Year	mmol per litre
1980	5.017
1981	5.012
1982	5.007
1983	5.001
1984	4.994
1985	4.985
1986	4.976
1987	4.969
1988	4.951
1989	4.936
1990	4.924
1991	4.918
1992	4.913
1993	4.91
1994	4.909
1995	4.905
1996	4.902
1997	4.904
1998	4.907
1999	4.911
2000	4.914
2001	4.914
2002	4.913
2003	4.911
2004	4.912
2005	4.913
2006	4.916
2007	4.922
2008	4.927