Comprehensive comparison of sampling methods for evaluating hexamethylene diisocyanate (HDI) in the air of an automotive collision repair facility

Abstract

Isocyanates are recognized as potent irritants and sensitizing agents. Accurate quantification of their airborne concentrations in occupational settings remains a significant analytical challenge due to their high chemical reactivity and semi-volatile nature, that is, their capacity to exist simultaneously airborne in both vapor and particulate phases. This study complements prior laboratory work by evaluating isocyanate sampling methods in an actual automotive repair facility. It investigates spatial distribution in samplers and assesses whether lab-based simulations yield comparable results to real-world conditions during HDI spray applications, supported by contextual analysis. This evaluation involved the comparison of three filter methods to the reference method—an impinger with a backup glass fibre filter (GFF) and 1,2-methoxyphenylpiperazine (MP) based on MDHS 25— during the application of HDI based polyurethane coatings : (1) Swinnex cassette 13 mm GFF MP (MP-Swin); (2) closed-face cassette 37 mm GFF (end filter and inner walls) MP (MP-37); and (3) denuder and GFF dibutylamine (DBA) (ISO 17334-1 Asset). Using a cascade impactor, the particle-size distribution (MMAD) was determined to be 15 µm. The analysis identified distinct patterns in the distribution of HDI and isocyanurate across the sampling sections. These patterns were similar to those observed in the laboratory study, but consistent with the larger particle size observed in the tested environment. SEM imaging revealed substantial coating droplet accumulation on filters, potentially hindering derivatization efficiency. Of all the methods tested, only the MP-Swin method showed a significant negative bias for HDI (-47%). All filter methods underestimated isocyanurate levels compared to the reference, with a bias ranging from -40% to -59%. Using a low-speed activator reduced biases, suggesting that high reactivity limits isocyanate derivatisation, which leads to underestimation of the measurement. Field results were more variable and partially contradicted laboratory findings, which had shown no significant bias between tested methods and the reference. These findings emphasize the importance of field extraction in airborne polyurethane sampling and caution against relying solely on filter methods when fast-reacting compounds are present. The study underscores the complementary value of field and laboratory comparisons and highlights the influence of sampling device design and chemical reactivity on accurate isocyanate quantification.