Isocyanic acid measurements in Swedish industries and environments

Isocyanic acid, the simplest of all isocyanates with the structural formula HNCO, has quietly become one of the more intriguing characters in modern air chemistry. It is not registered as a commercial substance under REACH and is rarely manufactured deliberately. Instead, it emerges as a by-product whenever nitrogen-containing materials are heated to the point of thermal degradation, which means it can appear almost anywhere that combustion or hot processing takes place. Cigarette smoke, biomass burning, and diesel and gasoline engines are all documented sources, and outdoor levels reported in the literature range from 0.018 to 1.8 micrograms per cubic metre. Yet despite its ubiquity, human health data remain strikingly limited, and until now no European environmental measurements had been published at all.

That gap is what motivated a team of Swedish occupational and environmental medicine researchers to carry out one of the most comprehensive surveys of isocyanic acid ever attempted. Writing in Environmental Science and Pollution Research, Gunilla Runström Eden of Sahlgrenska University Hospital and the University of Gothenburg, together with colleagues in Gothenburg and Örebro, collected 173 air samples across 16 different occupational and environmental settings between June 2020 and September 2021. The study was designed to compare relative concentrations across a wide range of environments rather than to conduct a traditional exposure assessment in a few select workplaces, and the results were benchmarked against the only occupational exposure limit for isocyanic acid in the world, the Swedish limit of 18 micrograms per cubic metre as an 8-hour mean and 36 micrograms per cubic metre over a 5-minute period.

The analytical approach relied on derivatisation during sampling with di-n-butylamine, followed by liquid chromatography mass spectrometry. Most measurements used the Supelco Easysampler denuder, while impingers with a higher flowrate of 1 litre per minute were deployed for very short tasks and for cigarette smoke measurements, where the higher flow better mimicked the act of smoking. Two samplers were coupled in series to catch possible breakthrough, field blanks were collected and used to correct the results, and all airflows were set and checked with digital flow meters before, during and after sampling. The detection limit was 0.006 micrograms per sample and the quantification limit 0.020 micrograms per sample. Personal samplers were placed in the breathing zone of workers wherever the COVID-19 pandemic allowed, but in many workplaces only stationary measurements were possible, with samplers placed within a metre of hypothesised sources in the newly investigated environments.

The highest concentrations were consistently found in industries using urea-based binders at elevated temperatures. At a company producing wooden laminate products, average personal exposure reached 48 micrograms per cubic metre with a maximum of 92 on the first measurement occasion, and repeated measurements in June were lower but still relatively high against the exposure limit, averaging 19 with a maximum of 30. Some stationary measurements were extreme, with the highest exceeding 264 micrograms per cubic metre. At a second building-material company, personal measurements characterised as a worst-case scenario averaged 2 micrograms per cubic metre with a maximum of 30. The team notes that these levels are in the same magnitude as reported 10 to 20 years ago in similar environments, with earlier studies ranging from below 4 up to 573 micrograms per cubic metre, suggesting little has changed in terms of exposure in this sector.

One of the most striking findings concerned mineral wool insulation during its initial heating phase. Glass wool and mineral wool are manufactured with binders containing urea, and Sweden’s large combined heat and power plants use these materials on a massive scale. At a brand-new facility, measurements taken over time as running-in temperatures increased showed emissions rising to a maximum of 62 micrograms per cubic metre, remaining elevated for a long period after start-up. The highest levels occurred at the first temperature rise above 200 degrees Celsius, consistent with earlier observations that isocyanic acid emission begins at 133 degrees, the point where urea starts to melt. With repeated temperature rises, levels stayed high but far below those measured during the first warming. During minor renovation of already insulated heating parts, by contrast, no detectable levels were found. The operations were aware of the risks and imposed personal protective equipment during start-up phases.

Thermal processing of polyurethane produced lower but spatially widespread concentrations. At a very large company with several hundred employees, a hot process involving polyurethane products sat centrally in a production hall of roughly 20,000 square metres. Close to the emission process, both diisocyanates and monoisocyanates could be detected, but further away only isocyanic acid remained detectable, indicating that it is not very reactive and can persist far from its source, in line with an estimated residence time of 8 hours in air. Concentrations were found throughout the building, though levels stayed below the Swedish exposure limit, with average personal exposure of 3 micrograms per cubic metre and a maximum of 5.

