Year	mmol per litre
1980	4.438
1981	4.421
1982	4.403
1983	4.386
1984	4.372
1985	4.357
1986	4.344
1987	4.33
1988	4.318
1989	4.306
1990	4.295
1991	4.281
1992	4.269
1993	4.261
1994	4.253
1995	4.244
1996	4.238
1997	4.233
1998	4.226
1999	4.221
2000	4.215
2001	4.209
2002	4.203
2003	4.194
2004	4.187
2005	4.182
2006	4.18
2007	4.179
2008	4.181