The eight compounds used in this study were collected at a U.S. ITO production facility from containers of feedstock materials or production processes (NIOSH, 2012 ). Previous evaluations showed that workers in the various departments at the ITO facility are exposed to mean respirable dust concentrations in the range of approximately 0.024–0.429L mg/m3, based on personal air sampling monitors (NIOSH, 2012 , 2013 ). Taking into account these airborne particle concentrations of 0.1 mg/m3 at breathing rate for an 8-h workday, deposition fraction (from an International Commission on Radiological Protection [ICRP] model for conducting airways based on particle sizes), and estimated human lung burden (based on cell culture treatments used in our studies), it was possible to estimate how many years of workplace exposure our doses represent. For cellular exposure experiments, particles were suspended in sterile filtered 1× phosphate-buffered saline (PBS) at various stock concentrations (1 mg/ml–10 mg/ml), vortexed, and diluted into cell culture media at final concentrations of 50 μg/ml or 1 mg/ml. The human lung burden was determined based on the 50-μg/ml dose being approximately 15 μg/cm2 (accounting for well volumes and surface areas) and the estimated surface area of the human airways being approximately 2300 cm2 (Mercer et al., 1994 (link)).
Therefore:
To determine how many years it would take to reach this amount, the following calculation was performed:
Therefore, the lower dose of 50 μg/ml represents approximately 3 yr of average workplace exposure. Using these same calculations, it was estimated the high dose of 1 mg/ml represents more of a career-long exposure (i.e., approximately 30–65 yr depending on department). Indium compound stocks were prepared fresh for experiments. Equal volumes of 1× phosphate-buffered saline (PBS) were used as control conditions for each experiment, and 1 mM potassium dichromate (Cr(VI)) was used as a positive control for experiments in which intracellular ROS were measured (Ye et al., 1999 (link)). Crystalline silica (Min-U-Sil 5 μm) was used as a control particle in the MTT and caspase activation assays as silica was shown to be cytotoxic in vitro (Allison et al., 1966 (link); Pfau et al., 2012 (link); Castranova, 2004 (link); Lison et al., 2009 (link)).