1) What it means.
Simple version - imagine I have 1 million bottles in my drinks cupboard (which alas I do not ). If one of them is vodka and the rest are gin, then the vodka is "1 part per million". If I go and swap some more gin bottles for vodka, so that I end up with 23 bottles of vodka and 999 977 gin, (so still a million in total), then the vodka is now "23 parts per million" 2) Relating it to other units
Returning to my hypothetical drinks cupboard - or a slightly more realistic version. If it actually contains 100 bottles, of which 85 are gin, 10 are whiskey and 5 are vodka. then I can scale it up to see what I'd get if I had a million in total. To scale 100 bottles up to 1 000 000 bottles, I have to multiply by 10 000. So I need to multiply each of the separate ones by 10 000 too. So that's 850 000 gin, 100 000 whiskey and 50 000 vodka. So now the vodka is 50 000 parts per million 3) Percentages and parts per million
OK, for a bit of variety - imagine a lot of chocolate bars, of which 3.5% are white chocolate If I actually had 100 chocolate bars, then that would mean 3.5 of them white chocolate If I had 1000 chocolate bars, that's scaled up by 10: so 3.5 x 10 = 35 white chocolate. To scale 100 chocolate bars up to 1 000 000, I'd need to multiply by 10 000. So 3.5 x 10 000 would give 35 000 white chocolate bars out of a million. So 35 000 parts per million. 4) Converting percentages to parts per million - general rule A percentage tells you how many "out of a hundred" you have. There are two ways to convert that up to parts per million (whichever method you prefer) (a) Think what you multiply by to scale 100 up to a million (the answer's 10 000). Then multiply the number in the % (eg 2.9%) by that OR
(b) Work out that % of a million. Eg to find 7.5% of a million, you do (7.5/100) x 1 000 000 = 75 000. So that's 75 000 parts in a million 5) Converting percentages to parts per billion
Exactly the same approach is with converting to parts per...