Year	mmol per litre
1980	4.105
1981	4.094
1982	4.084
1983	4.072
1984	4.056
1985	4.041
1986	4.026
1987	4.011
1988	3.999
1989	3.986
1990	3.973
1991	3.961
1992	3.948
1993	3.936
1994	3.925
1995	3.917
1996	3.911
1997	3.908
1998	3.909
1999	3.909
2000	3.91
2001	3.913
2002	3.917
2003	3.919
2004	3.921
2005	3.923
2006	3.924
2007	3.923
2008	3.925