Given that they occupy every other niche on earth, including deep sea vents, salt flats and the interior of human bodies, it should come as no surprise that bacteria are also found in human houses. Once inside, they can cause problems; airborne bacteria can be easily move between people and spread their corresponding diseases. Nowadays, with the proliferation of aircon units and indoor fans there are more ways than ever for airborne bacteria to be lifted up and spread around an indoor area.
A recent paper in PLOS ONE looks at how bacteria enter these indoor environments. There are various different routes they could potentially take: they could breeze in through open windows and doors, be actively carried on human skin or clothes, or be introduced through breathing, sneezing, or coughing in an enclosed space. The researchers chose a university classroom as their enclosed space, and performed quantitative analysis of the indoor bacteria, including those found on the floor, suspended in the air, and found in the aircon and ventilation units. These were then compared to phylogenetic libraries of bacteria found in the human skin microbiome, outdoor aerial bacteria, and indoor floor dust to see where the indoor bacteria were coming from.
The graph above shows the total particle mass found in the ventilation duct (DUCT), outdoor air (OUT) and indoor air (IN) during periods where the room was either occupied or vacant. Values are show as either PM10 (mass of particulate matter in particles 10 m in aerodynamic diameter or less) or PM2.5 (mass of particulate matter in particles 2.5 m in aerodynamic diameter or less). What this shows is that when the room is occupied, particulate matter, particularly larger particles (such as dust or dirt) are far more likely to be filling the room.
To further explore the effect of room occupancy on the effect of indoor particles they compared three situations - one where a single person walked in over the carpet, one where a single person walked in over a plastic sheeting covering the carpet (to prevent re-suspension of floor particles) and one where 30 people walked in over the plastic sheeting. The table below shows that the carpet was the major source for re-suspended large particles in the air, although with enough people large particles were still found floating around (unfortunately the experiment with 30 people was only carried out once, so there are no error bars).
Rather than looking simply at suspended particles and particle size, the researchers then looked at the actual bacterial content of these particles. The resuspended floor dust was, perhaps surprisingly, found to contain the most bacteria, compared with the ventilation duct, indoor air and outdoor air. As dust is formed from bits of dried human skin, it seems that the human skin microbiome may be most responsible for the propagation of indoor bacteria, and the effect is exacerbated the more people are inside and stirring up the dust.
What is interesting is that this suggests humans play two important roles in propagating indoor bacteria. Firstly, they are a major source of introducing it, through skin, hair, and other components of dust (further analysis in the paper shows the actually species of bacteria found in the dust, which correspond to those on human skin). Secondly by regularly occupying a house they stir the dust up, keeping the particles moving around in the air. If you want to reduce the amount of airborne bacteria, it isn't just the ventilation system that needs cleaning, it's wise to make sure the floor is swept as well!
Reference 1: Hospodsky D, Qian J, Nazaroff WW, Yamamoto N, Bibby K, et al. (2012) Human Occupancy as a Source of Indoor Airborne Bacteria. PLoS ONE 7(4): e34867. doi:10.1371/journal.pone.0034867
Credit link for featured image: lecture hall