Isocyanates are a frequent cause of occupational asthma. Specific inhalation challenges are often required to confirm the diagnosis. The inhaled concentration has to be assessed during this procedure. However, the respiratory retention of di-isocyanate has not, to the authors knowledge, been evaluated in humans.The existence of a closed-circuit apparatus, designed to carry out these challenges, makes it possible to assess the dose. The respiratory retention of hexamethylene diisocyanate (HDI) generated in vapour form, in both normal subjects (n=4) and subjects referred for the investigation of occupational asthma due to HDI in whom the diagnosis was excluded (n=5), was assessed. The latter group included four subjects with nonoccupational asthma.The HDI was generated at concentrations varying 5.1±15.2 ppb. The expired concentrations of HDI during such challenges varied 1.4±5.3 ppb. Therefore, the respiratory retention was 61±90%.To conclude, the majority of inhaled hexamethylene di-isocyanate vapour is retained within the airways and/on lung parenchyma. Eur Respir J 2000; 16: 729±730. Isocyanates are a common cause of occupational asthma (OA) [1]. Specific inhalation challenges with the causal agent are used to confirm the diagnosis in specialized centres [2]. For these tests, the inhaled concentration has to remain stable and below the threshold-limit-value (TLV) for a short-term exposure, that is <20 ppb in the case of hexamethylene di-isocyanate (HDI) [3]. However, the key factor that determines an asthmatic reaction is more likely to be the respiratory retention than the concentration. As far as we know, the proportion of inhaled HDI that is retained has not been determined.Therefore, an assessment was carried out of the dose of inhaled HDI using an apparatus that has been described previously [2]. This apparatus allows on-line information to be obtained about the inhaled and exhaled concentrations, thus enabling the respiratory retention to be assessed.
Material and methodsThe apparatus used for inhalation challenge procedures with HDI has been described [2]. Briefly, it includes three components: a generator of isocyanates, an exposure chamber and instruments for on-line assessment of concentration. The subject inhales isocyanate vapours at spontaneous tidal volume breathing through a mouthpiece connected to the central part of a plexiglass cylinder covered with Teflon, which is, in effect, the exposure chamber. Isocyanate concentrations were assessed with a GMD instrument (model 720-120; GMD Systems Inc., Pittsburgh, PA, USA) by connecting the port to the inhalation tube. The total length of the 2-cm-diameter tube, connecting the 9 exposure chamber to the mouth, was 26 cm. The inspiratory port was located at a distance of 12 cm from the subject's mouth while the expiratory port was 2 cm away from the mouth. Use of unidirectional valves (n=4) avoided contamination of the inspired and expired air samples. Assessments of concentrations were carried out in three phases: 1) The stability of the inspi...