Year	mmol per litre
1980	4.253
1981	4.244
1982	4.234
1983	4.221
1984	4.208
1985	4.195
1986	4.184
1987	4.171
1988	4.159
1989	4.146
1990	4.133
1991	4.121
1992	4.11
1993	4.103
1994	4.095
1995	4.09
1996	4.086
1997	4.085
1998	4.083
1999	4.083
2000	4.084
2001	4.084
2002	4.084
2003	4.087
2004	4.09
2005	4.092
2006	4.096
2007	4.103
2008	4.108