"INTRODUCTION Based upon data from several recently conducted controlled human exposure studies (1){2)(3)(4)(5)@ it has been demonstrated that asthmatic subjects exposed to SO2 respond with an Increase in airways resistance. It has been demonstrated further that when exposure is combined with exercise@ at a light to moderate level@ the magnitude of the SO2 induced Increase in airways resistance is greater. Based upon what is known concerning the asthmatic and within the context of the clinical definition of this disease this effect of SO2@ especially when combined with exercise@ is not unexpected. The current results of controlled human exposure studies in which exercising asthmatic subjects were exposed to SO2 during exercise can be divided into two groups: studies of subjects exposed via a mouthpiece which precludes nasal breathing and thus forces SO2 intake to be exclusively oral (encumbered breathing) or studies of subjects exposed via a facemask or in a chamber which permits oronasal (unencumbered) breathing. Under these circumstances the exercising asthmatic who Is exposed exclusively by mouthpiece (encumbered) represents the most severe or ""worst case"" exposure situation. In the study reported herein this group will be focused on initially. Their airways resistance responses will then be compared to those observed In asthmatics exposed in an unencumbered manner."