(1) "Normal" bronchial reactivity is demonstrated by an insignificant (less than twenty percent) fall from baseline FEV1 at test doses of histamine or methacholine, up to 16 mg/ml (continuous inhalation method) or up to 160 breath units (cumulative, repeated deep breath technique).

(2) "Mild" bronchial hyperactivity (BHR) is a significant (equal to or greater than twenty percent) fall in the FEV1 at test doses of 2.1-16 mg/ml, or 21-160 breath units.

(3) "Moderate" BHR is a significant (equal to or greater than twenty percent) fall in the FEV1 at test doses of 0.26-2 mg/ml, or 2.6-20 breath units.

(4) "Marked" BHR is a significant (equal to or greater than twenty percent) fall in the FEV1 at test doses equal to or less than .25 mg/ml, or 2.5 breath units.