A means of expressing extremely minute concentrations of substances in water or air. In the United States, the concentration is the number of units of the substance found in 1x10¹² units of water or air; an equivalent unit is nanograms per kilogram. Detection of concentrations this low, which was not possible until the late twentieth century, is limited to only a few types of chemical compounds. For example, one gram of sulfur hexafluoride, used as a tracer in studies of ocean mixing, can be detected in a cubic kilometer of sea water, a concentration of 1,000 parts per trillion.