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Imagine you’re eating a juicy swordfish steak and your friend tells you it
contains one part per million of the metal mercury
What does that mean?
If we’re talking about stuff that you don’t want to be eating, like mercury,
, parts per million – or ppm for short –
tells you how much of the bad stuff is in the good stuff,
especially if there’s just a small amount of the bad stuff there.
It’s a measure of concentration of one substance in another.
The trouble with ppm though
is that the value you get depends on what you are measuring.
imagine for instance that you have a rather large pool
containing one million
sea monsters
and with this writhing mass there are four and a half skunks
four and a half because one of them had an unfortunate interspecies encounter.
What then is the concentration of skunks here, in parts per million?
If you measure the number of animals, there are approximately four and a half parts per
million skunks to sea monsters.
But what if you are interested in the relative mass of skunks, rather than their number?
Assuming that each sea monster weights 15 metric tonnes, there are only
o.oo4 parts per million
skunk to sea monster
on a mass for mass basis –
all that’s changed is the way
we’re measuring things,
but the quantity we get in ppm is very different.
On the other hand, if we do the math based on smell
– and you’ll have to trust me here on how much a sea monster smells compared
to an enraged skunk (because believe me, these skunks will be enraged)
– there would be approximately 4,500 parts per million
skunk to sea monster.
So this is the challenge with using parts per million
to measure concentration.
The quantity only makes sense if you know what is being measured.
Fortunately, going back to the swordfish steak,
concentrations of hazardous substances are usually measured on a mass for mass basis.
So one part per million mercury in swordfish
is the same as saying one milligram of mercury for every kilogram of fish.
But it does pay to know what is being compared to what – just in case.
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