I think we are in danger of leaving the OP behind. CO2 retention is an important issue and another reason to use more helium. However, air doesn't have much CO2 in it - the CO2 is excreted by the body as a product of respiration. If the CO2 is exhaled then it is heading up the water column to the atmosphere. A small amount is re-inhaled as it is in the 2nd stage as the gas changes direction. The so called "dead space" of the regulator.
There's a big thread on it here:
http://www.scubaboard.com/forums/basic-scuba-discussions/224684-co2-retention.html
Dear Chrisch,
you are missing my point. You are absolutely correct that air does not contain much CO2. However, as a byproduct of respiration and oxygen metabolism, we are producing CO2.
When you are working hard in the water (swimming or breathing) and at increasing depths, you will produce more CO2 because of increased gas density, scuba equipment, fitness level, etc, ..you may not be able to expire all the CO2 you produced. That is how people get CO2 hits. Also, some people may be less sensitive to rising levels of CO2 and "naturally" retain. I have a whole lecture on this and I cannot explain it all in this forum.
However, to state that CO2 is not narcotic is incorrect. Moreover, CO2 plus N2 is synergistically narcotic! Although the use of Helium in recreational depths is usually less warranted, it may play a role however with increased work-loads as it decreases gas density. For comparison: air has a density of 1.2 vs He of 0.16. Now obviously you would not be breathing pure He. But you can do your own calculations for a He mix and compare it to Air.
So He serves two rather than just one major purpose regarding "clear heads":
1. It decreases Nitrogen narcosis by decreasing nitrogen in the mix
2. It improves work of breathing thus reducing CO2 production
Your dead space in your regulator is a minor player for OC. However a malfunctioning mushroom valve in CCR is quite a different story. But that is a different discussion all together.
Regards,