U |
| U.S. EPA | United States Environmental Protection Agency. |
| Uncertainty | A deficiency in knowledge concerning parameter values and the appropriate extrapolation of the significance of adverse health effects, to a situation involving different species and exposure conditions. Uncertainty can result from lack of knowledge, inherent variability (stochasticity), confounding effects, or imprecise measurements. Also see: Safety Factor. |
| Uncertainty Factor | One of several factors used in calculating the reference dose from experimental data. UFs are intended to account for (1) the variation in sensitivity among humans; (2) the uncertainty in extrapolating animal data to humans; (3) the uncertainty in extrapolating data obtained in a study that covers less than the full life of the exposed animal or human; and (4) the uncertainty in using LOAEL data rather than NOAEL data. |
| Unconfined Aquifer | An aquifer containing water that is not under pressure; the water level in a well is the same as the water table outside the well. |
| Upper Detection Limit | The largest concentration that an instrument can reliably detect. |
| UXO | Unexploded Ordinance. |
