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3bO™ Masks– in sizes, that fit all…

Put the credit card  under your nose vertically.
If the bottom of the card…

Size S: …exceeds the chin by more than 1 cm
Size M: …exceeds the chin by 1 cm
Size L: …meets the bottom of your chin
Size XL: …ends shorter than your chin

Incase of doubt, choose the bigger size.