What is happening in the lungs? During breath-holding and breathing less, the levels of gases made by the body will increase and oxygen levels will decrease. Recent discoveries show that carbon dioxide is not the only gas in exhaled breath, there is nitric oxide and organic gases such as pentane, ethane and propane. Nitric oxide seems to be very important in regulating the respiratory system. It may be that many elements are important in determining how long you can comfortably hold a breath - not only carbon dioxide.