Winter has arrived.  It’s cold outside.  Crank up the heat.  Warm yourself over the heat register and fill your lungs with a hundred thousand mold spores, that is, if mold is growing in your air ducts.  If not, get nice and toasty and enjoy.  But if your eyes start to itch or your nose starts to run and your asthma acts up, it’s time to grab a blanket.  You may have mold in your air ducts.  Most people believe that mold is a summer phenomenon.  That is only half of the truth.

It’s actually the summer air conditioning that kicks off the molds’ lifecycle in ductwork.  Mold spores germinate in pockets of condensing air. Moisture saturated dust and food debris (ever lose a French fry down the air vent in your kitchen? How about candy corn?) become the molds’ food source.  Spores present in the air attach to dust and food particles, germinate and form colonies.  A one inch diameter colony contains well over a million spores and depending on the number of cold-pocket anomalies in your air ducts, millions to billions of spores may be present.

Air duct mold exposure is often acute in the winter, especially early winter when we first switch on the heat.  That’s because air ducts contaminated with mold colonies are often wet with condensation from the summer AC, which retards spore release.  Moreover, molds grow vegetatively in the summer, converting dust and food particles into biomass.  The more biomass a mold builds the greater number of spores it can potentially produce.   In some respects, a mold’s life cycle is similar to a sea turtle’s cycle.    Perhaps one baby turtle out of a clutch of fifty eggs survives to adulthood.  A lot of turtles have to lay many eggs for the species to be successful.  Likewise, only three or four spores, out of millions, will grow into new mold colonies.  A mold’s lifecycle, like sea turtles, relies on sheer numbers to propagate the species.

We all know that mold needs moisture to grow.  Take away the moisture and the colony stops expanding.  When a mold colony detects a decreasing moisture gradient, for example, in the fall when we shut off the AC and open the window, it restricts vegetative growth and switches to spore production.  Jump ahead one month—the temperature drops, and we crank up the heat.  The mold colonies are now dormant.  But a massive amount of spores are still present.  They’re waiting for the hot air to dry out their stalks, releasing them from the mother colony like balloons in the wind.  That’s why early winter is problematic for air duct mold.  Every time we turn on the heat, a billion spores are pushed out of the air vents into our homes or offices.  Eventually most of the spores are released and the concentration of spores exiting the air duct drops significantly, that is, until summer when the whole cycle begins anew.