In sharp contrast, the newly investigated environments showed generally low or negligible levels. Across five restaurants, three food industries, linoleum welding, diathermy procedures, asphalt work and lawn mowing, concentrations were well below the exposure limit or below detection limits. Only one restaurant, where hamburgers were fried, generated concentrations above the detection limit, with a maximum of 0.6 micrograms per cubic metre, and the team suggests that poorer ventilation there and in home frying environments may have contributed. During linoleum mat welding at 300 to 350 degrees Celsius, a measurement directly in the formed smoke showed 6 micrograms per cubic metre, but personal and background samples detected nothing. At a dermatology clinic, isocyanic acid was detected during CO2 laser use at around 1 microgram per cubic metre but not during diathermy, a likely consequence of the temperature difference between the two techniques, roughly 400 degrees for the laser against about 100 for diathermy. Asphalt workers showed detectable but very low exposure with a maximum of 0.5 micrograms per cubic metre, and mowing a lawn with a petrol mower produced exposure just above 1 microgram per cubic metre. In 3D printing with thermoplastic polyurethane filament, emissions were confirmed inside one printer hood but not outside it, and a third printer and filament combination showed nothing at all.

The environmental measurements fill a significant European data gap. Outdoor air in Gothenburg, sampled at three locations with different traffic intensities over 20-hour periods, showed uniformly low concentrations between 0.14 and 0.25 micrograms per cubic metre, with a mean of 0.12, in the lower range of previously reported outdoor data and below recent measurements from China of around 0.7. Diesel engine exhaust sampled directly from exhaust pipes in an experimental setup ranged between 0.5 and 4.6 micrograms per cubic metre, with no clear pattern across engines, fuels or driving cycles, and the addition of urea-containing diesel exhaust fluid such as AdBlue did not generally raise emissions. The team concludes that internal combustion engines are an important source of the isocyanic acid found in urban air.

Cigarette smoke, however, stands in a category of its own. Direct measurements in cigarette smoke confirmed extremely high isocyanic acid levels of up to 4300 micrograms per cubic metre, while side-stream exposure measured at 1 to 5 micrograms per cubic metre was considerably lower. The smoke also contained extremely high levels of methyl isocyanate, over 6000 micrograms per cubic metre, findings consistent with literature dating back to 1965 and replicated in a larger 2025 study by the same group. The team notes that smoking a single cigarette provides an exposure above the Swedish 8-hour limit, making smoking one of the most significant personal exposures to this compound that most people will ever encounter.

The health implications remain the great unresolved question. Isocyanic acid is biochemically active, participating in a protein modification called carbamylation, and the isocyanate family is well known for causing asthma and other respiratory effects. Only one occupational study of monoisocyanate health effects has been identified, finding nasal symptoms and a small reduction in forced expiratory volume, and a more recent report described three workers exposed to decomposition products during hot iron cauterisation, with personal isocyanic acid measurements between 331 and 435 micrograms per cubic metre, two of whom had asthma, though the link to isocyanic acid is unclear. The Swedish limit itself is based on similarity to methyl isocyanate rather than on direct health data. The survey shows that higher isocyanic acid concentrations are mainly associated with industrial high-temperature handling of urea or polyurethane-based materials and with cigarette smoke, while everyone can expect exposure of around 0.2 micrograms per cubic metre outdoors and slightly higher indoors. The researchers caution that some environments were surveyed only once and that sampling times varied, so results should be used to rank environments rather than to determine whether limits were exceeded. A follow-up exposure assessment in select environments is planned, and the team stresses that better health effects data are urgently needed to understand what these exposures truly mean.

Subject of Research: Environmental Science and Pollution Research

Article Title: Isocyanic acid measurements in Swedish industries and environments

Article References: Eden, G. R., Storsjö, T., Johansson, A., Andersson, L., & Tinnerberg, H. (2026). Isocyanic acid measurements in Swedish industries and environments. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38302-w

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38302-w

Keywords: isocyanic acid, occupational exposure, air quality, industrial emissions, polyurethane, urea binders, cigarette smoke, Sweden, Isocyanic, acid, measurements, Swedish