Year	mmol per litre
1980	4.638
1981	4.629
1982	4.622
1983	4.615
1984	4.609
1985	4.609
1986	4.61
1987	4.605
1988	4.599
1989	4.588
1990	4.58
1991	4.573
1992	4.563
1993	4.552
1994	4.543
1995	4.533
1996	4.526
1997	4.521
1998	4.52
1999	4.518
2000	4.518
2001	4.515
2002	4.508
2003	4.503
2004	4.492
2005	4.489
2006	4.479
2007	4.465
2008	4.453