** make sure you read to the end - its the whole point!
Some years ago - maybe six - I wrote about mercury in fish - methylmercury - and shared a PPT and maybe even a paper I had written called, Good Fish Bad Fish.  The good is that when we eat fish in place of meat, we lower our levels of saturated fat.  Fish also is the best source for PUFAs and Omega 3s.  It can be bad because our waters are polluted, mostly from coal fired power plants (and other human causes of mercury emissions).  Our fish can be toxic.  For instance, swordfish and shark - even tuna steaks - are too high in methyl-mercury  to be consumed in any more than the rarest of circumstances.  (the bio-degradation process changes the mercury to methyl mercury)

Still many fish types - or the waters where the fish are found - do not have toxic levels.  The EPA among others, tests fish and water and a list is kept of the amount of mercury found in both.  There are many measures of this and that prompted today's post - in a way.  See them below.  Plenty of fish, including salmon and tilapia are very low in  mercury as measured in PPM,  parts per million.  It is recommended that we choose fish with less than .5 PPM of mercury. 
Check the list from the FDA  here. By the way, the NRDC now has a calculator with which you can estimate your mercury exposure by the type, amount and frequency of your fish consumption.  Here is the calculator.

rFd - Reference Dose - the amount set by the EPA that is safe in humans (safe and effective if its a drug)
NOEL - with human or animal subjects-  this is the level at which there are no observable effects
NOAEL - with human or animals - the level at which there are no observable adverse(bad) effects
PPM/PPB - how they measure toxins (here mercury) in fish and water.  Fish safety 1PPM and water is 2PPB (FDA and EPA)
MRL - Minimal Risk Level -this regards the amount of daily exposure that a human can endure without risk of adverse health
AL - Action Level - higher than MRL, signals that something must be done to correct the situation or harm may occur, i.e. the amount may be close to surpassing the rFd

Here is the point for today:When we look at our fish advisories and charts - what does PPM really mean?  Hard to wrap you mind around one part in a million isn't it?  Well - just last week I received my annual report from the city on the quality of my drinking water [yes!  people open and read those things sometimes]
That brochure provided 2  great examples of what PPB means  - yes that is billion, so I took that info to the internet and confirmed the translation into parts per million examples:
 One part per million is the same things as approx:
1 penny out of 10,000
1 inch out of 16 miles
1 minute in two years