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Toxicology Glossary - U

ulcer: Defect, often associated with inflammation, occurring locally or at the surface of an organ or tissue owing to sloughing of necrotic tissue

uncertainty factor:

1. In assay methodology, confidence interval or fiducial limit used to assess the probable precision of an estimate
2. In toxicology, value used in extrapolation from experimental animals to man (assuming that man may be more sensitive) or from selected individuals to the general population: for example,
   • a value applied to the no-observed effect level (NOEL); or
   • no-observed adverse effect level (NOAEL) to derive an acceptable daily intake or reference dose (RfD) (the NOEL or NOAEL is divided by the value to calculate the acceptable daily intake or RfD)

The value depends on the nature of the toxic effect, the size and type of population to be protected, and the quality of the toxicological information available
SNsafety factor
RT modifying factor, no-observed-effect-level, no-observed-adverse-effect-level, reference dose

unit risk (as used by the USEPA): Incremental upper-boundary lifetime risk estimated to result from lifetime exposure to an agent if it is in air at a concentration of 1 mg/m3 or in the water at a concentration of 1 mg/L
IRIS, 1986

upper boundary: Estimate of the plausible upper limit to the true value of a quantity: this is usually not a statistical confidence limit
IRIS, 1986

uptake: Entry of a substance into the body, into an organ, into a tissue, into a cell, or into the body fluids by passage through a membrane or by other means
PS absorption

urticaria: Vascular reaction of the skin marked by the transient appearance of smooth, slightly elevated patches (wheals, hives) that are redder or paler than the surrounding skin and often attended by severe itching