Year	mmol per litre
1980	5.299
1981	5.289
1982	5.269
1983	5.244
1984	5.219
1985	5.194
1986	5.173
1987	5.149
1988	5.123
1989	5.097
1990	5.073
1991	5.057
1992	5.036
1993	5.012
1994	4.988
1995	4.966
1996	4.946
1997	4.924
1998	4.898
1999	4.877
2000	4.852
2001	4.822
2002	4.791
2003	4.763
2004	4.736
2005	4.709
2006	4.68
2007	4.656
2008	4.632