- UEL, or UFL
- Upper explosive limit or upper flammable limit of a vapor or gas; the highest concentration (highest percentage of the substance in air) that will produce a flash of fire when an ignition source (heat, arc, or flame) is present. At higher concentrations, the mixture is too "rich" to burn. See also LEL.
- Microgram, one millionth of a gram.
- Tending toward decomposition or other unwanted chemical change during normal handling or storage.
- Unstable Reactive
- A chemical that, in the pure state, or as produced or transported, will vigorously polymerize, decompose, condense, or become self-reactive under conditions of shock, pressure, or temperature.
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