Year	mmol per litre
1980	4.497
1981	4.495
1982	4.497
1983	4.493
1984	4.484
1985	4.473
1986	4.462
1987	4.453
1988	4.445
1989	4.441
1990	4.437
1991	4.432
1992	4.431
1993	4.431
1994	4.431
1995	4.431
1996	4.432
1997	4.436
1998	4.438
1999	4.441
2000	4.447
2001	4.454
2002	4.461
2003	4.471
2004	4.481
2005	4.493
2006	4.502
2007	4.511
2008	4.52