Believe it or not, that's pretty advanced for the EPA, John! A couple of
years ago I came across one of their standards (lead dust?) that used units
of micrograms per square foot. Arghhh.
You might also be surprised to find out how the EPA defines "2.5 micrometer
particulate matter" ("PM_2.5" to them). It basically relates to a
distribution of sizes that are detectable by a certain, specified collector.
This is not entirely new in the world of specifications, but it doesn't mean
what many people would assume on the face of it. I.e., 2.4 um particles are
nearly as "nasty" as are 2.5 um particles.
Jim
On Wednesday, 2002 March 06 1821, kilopascal wrote:
> 2002-03-06
>
> The Environmental Protection Agency set average annual limits at 15
> micrograms per cubic meter in 1997, when it tightened its standards to
> include fine particulate matter - pollutants measuring less than 2.5
> micrometers. That is about 1/28th the width of a human hair.
>
>
>
> That must mean the author figured the human hair is 70 �m thick.
>
> Yes, the need to compare 2.5 �m to the thickness of the human hair is
> somewhat strange, as most people wouldn't know how thick hair is if their
> life depended on it. But, then again, one can see how thick hair is and to
> imagine that something can be 28 times smaller is awe inspiring. At least
> to the author.
>
> Also note the lack of using symbols. Why not write it as 15 �g/m^3?
>
> John
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