what is the critical short length of telomeres

-Nature news: Minimum telomere length defined for healthy cells
 --http://www.nature.com/news/2007/071003/full/449515a.html
 --2007 University of Cardiff UK
 --The critical telomere length was 12.8 repeats (of six base pairs) long, they found — any shorter and the chromosomes began to fuse together at their ends
 ---so that's 12.8*6 = 76.8 bp
 ---paper: The nature of telomere fusion and a definition of the critical telomere length in human cells
-according to Marinela Mendez in Maria Blasco's lab the length of critically short telomeres is about 10-15 kb for a mouse.  40 kb is short.  <3 kb is short for humans
-In one paper ([High-throughput telomere length quantification by FISH and its application to human population studies]), they consider <6 kb as short for humans.