Year	mmol per litre
1980	4.888
1981	4.895
1982	4.899
1983	4.897
1984	4.895
1985	4.891
1986	4.883
1987	4.874
1988	4.868
1989	4.864
1990	4.861
1991	4.866
1992	4.872
1993	4.877
1994	4.88
1995	4.885
1996	4.886
1997	4.886
1998	4.883
1999	4.879
2000	4.868
2001	4.856
2002	4.847
2003	4.834
2004	4.82
2005	4.807
2006	4.796
2007	4.786
2008	4.778