Year	mmol per litre
1980	4.193
1981	4.181
1982	4.169
1983	4.153
1984	4.129
1985	4.104
1986	4.083
1987	4.062
1988	4.044
1989	4.028
1990	4.021
1991	4.005
1992	3.994
1993	3.979
1994	3.963
1995	3.949
1996	3.933
1997	3.918
1998	3.906
1999	3.896
2000	3.89
2001	3.885
2002	3.881
2003	3.879
2004	3.878
2005	3.877
2006	3.876
2007	3.875
2008	3.875