Year	mmol per litre
1980	4.405
1981	4.392
1982	4.377
1983	4.361
1984	4.345
1985	4.329
1986	4.314
1987	4.298
1988	4.284
1989	4.269
1990	4.254
1991	4.239
1992	4.222
1993	4.209
1994	4.191
1995	4.178
1996	4.167
1997	4.154
1998	4.143
1999	4.133
2000	4.124
2001	4.117
2002	4.111
2003	4.105
2004	4.098
2005	4.091
2006	4.085
2007	4.076
2008	4.068