Exposures and urinary biomonitoring of aliphatic isocyanates in construction metal structure coating
Introduction
Reactive chemical resin systems based on isocyanates are used widely in diverse construction applications for their excellent performance characteristics, such as durability, resistance to chemical and physical agents, and optical transparency. Typical applications involving isocyanates include industrial metal structures coatings (bridges, exterior and interior surfaces of industrial storage tanks, water pipes, wind turbines), interior floor coatings, grouts and terrazzo applications, gluing, sealing, concrete bonding, and masonry work. The demand for these products continues to grow, in response to a strong growth in residential construction and the need for repairs in the aging infrastructure. More than 56,000 bridges in the United States are in need of repair or replacement, increasing the expected demand for industrial coating jobs (ARTBA, 2019; Kirk and Mallett, 2018). The number of construction painters accounted for ~380,000 in 2016, and is projected to grow by 6% (or 22,000 new jobs) in the next decade (BLS, 2018). Furthermore, as energy production shifts to renewable sources, the demand for building and maintaining wind turbines will continue to increase, and with it, the demand for industrial coatings. The general industrial coatings market is expected to reach $131 billion by 2022 (Pianoforte, 2019).
Painters exposed to isocyanates are at risk of developing occupational asthma and other isocyanate-related diseases. Isocyanates are potent respiratory sensitizers and have one of the lowest occupational exposure limits ever established (5 parts per billion, ppb time 8-hr time weighted average). Isocyanates continue to be a major occupational health problem (Reilly et al., 2019; Thore and Tiotiu, 2019) and a leading cause of occupational asthma (Goossens et al., 2002; Lefkowitz et al., 2015; Lockey et al., 2015; Redlich et al., 2007), in spite of the fact that their occupational toxicology and health have been studied continuously for more than half a century. In addition to asthma, exposures to isocyanates may also induce hypersensitivity pneumonitis, chronic obstructive pulmonary disease (COPD) and accelerated loss of pulmonary function, allergic and irritant contact dermatitis, rhinitis, irritation of the upper airways, eyes, and skin and occasional skin burns (Geier et al., 2018; Goossens et al., 2002; Wisnewski et al., 2006). Deaths from acute isocyanate expsoures have been reported, although fortunately they tend to be rare (NIOSH, 2006; Reilly et al., 2019; Thore and Tiotiu, 2019).
Occupational asthma continues to be the primary health concern of isocyanate exposures, in part because there is no cure for the disease; sensitized individuals may respond to extremely low isocyanate levels (as low as 1 ppb) at work or do-it-yourslelf consumer applications or cross-react to other isocyanates and amine components in the two-pack formulations; and their asthma may progress towards a generalized, non-specific asthma that may be triggered by other respiratory irritants and pollutants (Lockey et al., 2015; Redlich et al., 2007). At present there are no reliable biochemical tests for detecting isocyanate sensitization, especially in early stages, and clinical diagnosis of isocyanate asthma is complex, invasive and expensive. The current recommendation for the management of isocyanate asthma is complete avoidance or elimination of isocyanate exposures (Baur et al., 2012). However, this solution may come at the cost of significant loss of income for affected workers, while for many of them the isocyanate asthma symptoms do not improve significantly (Ruegger et al., 2014). Therefore, exposure reduction through effective exposure controls is an important intervention strategy, and one that could yield a better long-term outcome in reducing the risk of occupational asthma and preserving the income of these workers (Baur et al., 2012; Vandenplas et al., 2011).
Little is known about exposures and disease prevalence among construction trades that use isocyanates, in particular industrial coatings. Numerous studies have investigated isocyanate exposures and respiratory disorders (Pronk et al. 2007, 2009) during coating and painting tasks in the automotive and aerospace industries, and other manufacturing sectors that produce polyurethane products (England et al., 2001; Janko et al., 1992; Pronk et al., 2006; Reeb-Whitaker et al., 2012; Reilly et al., 2019; Sparer et al., 2004). Because construction differs in significant ways from aerospace and auto manufacturing industries, the knowledge base about exposures, chemistries, workflow and work practices from those sectors cannot be transferred readily to construction applications. This significant data gap on isocyanate exposures and effectiveness of exposure controls among construction workers must be addressed with relevant field exposure assessment and biomonitoring studies.
