Uncertainty Factors Factors used in the calculation of acceptable humans or environmental exposures.  They are applied to data from laboratory experiments or epidemiology studies.  Factors of 10 are normally used to account for such uncertainties in the data on which risk assessments are made.  Similar to safety factors. Unscheduled DNA Synthesis (UDS) The synthesis of DNA outside the normal mitotic process.  It is considered an indication of DNA damage and the first step in the process of mutagenesis.  The most commonly used test for UDS measures the uptake of tritium-labeled thymidine into the DNA of rat hepatocytes or human fibroblasts.