May interject something I learned when I was taking courses to get my SCUBA C-card. if you take longer breaths, pause briefly before exhaling and take your time to exhale, you will get will maintain a higher blood O2 level, and will take in less air (good for urban riding, a good scuba diver can make a tank last well over an hour and a half, I was only at 45 minutes last time I did a dive, that was about 5 or 6 years ago)