The polyurethane coating systems used in construction are often polymeric aliphatic isocyanates based on the 1,6- hexamethylene diisocyanate (1,6-HDI) monomer and/or isophorone diisocyanate (IPDI) referred to as pHDI and pIPDI, respectively (Supplemental Material (SI), Fig. S1). These formulations are often applied as two-part systems, comprised of the isocyanate hardener (part A) and the base part (part B), which is often a mixture of solvent blends, polyols, cross-linkers, and other additives. Current part A formulations contain only traces of the volatile monomer (HDI or IPDI, each typically at <0.1–1% in commercial formulations), with >99% of the isocyanate being higher oligomeric species (Bello et al., 2002b; Fent et al., 2009; Sparer et al., 2004). After the hardener is mixed with the base, the product is typically sprayed using a spray gun, or is manually applied using a roller or a brush. Painters are exposed to isocyanates through inhalation of their vapors (for volatile monomers) and airborne aerosols, as well as through direct dermal contact with the product or contaminated tools and surfaces. Comprehensive assessments of inhalation and dermal exposures, supplemented with exposure biomarkers are essential for identifying exposure sources, as well as for evaluating the efficacy of work practices and personal protective equipment (PPE) on reducing exposures. Urinary biomonitoring of isocyanate exposures in a number of studies has relied on measuring the corresponding diamines of monomers, such as HDI and IPDI (1,6-hexamethylene diamine, HDA) and isophorone diamine, IPDA) (Gaines et al. 2010a, 2011; Pronk et al., 2006) following aggressive acid hydrolysis. Because of its short clearance half-life of a few hours (Budnik et al., 2011; Liu et al., 2004) HDA monitoring can be valuable when evaluating the efficacy of exposure controls and PPE within a short time frame such as during a working shift.
We are presenting herein the results of a field investigation that was conducted as part of a larger study on reactive isocyanate systems among construction workers. The objectives of this work were (i) to characterize chemistries of and isocyanate exposures among painters during metal structure coating tasks in construction using simultaneous measurement of air and dermal exposures, and (ii) assess adequacy of existing work practices and exposure controls through field observations and through urinary biomonitoring pre- and post-shift. Results of this work can guide development of future intervention strategies for reducing isocyanate exposure among construction painters.
Section snippets
Sampling sites and participants
Workplace sampling was performed during topcoat applications at eight unique construction sites in the Northeast United States from May 2015 to October 2018. Thirty participants were recruited over ten field trips: 23 painters involved directly with product application through spraying, rolling or brushing, 5 helpers who performed product mixing and other auxiliary tasks, and two bystanders (managers). The majority of study participants were males (7% females). Among all participants, 80% were
Work practices and personal protective equipment (PPE)
Field observation data summarized in Fig. 2 indicate that the type and frequency of PPE used in workplaces visited varied widely between and within each site. The majority of participants (~61%) used half-face organic vapor cartridge (OVC) respirators, without particulate filter. One worker (3.6%) wore a full-face OVC respirator, another one wore an N95 (3.6%), while 32% of participants did not use any respirator. About 36% of workers wore disposable polymer dip coated gloves (coating on the
Discussion
Isocyanate-based formulations such as coating products are used widely in numerous construction trades. Although common, exposures to aliphatic isocyanates during coating applications in construction have not been characterized. In this work, we report our findings of a field study to assess inhalation and dermal exposures to isocyanates, their work practices, and the current status of exposure controls, among 30 painters from ten sites in the Northeastern USA performing metal structure coating
Conclusions
Exposure and biomonitoring results, in conjunction with field observations, support the overall conclusions that (a) substantial exposures to isocyanates occur during industrial coating applications among construction painters; and that (b) the current exposure controls are not sufficiently protective. Implementation of effective exposure control programs focusing on tasks performed in enclosed spaces and increased awareness about proper PPE use are warranted in order to reduce isocyanate
Acknowledgments
This research was supported by The Center for Construction Research and Training (CPWR) through the National Institue of Occupational Safety and Health (NIOSH) Cooperative Agreement Number U60–OH009762. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of CPWR or NIOSH. The authors would like to thank all study participants and our construction industry partners who facilitated workplace access. A special thank you goes to the graduate
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