Most of us in the developed world spend the majority of our time indoors, breathing clean-smelling air that is mechanically warmed or cooled and then perhaps filtered and recirculated. Windows are usually shut, especially in modern commercial structures such as hospitals. In the Unites States, the norm in patient rooms are inoperable windows, so when you are an in-patient, you have no choice about the source of your inhaled air.

Thanks to incredible new techniques that allow scientists to map the DNA and RNA "fingerprints" of bacteria, viruses, and fungi, we now know that these microbes are everywhere — in and on our bodies, in the air, water, buildings, and outdoors. Knowledge about these microbial ecosystems, referred to as microbiomes, is increasing by the week! We now know that in number, each of us is only from 10-30% human cells, the rest of us consisting of approximately 3 lbs of viruses, bacteria, and fungi, living on or inside us. Our microbiome impacts every aspect of our health, from digestion to psychological stability to immunological protection from infections and allergies.

We also now know that indoor and outdoor microbiomes are very different. The communities of bacteria, viruses, and fungi that reside indoors have much less diversity than those in the outside world. In addition, a strong link has now been found between the microbiomes of our skin and the population of microbes recovered from the inside of buildings.

In the hospital, this new information raises two very important questions. First, do these microbes contribute to the too familiar new infections that people get from being in the hospital, called health care-associated infections (HAIs), and second, how does indoor air management affect this microbiome of the built environment?

A year-long study using DNA and RNA analyses on the microbial populations of patients, staff, and the building at the University of Chicago hospital, beginning prior to opening in 2013, added invaluable understanding about human and indoor microbiome interactions. This study showed that when a patient first entered their room, their skin bacteria was distinct from that of the surrounding surfaces and air.

An especially interesting (and possibly alarming) finding was that antibiotic resistance genes were more abundant in the bacteria waiting on room surfaces than in skin bacteria of the newly admitted patients. Very quickly (within approximately 30 minutes) the patient skin and room microbial populations began to mingle and resemble each other. The bacterial populations established on the skin of patients were not very influenced by oral or intravenous antibiotic treatment. In a separate study, analysis of clinical outcomes of the patients in these hospital rooms showed that the number of patient HAIs was related to indoor air management, specifically to relative humidity (rh). Contrary to common thinking, low patient room rh was statistically and independently associated with higher rates of HAIs (submitted for publication).

These important revelations from the intersection of microbiome studies, patient records of HAIs, and indoor air measurements challenge us to reevaluate our previous mindsets. One incorrect, yet common, assumption is that dry indoor air is universally good for us. Numerous studies, including the well-known Sterling and Avundel chart from ASHRAE 1985, show that indoor rh from 40-60% is protective to humans and also limits the infectivity of many pathogenic viruses and bacteria. If we want to manage hospital IAQ to support patient and employee health, we must properly hydrate the air.

This is a revolutionary time management of the built environment. We once thought of microbes as only bad, disease-causing germs to be eradicated; we now know that they are also companions beneficial to our health and happiness. We cannot and should not try to eradicate them, but rather should learn to manage the built environment, and especially indoor air, to foster healthy cohabitation between organisms of all sizes — from single-celled bacteria to multi-organ humans.