By definition, an RfD assumes exposure over a lifetime and takes into account special considerations of sensitive populations such as pregnant women, children, infants and fetuses. Read the U.S. EPA’s definition of a reference dose on their website » The NAS Committee calculated a RfD by starting with a No-Effect Level and then applying a 10-fold safety factor to ensure protection of potentially sensitive populations. If this value was used to calculate a safe drinking water concentration, any additional adjustment to account for sensitive populations, such as using body weights and drinking water consumption rates for the sensitive population, would be unnecessary and in essence “double counting.”