Dude just look up photosynthesis, ok? co2 is not used the same way as oxygen for plants. This is not just for weed, but for ALL plants. As far as the potency goes, no expensive tests, they just used old fashioned smoking it as a gauge of potency. Here are a couple excerpts just from wikipedia on photosynthesis -
"photosynthesis uses carbon dioxide and
water, releasing
oxygen as a waste product"
"Carbon dioxide is converted into sugars in a process called
carbon fixation. Carbon fixation is a
redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert carbon dioxide into
carbohydrate, which is a
reduction reaction. In general outline, photosynthesis is the opposite of
cellular respiration, where glucose and other compounds are oxidized to produce carbon dioxide, water, and release chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments."