Despite its demonstrated efficacy in treating selected patients with acute respiratory failure, the application of NPPV is still plagued by a substantial failure rate, ranging from below 10% to above 40%. Mask intolerance ranks as one of the most common causes of failure. This is related to ill-fitting masks, mask discomfort (despite a properly fitted mask), excessively tightened straps, excessive air leaks around the mask or through the mouth, and claustrophobia.