further thoughts
being a big fan of radio fours "more or less" i decided to investgate the numbers behind these claims
how are we quantifying a billion? financial markets would say it is one thousand million but we aren't talking money so it must be a scientific billion or a million million or 1 000 000 000 000
now we have two ratios: zero parts per billion against zero parts per million. to compare we would have to multiply both numbers in each ratio by the same magnitude so that would be one million then.
oh err zero multiplied by one million is zero

so my water IS zero parts per billion brilliant
for that reason i'm oot