What is the respiratory retention of inhaled hexamethylene di-isocyanate?

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 di-isocyanate (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.