sources http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html varies sources http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html sources sources 20 years or may last considerably longer http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html 20 years or may last considerably longer sources http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html http://www.clir.org/pubs/reports/pub121/sec4.html