Summary

The two main sources of microorganisms indoors are the macroscopic occupants; people, pets, plants, and pests, and the environment outdoors, in particular – outdoor air.  When buildings are operated in a “closed mode”, they may utilize mechanical ventilation, recirculated air, and/or closed or inoperable windows (Figure 1).  If human occupants are present, operating a building in closed mode can increase the relative abundance of human-associated microorganisms in the space.  On the other hand, operating a building in an “open mode”, with open windows or doors, allows for more outdoor-associated microorganisms to be carried in with the breeze and change the microscopic community within (Figure 2, 3).  Our study in 2012 was one of the first to show the effect of ventilation source on the indoor microbiome, but this has been an overarching theme in a number of research studies since (e.g. 1, 2).  The understanding that the use of outdoor air as a means to change the indoor microbiome, and specifically make it less human-associated, raises some interesting possibilities for designing spaces for health.