Relationships between number, surface area, and mass concentrations of different nanoparticles in workplaces

Abstract

No consistent metric for measuring exposure to nanoparticles has yet been agreed upon internationally. This study seeks to examine the relationship between the number concentration (NC), surface area concentration (SAC), and mass concentration (MC) of nanoparticles in workplaces. Real-time NC_{20–1000 nm}, SAC_{10–1000 nm}, and respirable MC_{100–1000 nm} were determined for different nanoparticles. Concentration ratio (CR, activity: background), exposure ranking (ER), and between-metric correlation coefficients (R) were used to analyze the relationships between the three metrics. The ratio of cumulative percentage by number (APN) and cumulative percentage by mass (APM) was used to analyze whether the nanoparticle number is predominant, as compared with the nanoparticle mass. The CRs of NC_{20–1000 nm} and SAC_{10–1000 nm} for different nanoparticles at the corresponding work sites were higher than those of respirable MC_{100–1000 nm}. The ERs of NC_{20–1000 nm} for nano-Fe₂O₃ and nano-Al₂O₃ were the same as those of SAC_{10–1000 nm}, but were inconsistent with those of respirable MC_{100–1000 nm}. The order of correlation coefficients between NC_{20–1000 nm}, SAC_{10–1000 nm}, and respirable MC_{100–1000 nm} was: R_{SAC and NC} > R_{SAC and MC} > R_{NC and MC}. The ratios of APN and APM for nano-Al₂O₃ and grinding-wheel particles (less than 100 nm) at the same work site were 2.03 and 1.65, respectively. NC and SAC metrics are significantly distinct from the MC in characterizing exposure to airborne nanoparticles. Simultaneous measurements of the NC, SAC, and MC should be conducted as part of nanoparticle exposure assessment strategies and epidemiological studies.