The bugs we breathe

kw: bacteria, air quality

According to Jessica Green of BioBE (Biology and the Built Environment) at University of Oregon, a cubic meter of indoor air can contain up to 10 million bacteria. Other sources state "one million or more" or "a few million" microbes per cubic meter. This has a few interesting implications.

A cubic meter is 1,000 liters. A healthy adult has a lung capacity of about 4 (female) or 6 (male) liters, but we typically "process" about half a liter per breath, and take some 15 breaths per minute when resting. Thus it takes about 133 minutes, or more than two hours, to inhale (and exhale) a cubic meter of air. In that time, between one and 10 million bacterial cells get in, but few return except when we sneeze. How much mass is that?

Bacteria differ greatly in size. A few examples:

• E. coli: 0.6-0.7 fL (femtoliters)
• Staphylococcus: ~0.11 fL
• Streptococcus: ~0.14 fL
• Salmonella and Vibrio: ~2 fL

If there is a lot of Salmonella or Vibrio in the air, you're in trouble, so for the more usual microbes, we can expect an average volume of perhaps 0.3 fL. A femtoliter is one cubic micrometer, so it takes one billion fL to equal one cubic millimeter, and one trillion to equal one cubic centimeter (cc). Thus a million times 0.3 fL is 300,000/1 billion or 0.0003 cubic mm or 0.0000003 cc. More heavily occupied air, with 10 million/cu m, would then have 0.003 cubic mm or 0.000003 cc. One cc of water weighs a gram, and the specific gravity of bacterial cells is only a little greater than 1, so a cc of bacteria would weigh 1.0-1.1g.

OK, so in 2¼ hours of quiet breathing you take in between 0.3 and 3 micrograms of bacterial cells. In a day you take in ten times that. So those millions of bacteria don't amount to much, and our body pretty easily deals with them. Usually.

This doesn't take any account of viruses. They probably outnumber the bacteria ten to one; I haven't found any useful estimates. However, their mass would be much smaller.

Houses, at least in the suburbs, are a lot cleaner than they used to be. I remember as a child that when sunlight came through a window, we'd see a pretty strong sunbeam from the dust in the air. In recent years, I am more likely to see a few scattered dust motes, but no beam. Bacteria are too small to reflect enough light to see, even in full sunlight shining through an otherwise darkened room. I was wondering, could a simple filtration test determine what is in the air?

We'd want to separate larger particles (skin flakes, fibers shed form our clothes, and mineral dust) from bacteria, so it would take two filters, one with pores in the range of 10 microns, the other with submicron pores. With the two filters fitted into an appropriate fixture, one could then drag a cubic meter of air through them. Supposing each filter is a few cm across, this could take a while. But then you could examine the filters under a microscope.

Let's see, if each filter's area is 10 cm², the finer one ought to contain between 100,000 and 1 million cells per cm² or from 1,000 to 10,000 per mm². On average, the cells would be from 10-30 microns apart. Since they are in the range of a micron in size, it ought to be pretty easy to see them, right on the filter, with a SEM or even an optical microscope. Optical searching would be easier, though, if you used a stain that doesn't affect the filter paper.

If you were to wash them off and centrifuge the wash liquid, you'd have a mass of cells that would be easy to plate onto a slide and look at, whether stained or not. 1000X, even 400X, is enough to spot them. You could get an assay by weighing the dried residue. It just takes a scale that has good precision when weighing 0.3-3 micrograms! The cheapest one I can find online is a bit over $20,000. An optical microscope capable of looking at the filter paper at 1000X is less than $1,000.

Ten million, even one million, "germs" per cubic meter sounded like a lot when I first began to research this. Reduced to actual mass, though, it doesn't amount to much.