Year	mmol per litre
1980	4.268
1981	4.25
1982	4.23
1983	4.211
1984	4.194
1985	4.176
1986	4.156
1987	4.137
1988	4.116
1989	4.088
1990	4.057
1991	4.028
1992	3.995
1993	3.963
1994	3.934
1995	3.911
1996	3.891
1997	3.877
1998	3.866
1999	3.86
2000	3.857
2001	3.857
2002	3.858
2003	3.845
2004	3.834
2005	3.823
2006	3.816
2007	3.81
2008	3.805