Year	mmol per litre
1980	5.006
1981	4.996
1982	4.985
1983	4.973
1984	4.962
1985	4.951
1986	4.942
1987	4.936
1988	4.935
1989	4.935
1990	4.935
1991	4.937
1992	4.938
1993	4.939
1994	4.939
1995	4.936
1996	4.933
1997	4.931
1998	4.93
1999	4.931
2000	4.933
2001	4.934
2002	4.935
2003	4.935
2004	4.933
2005	4.93
2006	4.927
2007	4.922
2008	4.917