Gather around and think back to high school chemistry. Remember Graham's Law of Diffusion? Let's put on our thinking caps...

Picture a big room with a bunch of people of it, a party of sorts. It's mostly empty space, but given our personal space issues it seems quite full. Most people are wearing brown shirts (about 78%), some wear blue (20%), very few (0.03%) wear red, and a few other colors are represented in very small percentages¹. All of these people are moving in random directions at random speeds. Every so often they will collide; when they bump into each other, repulsive forces cause them to move in a new, random direction (as is the behavior of an ideal gas).

This is the air we breathe. More importantly, this is the air in which mushrooms grow.

Now, the door opens up and more people wearing red start entering the room. These people represent the addition of carbon dioxide gas, a byproduct from the respiration of the mushrooms. They all enter the room from the same location (the substrate), and they behave the same as the rest of our gaseous partygoers--they move at a random speed bouncing off others in random directions.

They do not instantly mix with the people that are already at this party. The rate at which a gas molecule intermingles with the rest of the gas in the container is inversely proportional to the square root of its molar mass (Graham's Law). Or, simply, you can say the speed a person can move through this crowd depends on his/her mass: lighter people cover more distance than heavier people in the same period of time. Most of the molecules at this party are N₂ and O₂. These party crashers that we need to keep our eyes on, the CO₂, have a greater molar mass than the N₂ and O₂ (44amu compared to 28 and 32). Eventually, they will diffuse out into the terrarium so that any part of the terrarium will contain an equal and homogenous mix of gas, but the continual biological production of CO₂ will result in a localized higher concentration of CO₂ in the air directly around the substrate.

But wait, some of you are saying, what about gravity? Does the greater mass of CO₂ cause it to settle at the bottom of the terrarium? Although carbon dioxide molecules have a greater mass than oxygen and nitrogen, gravity does not play a role in the behavior of gasses under these circumstances. The force of gravitational attraction is proportional to the masses of two bodies and inversely proportional to the distance between them squared. In terms of your own weight, will your scale register a lower weight on top of the kitchen table than the floor? No, although you have increased the distance between you and the center of the earth, it is such a small change compared to the enormous distance that already exists that no decrease in weight would be measured. The same goes for the tiny differences between the masses of the gas molecules in the terrarium. The kinetic energies (mass x velocity²) of the gas molecules play a much larger role in their diffusion and distribution. In fact, not until about 60 miles up (100km) in the earth's atmosphere do the lighter gasses become more prevalent².

The concentration of CO₂ is important because it part of the biological trigger that causes the fungus to fruit (as well as promote certain contaminants). This is why we fan our terrariums or take care to provide sufficient air exchange and circulation in our grow rooms. Because of the limited rate of diffusion of the carbon dioxide gas and continual metabolic production, we must provide "fresh" air through active means (such as a fan or manual fanning).

Looking at the molecular environment around our mushroom beds in this light should help you understand what actions you should take to promote the most beneficial environmental parameters, as well as put some old myths to rest.

1 This assumes clean, dry air at sea level. Real life samples would include some percentage of H₂O vapor and other strange stuff.

2 http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html