The CSF is used in EHRA to estimate the upper-bound probability that cancer will develop over a lifetime of exposure to a chemical at a specific level of intake. It is the slope of a linear extrapolation from the bound estimate of a POD dose to zero. The URF can be derived directly from inhalation or drinking-water studies depending on which media is being assessed. Where such data is not directly available, these unit risks can be derived by converting an oral CSF with units of mg/kg/d–1 to a concentration of the substance in air, water or other media. These extrapolations often assume default intake rates for the specified media (e.g. an inhalation rate of 20m3 per day of air, or ingestion of 2 L/day of water and a body weight of 70 kg). The conversion equation most commonly used is: Inhalation URF (µg/m3)–1 = (BW = body weight) Where different default values are used (e.g. those recommended in the Australian exposure factor guidance document – see Chapter 4), it may be necessary to adjust CSFs derived by the US EPA or in Integrated Risk Information System (IRIS) databases. Recent US EPA (2009a) guidance (RAGS-F) indicates that inhalation risks should be assessed by calculating an exposure-adjusted air concentration, which is then used in EHRA risk characterisation equations. This means that exposure estimates are no longer adjusted based on inhalation rate or body weight, and the only difference in risk estimates between a child and an adult is the exposure time. The application of these risk factors is to calculate the probability of a finite increase in cancer risk over a lifetime, according to the equation: Increased lifetime cancer risk (ILCR) = chronic daily intake (mg/kg/d) × CSF (mg/kg/d)–1 or Increased lifetime cancer risk (ILCR) = exposure concentration × URF
The CSF is used in EHRA to estimate the upper-bound probability that cancer will develop over a lifetime of exposure to a chemical at a specific level of intake. It is the slope of a linear extrapolation from the estimate of a POD dose to zero. The URF can be derived directly from inhalation or drinking-water studies depending on which media is being assessed. Where such data is not directly available, these unit risks can be derived by converting an oral CSF with units of mg/kg/d–1 to a concentration of the substance in air, water or other media. These extrapolations often assume default intake rates for the specified media (e.g. an inhalation rate of 20m3 per day of air, or ingestion of 2 L/day of water and a body weight of 70 kg). The conversion equation most commonly used is: Inhalation URF (µg/m3)–1 = (BW = body weight) Where different default values are used (e.g. those recommended in the Australian exposure factor guidance document – see Chapter 4), it may be necessary to adjust CSFs derived by the US EPA or in Integrated Risk Information System (IRIS) databases. Recent US EPA (2009a) guidance (RAGS-F) indicates that inhalation risks should be assessed by calculating an exposure-adjusted air concentration, which is then used in EHRA risk characterisation equations. This means that exposure estimates are no longer adjusted based on inhalation rate or body weight, and the only difference in risk estimates between a child and an adult is the exposure time. The application of these risk factors is to calculate the probability of a finite increase in cancer risk over a lifetime, according to the equation: Increased lifetime cancer risk (ILCR) = chronic daily intake (mg/kg/d) × CSF (mg/kg/d)–1 or Increased lifetime cancer risk (ILCR) = exposure concentration × URF bound
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