A breathing therapy system, the system comprising: a quantitative colorimetric detection system (100, 103, 300) comprising: a gas inlet (102, 303) configured to receive at least a portion of a user's exhaled air; and a colorimetric indicator (104, 116) in fluid communication with the gas inlet (102, 303) for measuring a user's end-tidal CO 2 levels based on a color change of the colorimetric indicator (104, 116) resulting from exposure to the user's exhaled air; and one or more processors programmed to: during a first time period: determine the user's end-tidal CO 2 levels based on the color change of the colorimetric indicator (104, 116) resulting from exposure to the user's exhaled air; and determine the user's respiration rate; determine a therapy protocol based on the user's end-tidal CO 2 levels determined during the first time period or based on the user's respiration rate determined during the first time period; and during a second time period: outputting in accordance with the determined therapy protocol a set of visual and/or audio cues from the quantitative colorimetric detection system (100, 103, 300) with instructions for the user to adjust their breathing pattern to coincide with the cues to thereby modify the user's exhaled CO 2 levels; and a display screen (110, 126, 316, 1526) for outputting the set of visual cues and/or an audio device for outputting the set of audio cues. a quantitative colorimetric detection system (100, 103, 300) comprising: a gas inlet (102, 303) configured to receive at least a portion of a user's exhaled air; and a colorimetric indicator (104, 116) in fluid communication with the gas inlet (102, 303) for measuring a user's end-tidal CO 2 levels based on a color change of the colorimetric indicator (104, 116) resulting from exposure to the user's exhaled air; and one or more processors programmed to: during a first time period: determine the user's end-tidal CO 2 levels based on the color change of the colorimetric indicator (104, 116) resulting from exposure to the user's exhaled air; and determine the user's respiration rate; determine a therapy protocol based on the user's end-tidal CO 2 levels determined during the first time period or based on the user's respiration rate determined during the first time period; and during a second time period: outputting in accordance with the determined therapy protocol a set of visual and/or audio cues from the quantitative colorimetric detection system (100, 103, 300) with instructions for the user to adjust their breathing pattern to coincide with the cues to thereby modify the user's exhaled CO 2 levels; and a gas inlet (102, 303) configured to receive at least a portion of a user's exhaled air; and a colorimetric indicator (104, 116) in fluid communication with the gas inlet (102, 303) for measuring a user's end-tidal CO 2 levels based on a color change of the colorimetric indicator (104, 116) resulting from exposure to the user's exhaled air; and one or more processors programmed to: during a first time period: determine the user's end-tidal CO 2 levels based on the color change of the colorimetric indicator (104, 116) resulting from exposure to the user's exhaled air; and determine the user's respiration rate; determine a therapy protocol based on the user's end-tidal CO 2 levels determined during the first time period or based on the user's respiration rate determined during the first time period; and during a second time period: outputting in accordance with the determined therapy protocol a set of visual and/or audio cues from the quantitative colorimetric detection system (100, 103, 300) with instructions for the user to adjust their breathing pattern to coincide with the cues to thereby modify the user's exhaled CO 2 levels; and during a first time period: determine the user's end-tidal CO 2 levels based on the color change of the colorimetric indicator (104, 116) resulting from exposure to the user's exhaled air; and determine the user's respiration rate; determine a therapy protocol based on the user's end-tidal CO 2 levels determined during the first time period or based on the user's respiration rate determined during the first time period; and during a second time period: outputting in accordance with the determined therapy protocol a set of visual and/or audio cues from the quantitative colorimetric detection system (100, 103, 300) with instructions for the user to adjust their breathing pattern to coincide with the cues to thereby modify the user's exhaled CO 2 levels; and a display screen (110, 126, 316, 1526) for outputting the set of visual cues and/or an audio device for outputting the set of audio cues.