An integrated assessment of nanoparticle exposures in the ceramic industry

Abstract

Exposure to nanoparticles has been associated with adverse effects on human health. When the exposure route is inhalation, nanoparticles can cause pulmonary inflammation which may be more severe than from fine particles, while significant associations between nanoparticles and cardiovascular morbidity have also been also observed. Consequently, industrial processes which release airborne nanoparticles into workplace air have become an issue of growing concern with regard to occupational exposure and potential health hazards for workers. Nanoparticles emitted by industrial activities may be engineered and used as input/output in a process, or formed unintentionally as a result of a given industrial activity and are referred to as process-generated nanoparticles. The latter have high probability to be released from high energy processes (e.g. thermal) and to impact exposure in various scenarios at diverse industrial settings. The present PhD Thesis has three main objectives: to identify nanoparticle sources in industrial settings and characterize their release mechanisms, to characterize the different exposure scenarios, and to assess the effectiveness of mitigation strategies for exposure reduction. These objectives were addressed by applying an integrated assessment of nanoparticle exposures in real-world scenarios. All of the industrial scenarios studied were related to the ceramic industry (e.g. laser ablation of ceramic tiles, thermal spraying of ceramic coatings). The results obtained are presented in the form of four scientific papers. The first scientific publication (1) identified pulsed laser ablation of ceramic tiles as a source of process-generated nanoparticles. This work studied the mechanisms controlling nanoparticle formation and release during ablation of different types of ceramic tiles. High particle number concentrations were detected (3.5×104 to 2.5×106 cm-3) for all of the tiles and with both lasers assessed (near- and mid-infrared). Different particle release mechanisms were identified: during ablation with the near-IR laser particles were emitted through melting and nucleation, while emissions from the mid-IR laser were attributed to melting and mechanical shockwaves. Particle number and mass emissions were dependent on the tile surface characteristics as well as the laser parameters. The second publication (2) characterized nanoparticle emissions and their impact on exposure during thermal spraying of ceramic coatings in a real-world industrial scenario. High particle number (>106 cm−3; 30–40 nm) and mass (60–600 μgPM1 m−3) concentrations were recorded inside the spraying booths, which impacted exposure in the worker area (104–105 cm−3, 40–65 nm; 44–87 μgPM1 m−3). Irregularly-shaped, metal-containing particles (Ni, Cr, W) were sampled from the worker

area and a direct link between the spraying activity and exposure was established. In terms of particle number count, 90% of the particles were nanoparticles with sizes 26–90 nm. The third publication (3) discussed the hygroscopic properties of these nanoparticles, which were monitored online with an HTDMA. The nanoparticles emitted were found to take up moderate amounts of water when exposed to elevated relative humidity (87% RH), with their hygroscopicity being distinguishably lower compared to that of the atmospheric background aerosol particles present in the workplace air. Thus, particle hygroscopicity was identified as a useful metric to discriminate process-generated from background particles in workplace air. Finally, the fourth scientific publication (4) quantified the effectiveness of mitigation strategies implemented during four different exposure scenarios under real-world settings. The nanoparticle removal efficiency of source enclosure combined with local exhaust ventilation was quantified to range between 65-99%, The highest efficiency was achieved by the combined use of a strong local exhaust ventilation with full enclosure (99.8%), tested during thermal spraying. Source substitution achieved a 91.5% reduction of exposure concentrations in the worker breathing zone, lower than the expected 100% due to interference from simultaneous sources. Mask respirators managed to reduce worker exposure by 86.7%, whereas source isolation reached maximum efficiency of 84.4%. The results highlight the interdependence of different mitigation strategies (e.g., LEV and source enclosure), which are frequently implemented simultaneously in real-world industrial scenarios. The mitigation measures for exposure reduction have proven to be more efficient when tailored to each specific industrial scenario. The combination of key information and analyses deriving from the experimental scenarios assessed allowed to extract conclusions and recommendations with direct application to the industrial sector under study.

La exposición a nanopartículas se asocia con efectos adversos en la salud humana. En consecuencia, las actividades industriales que liberan nanopartículas al aire en entornos laborales se han convertido en un problema de creciente relevancia desde el punto de vista de salud ocupacional y de los riesgos potenciales para la salud de los trabajadores. Las nanopartículas emitidas por actividades industriales pueden diseñarse y utilizarse como materia prima en procesos industriales, o formarse involuntariamente como resultado de una actividad industrial dada (estas últimas, denominadas con frecuencia nanopartículas de proceso). Las nanopartículas de proceso tienen una alta probabilidad de liberarse durante actividades altamente energéticas (por ejemplo, procesos térmicos) y de generar situaciones de exposición laboral en diversos escenarios y entornos industriales. La presente tesis doctoral tiene tres objetivos principales: identificar fuentes de emisión de nanopartículas en entornos industriales y caracterizar sus mecanismos de liberación, caracterizar los diferentes escenarios de exposición, y evaluar la efectividad de las estrategias de mitigación para la reducción de la exposición. Estos objetivos se abordaron mediante la aplicación de una evaluación integrada de la exposición a nanopartículas en escenarios industriales reales (no en laboratorio). Todos los escenarios industriales estudiados estaban relacionados con procesos térmicos asociados a la industria cerámica (por ejemplo, ablación por láser de baldosas cerámicas, proyección térmica de recubrimientos cerámicos). Los resultados obtenidos se presentan en forma de cuatro publicaciones científicas. La primera publicación científica (1) identificó la ablación por láser pulsado de baldosas cerámicas como una fuente de nanopartículas de proceso. La segunda publicación (2) describió las emisiones de nanopartículas y su impacto en la exposición durante la proyección térmica de recubrimientos cerámicos en un escenario industrial. La tercera publicación (3) analizó las propiedades higroscópicas de estas nanopartículas, que se monitorearon en línea con un HTDMA. Finalmente, la cuarta publicación científica (4) cuantificó la efectividad de las estrategias de mitigación implementadas en cuatro escenarios de exposición diferentes, en entornos industriales bajo condiciones reales de operación. La combinación de información clave y de los análisis derivados de los escenarios experimentales evaluados permitió extraer conclusiones y recomendaciones con aplicación directa al sector industrial cerámico.