With regard to nomenclature issues, studies on three different test substance particle-types are discussed in the following sections. Pigment-grade TiO2 particles, are generally considered in the mean, primary particle size range > 100 nm. Pigment-grade or fine-sized TiO2 particulates are generally in the size range of 250 – 400 nm. In addition, the crystal structure of the pigment-grade particle types were predominantly of the “rutile” type and generally have a measured surface area of 5-6 m2/g. In contrast, the “ultrafine TiO2 test substances used in the sub chronic (Bermudez et al, 2004) and Heinrich et al, (1995) had a crystal structure composition of 80% anatase and 20% rutile, with a measured surface area of ~ 53 m2/g, and a primary particle size (TEM) of 25 nm. It is important to note that the aerosolised distribution of ultrafine and pigment-grade TiO2 particles included primarily agglomerated particulates. Carbon black particles are generally found in the <100 nm range. In the Elder et al (2005) study, the primary particle size of the high surface area carbon black test substances was approximately 16 nm, corresponding to a specific surface area of 300 m2/g. The primary particle size and specific surface area of the “lower” surface area carbon black test particle was approximately 70 nm and 37 m2/g, respectively.