Breakdown of study procedure: Participants will undergo three (potentially four) methacholine challenges, one with each nebulizer in randomized order. An additional methacholine challenge may be performed for screening purposes if the participant has not undergone methacholine challenge testing in one of the participating labs in the previous 6 months. The screening challenge will be performed with the standard nebulizer used at the given testing site (i.e. the Wright or the Bennett-Twin).