RfD is defined by the EPA as, “an estimate of a daily oral exposure to the human population (including sensitive subgroups such as children) that is not likely to cause harmful effects during a lifetime.”

See the definition of reference dose on the EPA website »

EPA’s definition is based on the assumption that exposure could occur throughout a lifetime and takes into account all stages of life. The definition also takes into account “sensitive subgroups,” such as pregnant women, infants, children and fetuses.

It is sometimes misunderstood to mean that doses above the RfD are unsafe. This is not correct. The RfD incorporates a number of safety factors to ensure the value is health-protective. When a dose exceeds an RfD value, it does not mean that an adverse effect will occur. Learn more about safe water from the EPA »

The RfD is not a regulatory standard —- it is one of the building blocks assembled into a regulatory standard, and should not be misinterpreted as serving the purposes of a regulatory standard.