Erratum
After publication of the article [1] it was brought to our attention that resistance values of Breezahler (0.15 cmH2O/L/min) and Handihaler (0.22 cmH2O/L/min) are incorrect. (page 4 right column, lines 14 and 16 from below). The correct resistance values of Breezhaler and Handihaler are 0.07 and 0.16 cmH2O/L.min-1, respectively. We apologise for the inaccuracy.
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