Year	mmol per litre
1980	4.507
1981	4.493
1982	4.477
1983	4.463
1984	4.45
1985	4.436
1986	4.424
1987	4.411
1988	4.395
1989	4.381
1990	4.369
1991	4.359
1992	4.348
1993	4.34
1994	4.334
1995	4.331
1996	4.33
1997	4.331
1998	4.332
1999	4.333
2000	4.334
2001	4.333
2002	4.332
2003	4.333
2004	4.335
2005	4.337
2006	4.342
2007	4.349
2008	4.358