| I've often been asked about mold as a factor in underground tunnels and ducts. I know very little about mold, so don't take anything of what I say here as a reliable absolute. Mold can be a problem, particularly for people with athsmatic tendancies. Mold is everywhere - more mold is worse, and there are many different types of mold, some nastier than others. e.g. moldy bread or cheese doesn't bother me, it's just a bit unappetizing. Fatalities directly attributable to mold seem extremely rare. Mold likes wood, cloth, carpeting - all such being fibrous things, so keep such out of tunnels and ducts! As mold is a "living thing", it needs moisture to survive, and so is most common in bathrooms. And yes, mold in tunnels and ducts can easily become airborne when dry (thus light). I caught something in one of the links below suggesting that moving cold air thru ducts/tunnels increases the probability of mold. What one needs to understand from such is "keep your powder dry"! Build tunnels and ducts of impervious materials such as plastics, concrete or metal, and protect exterior openings (the cooling intake tower) so that rain cannot enter. It occurs to me that evaporative coolers, used throughout the arid Southwest, would probably have far greater mold potential than a dry duct or tunnel. Many millions of those are in use, and the world goes on. It does cause me to think about the possible effects of inducing water into the tunnels/ducts by using the wet aspect of an evaporative cooler when using such to help airflow. It also appears that mold in ducts and tunnels is probably treatable, chemically or by cleaning. Perhaps you've noticed a whole new industry of little businesses that want to vacuum your heating/cooling ducts? Might be helpful to talk to one, and hope he/she is beyond just selling their services! Some links useful for more in-depth study: | 
