Bronchiolar aerosol deposition and clearance.

We have often, and over a long time, thought of the "small airways" as a mystery zone or a quiet zone [1]. Relative to central bronchial generations, the total crosssectional area of each bronchiolar generation is large. Airflow resistance should in consequence be low. Considerable changes due to disease could, thus, occur with little chance of being detected by conventional lung function testing. This is the basic idea of a "quiet zone" at risk of insidious damage. The concept was strengthened by the finding that early damage from cigarette smoking occurs within these airways [2]. However, even for the normal lung, there is morphological evidence that internal bronchiolar diameters appear to act as a major determinant of ventilatory function [3]. Conventional spirometry supposedly reflects "large airways" function, but is significantly related to specific pathological scores of small airways disease in patients with minimal airways obstruction [4]. Sustained exploration of "small airways tests" has suggested that several such tests can respond sensitively to bronchiolar inflammation [5]. Sadly, these tests generally lack predictive ability. In particular, they cannot foretell which smokers will develop overt disease [5]. Considerable attention has been paid to the small airways in asthma. Even when free from symptoms, asthmatics may well have persisting evidence of a small airways abnormality, at least partially reversible by bronchodilator administration [6]. A severe attack is likely to be accompanied by highly significant bronchiolar abnormalities including inflammation and lumen occlusion [7]. A different focus of attention on the small airways has recently come from the radiation safety field. Driven by concerns over risks from long-lived radioactive particulates retained in workers' lungs, a new respiratory tract model from the International Commission on Radiological Protection (ICRP) explicitly treats the bronchi and the bronchioles as separate zones [8]. These two zones are held to differ not only in their deposition and clearance characteristics but also in terms of the cellular sites considered to be crucially radiation-sensitive. New methods of investigating the small airways are, therefore, to be welcomed. In this issue of the Journal, SVARTENGREN et al. [9] describe their evaluation of small airways deposition and clearance. The findings and approach of this study offer a chance to reconsider the "mystery zone" status of the small airways and to look again at their defence role in the lungs. Deposition by very slow inhalation