Humans in the developed world spend more than 90 percent of their lives indoors, where they breathe in and come into contact with trillions of life forms invisible to the naked eye: microorganisms. Buildings are complex ecosystems that are an important source of microbes that are good for us, and some that are bad for us. What determines the types and distributions of microbes indoors? Buildings are colonized by airborne microbes that enter through windows and through mechanical ventilation systems. And they are brought inside by humans and other creatures. The fate of microbes indoors depends on complex interactions with humans, and with the human-built environment. And today, architects and biologists are working together to explore smart building design that will create healthy buildings for us.
We spend an extraordinary amount of time in buildings that are extremely controlled environments, like this building here — environments that have mechanical ventilation systems that include filtering, heating and air conditioning. Given the amount of time that we spend indoors, it's important to understand how this affects our health. At the Biology and the Built Environment Center, we carried out a study in a hospital where we sampled air and pulled the DNA out of microbes in the air. And we looked at three different types of rooms. We looked at rooms that were mechanically ventilated, which are the data points in the blue. We looked at rooms that were naturally ventilated, where the hospital let us turn off the mechanical ventilation in a wing of the building and pry open the windows that were no longer operable, but they made them operable for our study. And we also sampled the outdoor air.
If you look at the x-axis of this graph, you'll see that what we commonly want to do — which is keeping the outdoors out — we accomplished that with mechanical ventilation. So if you look at the green data points, which is air that's outside, you'll see that there's a large amount of microbial diversity, or variety of microbial types. But if you look at the blue data points, which is mechanically ventilated air, it's not as diverse. But being less diverse is not necessarily good for our health. If you look at the y-axis of this graph, you'll see that, in the mechanically ventilated air, you have a higher probability of encountering a potential pathogen, or germ, than if you're outdoors.
So to understand why this was the case, we took our data and put it into an ordination diagram, which is a statistical map that tells you something about how related the microbial communities are in the different samples. The data points that are closer together have microbial communities that are more similar than data points that are far apart. And the first things that you can see from this graph is, if you look at the blue data points, which are the mechanically ventilated air, they're not simply a subset of the green data points, which are the outdoor air.
What we've found is that mechanically ventilated air looks like humans. It has microbes on it that are commonly associated with our skin and with our mouth, our spit. And this is because we're all constantly shedding microbes. So all of you right now are sharing your microbes with one another. And when you're outdoors, that type of air has microbes that are commonly associated with plant leaves and with dirt.
Why does this matter? It matters because the health care industry is the second most energy intensive industry in the United States. Hospitals use two and a half times the amount of energy as office buildings. And the model that we're working with in hospitals, and also with many, many different buildings, is to keep the outdoors out. And this model may not necessarily be the best for our health. And given the extraordinary amount of nosocomial infections, or hospital-acquired infections, this is a clue that it's a good time to reconsider our current practices.
So just as we manage national parks, where we promote the growth of some species and we inhibit the growth of others, we're working towards thinking about buildings using an ecosystem framework where we can promote the kinds of microbes that we want to have indoors. I've heard somebody say that you're as healthy as your gut. And for this reason, many people eat probiotic yogurt so they can promote a healthy gut flora. And what we ultimately want to do is to be able to use this concept to promote a healthy group of microorganisms inside.