This paper assesses the fundamental assumption that multiple uncertainty factors (UFs) are independent of each other and therefore may be combined through a multiplicative scheme. It is argued that a lack of independence is seen in several cases such as that between the interindividual and less-than-lifetime UFs with respect to an age component and results in an error in double counting of UFs. The paper proposes revised UF values based on the concept of the relationship of independent and interdependent UFs. This uncertainly factor revision is then applied to a large number of contaminants for which EPA either has or has proposed MCLs or DWELs. © 1993 Academic Press. All rights reserved.
