In a 2006 study, it was found that women with PMS experience a much greater decline in blood carbon dioxide in the premenstrual phase than women who do not experience symptoms. As progesterone increases and blood Carbon Dioxide (CO2) decreases, symptoms appear. When the luteal phase ends, progesterone decreases, CO2 levels normalise, and symptoms disappear. The study concluded that women with PMS have higher than normal sensitivity to CO2, perhaps caused by progesterone, resulting in ‘pronounced hyperventilation’ - and that the hyperventilation is responsible for the symptoms. The authors commented that because the breath can be easily manipulated using a variety of techniques, including respiratory biofeedback training, it would be promising to explore this phenomenon in more detail in order to understand the mechanisms underlying PMS, and to find more effective ways to treat it at least for some women.