IUPAC Glossary of Terms Used in Toxicology - Terms Starting with U

ulcer
Defect, often associated with inflammation, occurring locally or at the surface of an organ or tissue owing to sloughing of necrotic (see necrosis) tissue.

ultrafine particle
Particle in air of aerodynamic diameter less than 100 nm.
Note: As a group, ultrafine particles are referred to as PM_0.1 (100 nm is 0.1 μm).

uncertainty (in metrology)
Parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used.
Note: The parameter may be, for example, a standard deviation or the half-width of an interval, having a stated coverage probability.
[7]

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 (ADI) or tolerable daily intake (TDI).
   Note: The NOEL or NOAEL is divided by the value to calculate the ADI or TDI.
   Aee also modifying factor, safety factor

unit risk
Upper-bound excess lifetime cancer risk estimated to result from continuous exposure to an agent at a concentration of 1 μg L^-1 in water, or 1 μg m^-3 in air.
Note: The interpretation of unit risk is as follows: if unit risk = 1.5 x 10^-6 μg L^-1, 1.5 excess tumors are expected to develop per 1 000 000 or 10⁶ people if exposed daily for a lifetime to 1 μg of the chemical in 1 litre of drinking water.

upper boundary
Estimate of the plausible upper limit to the true value of a quantity.
Note: This is usually not a statistical confidence limit.

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.

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.