Atmospheric Tracer Safety
Questions 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.
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.