Year	mmol per litre
1980	4.991
1981	5.004
1982	5.014
1983	5.026
1984	5.038
1985	5.053
1986	5.064
1987	5.076
1988	5.087
1989	5.102
1990	5.118
1991	5.132
1992	5.152
1993	5.169
1994	5.181
1995	5.192
1996	5.202
1997	5.213
1998	5.224
1999	5.233
2000	5.237
2001	5.246
2002	5.252
2003	5.256
2004	5.247
2005	5.242
2006	5.237
2007	5.235
2008	5.233