Year	mmol per litre
1980	4.701
1981	4.698
1982	4.696
1983	4.693
1984	4.689
1985	4.686
1986	4.685
1987	4.683
1988	4.679
1989	4.676
1990	4.676
1991	4.675
1992	4.677
1993	4.68
1994	4.685
1995	4.69
1996	4.678
1997	4.671
1998	4.667
1999	4.666
2000	4.661
2001	4.646
2002	4.622
2003	4.597
2004	4.57
2005	4.546
2006	4.524
2007	4.502
2008	4.482