As the Proposed HON Rule depends on technically accurate and biologically plausible measures of “unacceptable residual cancer risk” the following comments identify and clearly describe the specific unaddressed flaws in the IRIS assessment that produced risk specific concentrations (RSCs) so small relative to background exposure as to provide no utility in managing the residual risk posed by facility EtO emissions. These comments also provide support for alternative risk assessments, such as the TCEQ assessment that provides biologically plausible RSCs and health-based action levels to identify and manage unacceptable residual cancer risk under the HON amendments for EtO.