We present a toxicological assessment of five carbon nanomaterials on human fibroblast cells in vitro. We correlate the physico-chemical characteristics of these nanomaterials to their toxic effect per se, i.e. excluding catalytic transition metals. Cell survival and attachment assays were evaluated with different concentrations of refined: (i) single-wall carbon nanotubes (SWCNTs), (ii) active carbon, (iii) carbon black, (iv) multi-wall carbon nanotubes, and (v) carbon graphite. The refined nanomaterial that introduced the strongest toxic effect was subsequently compared to its unrefined version. We therefore covered a wide range of variables, such as: physical dimensions, surface areas, dosages, aspect ratios and surface chemistry. Our results are twofold. Firstly, we found that surface area is the variable that best predicts the potential toxicity of these refined carbon nanomaterials, in which SWCNTs induced the strongest cellular apoptosis/necrosis. Secondly, we found that refined SWCNTs are more toxic than its unrefined counterpart. For comparable small surface areas, dispersed carbon nanomaterials due to a change in surface chemistry, are seen to pose morphological changes and cell detachment, and thereupon apoptosis/necrosis. Finally, we propose a mechanism of action that elucidates the higher toxicity of dispersed, hydrophobic nanomaterials of small surface area.