 
Atmospheric Tracer Safety
Sulfur HexafluorideQuestions sometimes arise concerning the safety of releasing SF6 into the atmosphere, particularly in an urban environment. SF6 is a colorless, odorless, non-toxic, inert gas that has been commonly used as an atmospheric tracer since the mid-1960s. It is also commonly used as a gaseous insulator in high voltage electrical equipment, as a cover over molten aluminum and magnesium to prevent contact with air, in foam insulation, in gas filled athletic shoes, tennis balls, loudspeakers, shock absorbers, sound-insulating windows, in the semiconductor industry, and in many other applications. SF6 is an extremely stable compound with a long atmospheric lifetime and a low global atmospheric background concentration of approximately 3 parts per trillion (ppt). SF6 has no adverse health impacts at the concentrations typically released for atmospheric studies. The American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value, Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit, and National Institute for Occupational Safety and Health (NIOSH) Recommended Exposure Limit are all 1000 parts per million (ppm), set as a conservative precaution against possible oxygen deficiency in enclosed spaces. During the actual tracer study, a small quantity of SF6 (usually less than 5 to 20 grams per second) is released to provide measurable downwind maximum concentrations less than 1 ppm through the area. The maximum tracer concentrations are therefore about 1,000 times smaller than health agency exposure limits and decrease very rapidly with distance from the tracer source. For any particular release, it is planned that concentrations of SF6 usually would range from approximately 0.1% of the OSHA limit (or 1 part per million) near the source, decreasing to 1 part per billion (or a decrease of over a factor of 1,000) at one mile from the source. The scientists who operate the tracer source and the samplers do not have to take any special precautions such as wearing protective clothing or masks, despite working within the regions of maximum concentrations. SF6 atoms do not chain react with ozone, and therefore do not have the stratospheric ozone depletion impacts associated with conventional chlorofluorocarbon compounds. SF6 is considered to be a greenhouse gas, although its contribution to global warming is small compared to the primary anthropogenic sources: carbon dioxide, methane, and nitrogen oxide. The primary sources of SF6 are circuit breakers, switchgear and other electrical equipment. Recent (2001) U.S. emissions of SF6 compounds are reported to be 4.7 million metric tons (carbon equivalent) and are estimated to contribute 0.12% to the global warming potential. Typical tracer releases yearly add less than 0.0044 x 10-6 million metric tons (carbon equivalent) of SF6, which would represent 2.4x10-4% of the global warming potential of U.S. emissions. Consequently, the contribution of this experiment to the total global warming potential is inconsequential.
Adapted from: Wilkes, B., and L. Ritchie. 2002. Joint Urban 2003 atmospheric dispersion study, Oklahoma City, July 2003, purpose and sponsors of this study. National Nuclear Security Administration. 2 pp. Dunigan, P. F. X. 2003. Categorical exclusion for an urban-scale atmospheric dispersion study in Oklahoma City. Pacific Northwest National Laboratory, Richland, WA. 17 pp. Hicks, B.B. 2004. Categorical exclusion (CE) for an atmospheric dispersion study at the Pentagon in Washington, D.C. NOAA Air Resources Laboratory, Silver Spring, MD. 3 pp.
PerfluorocarbonsPerfluorocarbon tracers (PFTs) comprise a series of compounds that are in the generic class of perfluoroalkylcycloalkanes. These are typically four, five, or six carbon member ring compounds with one, two, or three methyl groups attached to the ring. They are colorless, odorless, biologically inert, non-toxic and non-combustible gases. PFTs have been used worldwide as tracers and in many other applications for decades by both government and private organizations. The tracers themselves have regulatory acceptance and are used commercially (e.g., detecting leaks in underground power cables). The gases are chemically inert and nonflammable, and will be injected at low concentrations (hundreds of ppm range). The compounds are fully fluorinated (saturated with fluorine), contain only carbon and fluorine, and have no unsaturated bonds. For these reasons, PFTs are extremely stable, both chemically and physically. In the pure state, PFTs are clear, slightly viscous liquids, boiling between 45 to 130 C, with a density about 1.75 times that of water. Being chemically extremely stable also makes them biologically very inert. PFTs can be inhaled and ingested with no concern. For example, in the article "The (Liquid) Breath of Life" (Science, Vol. 245, pp. 1043-1045, 8 Sept. 1989), a perfluorocarbon fluid in the lungs of a premature infant can facilitate normal breathing in the immature lung membranes. The article also mentions the use of per-fluorocarbons as artificial blood substitutes (p. 1045) as does the article "Different Paths to a Human Blood Substitute" (NY Times, p. D5, 8 July 1992). Since the PFTs are safe for human exposure at high concentration, there is certainly no concern for their levels in tracer technology applications. PFTs also have no environmental consequence. Their fluorine atoms do not chain react with ozone; therefore PFTs do not affect the ozone layer as do conventional chlorofluorocarbon compounds (CFCs). The sum total ambient air background concentration of all CFCs plus carbon tetrachloride (130 ppt) and methyl chloroform (135 ppt) exceeds 1,000 ppt (parts-per-trillion), whereas the ambient background of all PFTs is about 0.03 ppt, about 30,000 times smaller. In building ventilation studies, for example, the indoor air concentration of PFTs is always 10 times smaller than the total CFC levels that are inhaled continually. Additionally, the amount of PFTs released in tracer studies is a tiny fraction of that used in industry and in other applications annually. There are currently no regulatory restrictions on the use or emission of these gases. For perspective, an atmospheric tracer experiment will use gram quantities of perfluorocarbons, the aluminum refining industry estimated that primary aluminum production resulted in the release of ~8000 tons of CF4 and C2F6 in 2000 (Perfluorocarbon Emissions Reduction Programme 1990-2000, International Aluminium Institute, 2001).
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