like air temperature, humidity and amount of airflow through the various rooms.
Using high-throughput sequencing of the bacterial 16S rRNA genes of the bacteria captured in the water, they were able to measure the abundance of bacteria as well as species diversity. Between the three environments (mechanically ventilated, window ventilated, and rooftop outside) the airborne bacterial cell density was roughly between 500,000 and 2.5M cells per cubic meter of air sucked through the BioSamplers. Interestingly, the density of cells between these three different environments did not vary significantly. That is, the abundance. However, the highest diversity of bacteria was recorded outside, on the rooftop, with the lowest diversity recorded indoors in the mechanically ventilated rooms.
Interestingly, the mechanically ventilated rooms had the greatest relative abundance of potential pathogenic bacteria, when compared to either window ventilated rooms or the rooftop outside. In addition, both indoor settings (mechanical and window) also contained bacteria floating in the air that are known to be associated with humans (our regular bugs). Neither the potential pathogenic or human-associated bacteria recorded in both indoor settings were observed in any great abundance from the outdoor samples. The outdoor samples were dominated by bacteria known to occur in soil and water. These soil and water bacteria were “relatively rare or absent indoors” according to the researchers.
The study is fascinating in that it demonstrates how much architectural design and our modern lifestyle changes the microbial ecology of our surroundings. In the case of the Himba, inside is just like the outside – and so it has been for nearly all of human existence. Mechanically ventilated rooms, with filters and return air registers, have an ecologically distinct set of microbes than that found outside on the rooftop. Turns out, the window ventilated rooms have microbial communities somewhere in the middle between outside and those mechanically vented rooms. Also, the outdoor air – though dominated by bacteria found in soil and water – demonstrated a greater diversity of overall bacteria than either indoor setting.
The most important finding of this study is that both indoor environments were dominated by human commensals (known to occur on humans) and known pathogens. The researchers also found that airflow rates through the room had a significant impact on the “pathogen load” of the room. That is, as the air moved in and out of the room via the mechanical ventilation or the natural airflow, the pathogen load was diluted down. It also appeared that temperature and humidity impacted bacterial community composition of the indoor settings as well.
At the moment, while the researchers can rule out the outdoor air as a significant source for the pathogenic and human-associated commensals measured in the rooms, the actual source of these microbial species is not certain, but likely comes from patients and others in the hospital as well as various surfaces – all pushed along with rising and falling temperature and humidity.
Though the researchers did not find any appreciable differences in the pathogen load between mechanical and naturally ventilated rooms, they did note that airflow reduces that load. They also suggest, “that reducing direct contact with the outdoor environment may not always be an optimal design strategy for bacterial pathogen management.” In other words, current architects and engineers design buildings for comfort by controlling light, temperature, humidity and airflow – but in the future should consider the management of microbial community as well
Though the researchers do not elaborate on why we should not reduce our exposure to the outside, they are implying that “increasing” exchange and airflow with the outside and its soil and water dominated microbial communities may not only reduce the pathogen load even further – and serve as a more economic and ecological friendly way to think about built environments – but reintroduce us to some Old Friends that our transition from village to urban life have scrubbed away.
Seems Florence Nightingale may be more correct than knowable at the time when she said over 150 years ago that open windows were the hallmark of a healthy hospital ward.
Think I will open that window now.
Jessica Green: Are we filtering the wrong microbes?