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High-level disinfectants such as ortho-phthalaldehyde (OPA) have increasingly been linked to various adverse health outcomes including occupational asthma, contact dermatitis and anaphylaxis. However, environmental exposure characterisation studies of aldehydes such as OPA, are limited. The aim of this study was to conduct a detailed exposure characterisation study of health care workers (HCWs) exposed to aldehydes.A total of 269 full-shift passive personal samples were collected from HCWs randomly selected from 17 different clinical departments in a tertiary hospital. Passive sampling used TraceAir AT580 monitors (Assay Technology, Livermore, CA), which were analysed for OPA and formaldehyde.OPA was detectable in 6 (2%) samples from gastrointestinal (GI) unit, with a median of 0.009 ppm (range: 0.005–0.027). HCWs with detectable OPA levels had a longer duration of OPA use (OR=1.28; 95% CI: 1.10 to 1.50). Formaldehyde was detectable in 103 (38%) samples with a median of 0.004 ppm (range: 0.003–0.027). Three (1%) samples had formaldehyde levels higher than the NIOSH recommended exposure limit (REL) of 0.016 ppm time-weighted average.In this study, OPA levels were on average 10-fold higher than in similar settings elsewhere. Detectable OPA in the GI unit corroborates findings of workplace inspections conducted during the exposure measurements, which found high level of OPA usage among sterilising operators and registered nurses. Formaldehyde levels were on average 10-fold lower than studies in pathology and anatomical laboratories elsewhere but were comparable to average levels in US general buildings. The most likely source of exposure is probably related to exposure to formaldehyde (10%) solution used for specimen preparation in most departments, residue evaporation from formaldehyde contaminated surfaces and other general indoor sources. The study concluded that mean detectable exposures to OPA are higher and more isolated than more widespread low-level formaldehyde exposures.