The results "are strong testimony for the efficiency of the train pumping system for ventilation," principal investigator Norman R. Pace of the University of Colorado, Boulder, said in a statement. "I was impressed by the similarity of subway and outdoor air."
The wind one feels while walking across a subway grate in New York as the subway clatters beneath also demonstrates just how effective that system is, Pace said.
The only obvious differences in the subway's microbial population are the somewhat higher proportion of skin microbiota, and the doubled density of the fungal population, which Pace suggested might be due to rotting wood.
The researchers used a high-tech mechanism to collect air at around 300 liters per minute, a big jump on the previous state of the art, which swallowed 12 liters per minute. That enabled collecting sufficient volume of air to take the bacterial census within 20 minutes, instead of hours, Pace said.
Pace said the microbial content of subway air was unknown before now.
The findings were published in the journal Applied and Environmental Microbiology.