Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p3
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 3/8)
Character Range: 1037755–1040583

TDI   mg/kg-day  An estimate of the intake of a substance that can occur over a lifetime without appreciable health risk (WHO 1994). TDI is generally used when a chemical is a food or environmental contaminant. Like ADI, TDI is conceptually similar to RfD.
Reference dose RfD           mg/kg-day  An estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral intake of a substance by the human population (including sensitive populations) that is likely to be without an appreciable risk of deleterious effects during a lifetime. It can be derived from a NOAEL, LOAEL or benchmark dose, with uncertainty factors generally applied to reflect limitations of the data used (IRIS). Generally used in non-cancer health assessments.
Reference concentration RfC  mg/m3      An estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation intake of a substance by the human population (including sensitive populations) that is likely to be without an appreciable risk of deleterious effects during a lifetime. It can be derived from a NOAEL, LOAEL or benchmark concentration, with uncertainty factors generally applied to reflect limitations of the data used (IRIS). Generally used in non-cancer health assessments.

CSFs are typically calculated for the assessment of carcinogenic end-points for genotoxic carcinogens (that is, non-threshold compounds). A slope factor is a plausible upper-bound estimate of the probability of a response per unit of intake of a chemical over a lifetime. The slope factor is used in risk assessments to estimate an upper-bound lifetime probability of an individual developing cancer as a result of exposure to a particular level of a potential carcinogen (US EPA 1989). This approach is also known as the 'linear' approach which implies a proportional (linear) relationship between risk and dose at low doses. CSFs assume that there is no level of exposure to carcinogenic chemicals that does not pose a finite probability, however small, of generating a carcinogenic response.
A URF is an expression of carcinogenic potency in concentration terms, such as probability of cancer per 1 μg/L of drinking water or probability of cancer per 1 μg/m3 in air. Generally, the drinking water URF is derived by converting a CSF from units of mg/kg-day to units of μg/L, and an inhalation URF is developed directly from a dose-response analysis using equivalent human concentration already expressed in units of μg/m3 (US EPA 2005a). Derivation of a URF often assumes a standard intake rate (for example, inhalation of 20 m3 of air per day or ingestion of 2 L of water per day) and body weight (for example, 70 kg). When a theoretical upper-bound cancer risk estimate is calculated using a URF instead of a CSF, it is often termed the unit risk.
Unit risk