The test is simple, non-invasive, and is performed after a short period of fasting (typically 8–12 hours). Hydrogen breath tests are based on the fact that there is no source for hydrogen gas in humans other than bacterial metabolism of carbohydrates. Even though the test is normally known as a "hydrogen" breath test, some physicians may also test for methane in addition to hydrogen. Many studies have shown that some patients (approximately 35% or more) do not produce hydrogen but actually produce methane. Some patients produce a combination of the two gases. Other patients, who are known as "non-responders", don't produce any gas; it has not yet been determined whether they may actually produce another gas. In addition to hydrogen and methane, some facilities also utilize carbon dioxide (CO2) in the patient's breath to determine if the breath samples that are being analyzed are contaminated (either with room air or bronchial dead space air).