Year	mmol per litre
1980	4.446
1981	4.434
1982	4.425
1983	4.419
1984	4.414
1985	4.411
1986	4.405
1987	4.401
1988	4.397
1989	4.395
1990	4.392
1991	4.389
1992	4.388
1993	4.387
1994	4.388
1995	4.391
1996	4.396
1997	4.404
1998	4.411
1999	4.418
2000	4.427
2001	4.437
2002	4.449
2003	4.46
2004	4.472
2005	4.484
2006	4.497
2007	4.51
2008	4.523