Pets Tracking Mold Into the House?

Probably. Our expert tells you whether it's dangerous

by Thad Godish, Ph.D

Q. What is your professional opinion on pets being carriers/ transporters of mold spores to the inside of the home? Patricia, Georgia

A. There is, of course, no direct scientific evidence to confirm that pets can and do bring mold spores into homes. However, it is quite likely since pets like humans track dirt into a house on their paws (our shoes and feet).

Soil contains large quantities of mold (because the organic matter present is a good source of food). The more organic matter (leaf/grass debris, etc.) the more mold will be present (It is mold's job to decompose dead plant and animal matter).

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Outdoor air has during the summer and especially the fall very high levels of mold spores. These spores land on surfaces including people and pets who/which can bring them indoors where they are incorporated into house dust. They can become airborne when such house dust is disturbed. Of course mold spores come into houses through open windows affecting dust mold levels and airborne mold numbers for months after entry.

Indirect evidence for pet transport of mold/mold spores comes from studies of lead-based paint and childhood lead exposures. Pets and children are known to carry lead contaminated soil indoors from contaminated soil around building exteriors and thus cause increased indoor lead levels as well as exposures to children.

The question that I think you have implied is "Do pets carry mold into our houses and if so, how significant a problem/concern is it?"

I have answered the first part of your question. Now here is the second. In the average house it is very doubtful that pets significantly increase indoor mold levels as a consequence of their going outdoors and then coming back in (certainly not more so than their masters).

However, it has been my experience that very active pets can cause mold spores in house dust to become re-suspended and produce significant short-term increases in airborne mold levels (I saw this once with two large dogs which in their excitement in a very small space caused a significant spike in airborne mold levels.

Previously I made reference to the fact that people can bring mold in from the outdoors and thus cause increased airborne mold levels. I have seen significant increases in airborne mold in school classrooms during occupied hours as compared to those sampled after hours. It is likely that a good part of the increased levels during occupied hours is due to student activity. It is also likely that some of it is due to the fact that students are bringing mold in. In some classrooms I have actually smelled mold on some students' coats or jackets.

Indoor Environmental Quality (2004), Thad Godish Ph.D., C.I.H

First Published: January 2005
Updated: February 2007

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