The methacholine challenges performed with the Wright and Bennett-Twin nebulizers will follow the two-minute tidal breathing protocol as outlined in the 1999 American Thoracic Society Guidelines for Methacholine Challenge Testing. The testing protocol used with the Aeroneb solo will be identical save for a 1 min inhalation period instead of 2 min. Each challenge will be stopped when a participant's forced expiratory volume in 1 second (FEV1) drops at least 20%. This will allow for the determination of the participant's PC20 (provocative concentration of methacholine causing a 20% fall in FEV1), which will subsequently be converted to a PD20 (provocative dose) value. A minimum 24-hour washout period between challenges will be enforced.