Year	mmol per litre
1980	4.108
1981	4.096
1982	4.083
1983	4.071
1984	4.059
1985	4.046
1986	4.029
1987	4.014
1988	4
1989	3.985
1990	3.968
1991	3.954
1992	3.939
1993	3.924
1994	3.907
1995	3.895
1996	3.886
1997	3.881
1998	3.878
1999	3.877
2000	3.877
2001	3.879
2002	3.881
2003	3.885
2004	3.888
2005	3.893
2006	3.898
2007	3.901
2008	3.904