The correlation of particle surface area dose and biological activity in vivo, suggesting increasing toxicity with decreasing particle size at equal concentrations, has been demonstrated in a large number of studies with various particulate materials.
Oberdörster et al (1994) performed a sub-chronic inhalation study with ultrafine (20 nm) and fine (250 nm) anatase TiO2 at similar mass concentrations (23.5 and 22.3 mg/m³ respectively). After the exposure period, mass lung burden differed only slightly between the two materials, but ultrafine? <== CHECK (nano) TiO2 caused prolonged retention half-time and macrophage clearance rate (3-and 8 fold, respectively). Moreover, significantly more severe inflammatory responses and changes of lung morphology were found after exposure to nano TiO2. From the deposited nano and fine particles, 44 % of the nano particles was determined in the interstitial space, whereas there were only 13 % of the fine TiO2 particles (section on Translocation). When establishing dose-response relationships on the basis of either mass, volume or surface area for these effects, the authors indicate that “surface area” and not volumetric load as the appropriate dose metric for “ultrafine” particles in correlation with the examined endpoints.
In a similar way of establishing dose-response relationships for observed effects, Tran et al (2000) analysed sub-chronic data of TiO2 and BaSO4 (Cullen et al, 2000). The numbers of polymorphonuclear cells (PMN) in the lung were plotted in relation to the corresponding lung burdens expressed in terms of mass, surface area, and particle numbers. The results indicated that surface-area burden was the most likely of the three measures to explain the difference in the numbers of PMN between the dusts suggesting a threshold for PMN recruitment at a lung burden area of 200 to 300 cm². Tran et al also analysed published data by Driscoll (1996) and Oberdörster et al (1994) concluding to be in line with the results of Cullen et al (2000) and Tran et al (2000). Considering that particle surface area incorporates both particle number and their size, it was postulated that there is a chain of biological responses following deposition of large particle surface area, which may lead via an “activated environment” to immobility of alveolar macrophages, and to the overload condition.
Further comparisons were performed by Sager et al (2008, 2009) who examined the pulmonary response of ultrafine (nano) versus fine titanium dioxide and carbon black (CB) after intratracheal instillation to rats. They compared the dose-response relationship of nano and fine material either on a mass-based or a surface-based dose metric. It was observed that on a mass dose basis nano sized particles gave a 30-100 fold higher response than the fine-sized particles of the same composition in the examined parameters. However, when the dose is normalised to surface area the difference of the same sets of parameters was about an order of magnitude lower (Sager et al, 2008).
In general, a number of studies over the past 15 years, suggest that the smaller the particle (the greater the surface area dose) the greater the induced inflammatory response. Donaldson et al (2008) identified a threshold value for onset of inflammation of 1 cm² particle surface area burden per 1 cm² of proximal alveolar region (PAR), (Donaldson et al, 2008). The relevance of surface area is discussed particularly in regard to toxicity of nano-particles because at the same mass load surface area increases as particle size decreases. Furthermore, the specific characteristic of engineered nano-particles is very often determined by surface functionalization which may significantly influence their toxicity. Rushton et al (2010) showed good correlation between in-vivo and in-vitro signals when using a surface area (BET-values) based dose metric when testing nanoparticles.