I remember when I was a kid seeing a commercial about a new type of storage bag.You stuffed your clothes into it, attached a vacuum hose, then sucked out the air from the bag. The bag would crumple down and save space on your stored items.
When you needed to remove those items from storage, they would come out as clean as when you put them in. Unless you stored damp clothes, then you’d have mildew problems.
Proper storage of your cannabis is just as critical to keeping it fresh, but there are more dangerous consequences to damp cannabis than clothes getting musty.
Cannabis retains a significant amount of water in its flowers when it’s harvested. While growers go through the process of drying and curing their flowers, there is a danger looming that human perception cannot discern.
When plants are harvested for distribution, a cursory physical examination might not show signs of bacterial or fungal growth. But, incomplete curing and leftover water content can encourage growth later on.
Recognizing this problem, many places require a test beyond the physical examination; they measure the water activity (AW). This is different from a measure of moisture content. The AW provides us insight into how much of the moisture content is accessible to microbial growth.
On a scale of 0-1, an AW score of 0.6 or lower indicates a low likelihood of microbial growth to occur. Slightly above that, there is chance of slow microbial growth over time.
According to a study done by W.D. Grant, most pathogens need an AW score of 0.9 or above for viable reproduction conditions. 
Because cannabis is routinely dried, cured, and then rapidly heated before consumption, it’s difficult for most microbes to survive this environment on cannabis flowers. However, Salmonella, Aspergillus, and Clostridium botulium are all capable of successfully reproducing on cannabis flowers. 
What Holmes and co-authors determined was that any cannabis sample with a AW ≥ 0.65 should be returned to the distributor and they recommended that growers aim for AW ≤ 0.6. Otherwise, your cannabis might look like how a wet shirt would if you placed it in a shrinkable bag for five months.
- Grant, W. D. “Life at low water activity.” Trans. R. Soc. B, Biol. Sci,2004, 359, 1249-67.
- Holmes, et al., “Microbiological Safety Testing of Cannabis.” Cannabis Safety Institute, 2015.