The sizing guide is a guideline only and it is recommended that you email your Biometric data to have your mask size confirmed. Under the Consumer Contracts Regulations June 2014, a mask is considered as ‘personal’ item and for hygiene reasons Respro are unable to exchange any mask that has been removed from its sealed packaging and fit attempted. This poses a contamination threat and subsequent product liability issue. If however, Respro have advised you of the size to purchase and if it is subsequently deemed the wrong size then Respro will exchange it for the correct size free of any charge. Respro does not offer this service if a mask is purchased from a third party supplier.