 This is an open label, investigator-sponsored, pilot study. Subjects agreeing to participate in the study and meeting the eligibility criteria assessed at the screening visit will be enrolled in the study. The length of time between screening and treatment will last between 0 days to a maximum of 12 weeks. Subjects who enter the screening phase during a cluster headache episode and meet the study eligibility criteria can immediately enter the treatment phase and may opt to treat their cluster headache episode in the clinic. Subjects who are not in a cluster headache episode, who meet initial screening eligibility criteria, can remain in the screening phase for up to 12 weeks until their next cluster episode begins. Upon initiation of a cluster headache episode, subjects will enter the treatment period. Subjects will be trained on the proper use of the hand-held dispenser containing carbon dioxide CO2 calibrated to deliver 0.5 standard liters per minute SLPM. This dispenser will be provided for use in the clinic or home. Subjects will be instructed to use the nasal CO2 dispenser, 10 secondsnostril, as needed up to 6 times to treat one attack. Each dose must be separated by 3-5 minutes. Subjects should treat only one attack in a 24-hour period. Subjects may treat up to three cluster headache attacks during the treatment phase of this study. One hour after the first dose, subjects can choose to treat with investigator-approved rescue medication. Subjects will be asked to complete an online diary after the completion of the dosing. Diary assessments will collect pain severity, nasal CO2 usage, acute medication usage, satisfaction of treatment, number of cluster attacks, and unusual symptoms. Subjects will be contacted by phone within 3 days of the first use of the nasal CO2 dispenser to assess adverse events AEs and medication usage. A total of 25 subjects will enter the treatment period and be instructed to treat up to 3 cluster headaches with nasal CO2. Within 7 days of treating their last cluster headache episode, subjects will return for an end of study visit. 