Miners called it Blackdamp, maybe we should all buy canaries!

How about the actual type and thickness of the plastic container and it's gas permeability? Here's a chart I found about the properties of plastics.

The following chart compares characteristics for the most common types of resins used for plastic packaging. However, most data refers to tests done on sheets of resin (film) rather than rigid plastic bottles. For best results, Alpha Packaging recommends that our customers always test their product in actual bottles to ensure compatibility.

*MVTR stands for Moisture Vapor Transmission Rate in g-mil/100in. 2/24hr. MVTR is a measure of the passage of gaseous H₂O through a barrier. The lower the rate, the longer the package protects its contents from moisture and ensures the moisture content of the product remains the same.
**O₂ and CO₂ stand for Oxygen Transmission Rate (OTR) and Carbon Dioxide Transmission Rate (COTR) in cm3-mil/m2/24hr. OTR and COTR are measures of the amount of gas that passes through a substance over a given period. The lower the readings, the more resistant the plastic is to letting gasses through.

FYI......My fig propagation experiments have included needle injection of pure Carbon Dioxide and Oxygen INTO Baggies and INTO rooting containers. I never got around to injecting pure Nitrogen. Will not be sharing all of my results on these experiments as I intend to publish some of my work one day. 

Dan
Semper Fi-cus

Dan I'd bet the O2 would have the effect of retarding mold growth.  I can imagine that the N would be pretty inert as figs don't have the structures necessary to convert gaseous N to a useful form (and wouldn't require much in the way of N til green growth resumed, which one wouldn't want until rooting had occurred)

Just my thoughts on the subject....

~Chills

........you got it Chills.

Dan
Semper Fi-cus

Dan, being no scientist I just do things from little bits of info that I gather at random from here and there. So my idea to make "air cups" was a sound idea and might help attribute to my success in rooting cuttings. My idea of using upside down cups have proven to be a plus because of the increased rooting area in the bottom of the inverted cup and the easy of transferring to a larger pot without disturbing the roots. But with what you're saying about trapped CO2, perhaps unknowingly, the best part of my idea might be the numerous holes around the cup to help move oxygen in and CO2 out. Below are three pictures of such a air cup with the resulting roots. As you can see by the date of 3/15/11 on the cup, it's only been 14 days since I put in the cup with only root initials showing. These aren't the only ones but the most photogenic of my rooting trees. The first two are of both sides of the same cup.

Thank you Dan for sharing all of the things you have with us.
"gene"

Gene,

Some rooters are under the misunderstanding that holes in rooting cups and placing them on wire racks are only needed for excess moisture drainage. If you try to root in "tall" cups or a tall containers and don't have some ventilation holes on the sides too, it can take a long long time before you see some roots develop. You just have to get some air in the rooting zone and have a way for the carbon dioxide that is created as the roots are forming ......to get out of there. People need to be aware that the acid gas carbon dioxide can be causing you to have some rooting problems. That's a good idea you have there Gene. 

Dan
Semper Fi-cus