I saw your response to a question about black particle deposition, and have one of my own. I do some mold inspections and was called into an apartment with electric baseboard heat to determine if there is a mold problem. The pattern of blackening on the walls, above the heaters, around furniture, at wall studs and around plastic fixtures looked like soot. The owner said the tenant smoked and he thought she probably burned candles, but didn't know. She was complaining of respiratory problems.

     She had taken her own swabs of a dirty ceiling fan and sent it to a lab, which counted 320,000 total CFUs, mostly yeast and Penicillium, with  a little Mucor. The owner took a tape-lift and sent it to another lab, which found no spores and thought it looked like soot.

     I went in and took tape-lifts AND swabs (I used aseptic technique and took the tape-lift from the same blackened area and took another on a wall onside a closet to serve as a sort of control).

    I was not surprised that the tape-lifts (I took duplicates so I could look
myself and send it to EM Lab) came out negative for mold spores and appeared to have tiny particles, smaller than 1 micrometer diameter, many clumped together randomly.

    But what I do not understand is the result of the swab culture. The wall with the heaviest black deposit had 12,000 Aureobasidium CFU's per square inch, plus 9,600 yeast /in² and the closet  wall had 1800 Penicillium CFU /in².I did not expect to see so much mold in either location. I guess a sporangium could have floated in and been picked up by the swab and then burst, releasing its thousands of spores into the dilution water?

    Or, maybe black particle deposition can be a contributing factor to indoor mold growth? If the soot is a carbon source, could mold grow in it? The place smells a little musty but there is no obvious moisture or visible mold growth (even with a microscope). Have you ever heard of anything like this before? Do you have any suggestions? Sara, New York

 

    The fact that the interior smells musty indicates that a mold problem is present somewhere in the building. I have seen a number of buildings with no obvious mold growth on surfaces bur nevertheless had significant airborne mold concentrations and sometimes a musty smell. These notably were in such less obvious places such as crawlspaces and wall cavities behind brick veneer wall cladding. 

    Mold spores are particles and behave as such. They are subject to similar forces as other particles such as soot carbon (albeit the latter are much smaller and do behave somewhat differently. Soot particles can settle out due to gravitational forces , diffuse to surfaces randomly, or diffuse to surfaces along thermal gradients (thermophoresis). It is the last process responsible for the soot particle deposition patterns you report. It is possible that to a limited extent that small mold spores can also be deposited onto surfaces by thermophoresis. Aureobasidium and yeast have similar thin-walled spores with likely lower mass densities than other mold spores. These properties may increase the likelihood of non-gravitational deposition such as diffusion and thermophoresis. 

    There may be other factors involved as well. My experience with Aureobasidium is that it often grows on surfaces subject to condensation (shower areas, insulated pipes). See the following images which show Aureobasidium growing on the ceiling of a shower area. 

The areas of soot/particle deposition by thermophoresis are typically cooler and more prone to condensation. Carbon particles are moderately

hydrophilic and thus can concentrate moisture. Though hydrophilic, carbon particles can also readily give up or release water vapor. Given these facts your observations make sense as far as finding Aureobasidium and yeast present and maybe even Penicillium.

    Though the thermoporetic deposition may be dominated by dark carbon particles there is likely to be many other particle types present including those that contain organic carbon. Such particles would be a much better carbon source for mold growth than the inorganic carbon of soot. 

June 18, 2004

Indoor Environmental Quality (2000), Thad Godish Ph.D., C.I.H

Direct E-mail 00tjgodish@bsu.edu