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Radon: At ordinary temperatures radon is a colorless gas. When cooled below the freezing point, radon exhibits a brilliant phosphorescence which becomes yellow as the temperature is lowered and orange-red at the temperature of liquid air. The main hazard is from inhalation of the element and its decay products which are collected on dust in the air. It is the heaviest known gas. Radon is also present in some spring waters.(1)

Pico curies per liter (PCi/L): Unit of measure used to express the results of radioactivity tests in air and water. For radon gas, one Pico curie per liter is the amount of radon in the air so that 2.2 atoms of radon decay during one minute in one liter of air. (2)

U Tube Manometer: A manometer consisting of a U-shaped glass tube partly filled with a liquid of known specific gravity; when the legs of the manometer are connected to separate sources of pressure, the liquid rises in one leg and drops in the other; the difference between the levels is proportional to the difference in pressures and inversely proportional to the liquid's specific gravity. Also known as liquid-column gage. (3)

(1) Copyright 1993-2009 Mark Winter [The University of Sheffield and WebElements Ltd, UK (2) http://www.health.state.ny.us/environmental/glossary/ (3) http://www.answers.com/topic/u-tube-manometer