Year	mmol per litre
1980	4.22
1981	4.208
1982	4.196
1983	4.184
1984	4.175
1985	4.166
1986	4.16
1987	4.151
1988	4.146
1989	4.14
1990	4.135
1991	4.125
1992	4.118
1993	4.116
1994	4.113
1995	4.11
1996	4.109
1997	4.108
1998	4.105
1999	4.105
2000	4.103
2001	4.102
2002	4.101
2003	4.096
2004	4.094
2005	4.093
2006	4.097
2007	4.103
2008	4.11