Year	mmol per litre
1980	4.356
1981	4.339
1982	4.324
1983	4.311
1984	4.299
1985	4.291
1986	4.285
1987	4.28
1988	4.275
1989	4.272
1990	4.272
1991	4.271
1992	4.271
1993	4.272
1994	4.272
1995	4.273
1996	4.277
1997	4.286
1998	4.298
1999	4.289
2000	4.282
2001	4.277
2002	4.268
2003	4.258
2004	4.244
2005	4.232
2006	4.218
2007	4.209
2008	4.204