Year	mmol per litre
1980	5.421
1981	5.404
1982	5.388
1983	5.372
1984	5.355
1985	5.337
1986	5.32
1987	5.305
1988	5.294
1989	5.284
1990	5.275
1991	5.267
1992	5.255
1993	5.24
1994	5.22
1995	5.193
1996	5.171
1997	5.151
1998	5.136
1999	5.117
2000	5.098
2001	5.079
2002	5.061
2003	5.049
2004	5.037
2005	5.032
2006	5.025
2007	5.02
2008	5.016