130 ppm, by my shaky mental math

(I got 130 ppm, which puts me in the clear, as far as the EPA regs go, but I have little confidence in my math skills.)

Hey danf,

I love Engineer's Toolbox, and here's what they say:

weight percent (%wt) = 100 * mass of contaminant/mass of solution (mc/ms)

ppm = 1 million * mc/ms

so ppm = %wt * 10,000

In your case, that would be 130 ppm.

The whole deal is sketchy. Based on some tests done in New Jersey adjacent to some pretty contaminated sites.

But the EPA soil levels for play areas are 400 ppm. This, on top of the fact that the "green" color field material is zero, as is the white. The 130ppm is only on the yellow striping, which is the outline of the soccer field on this surface.

Then the question about whether any of the lead is dislodgeable comes into play.

I feel better now.

Um. Glad someone else could help ya.

Math is hard.

lol, deborah, it's not math that's the hard part, it's the fucking arithmetic that'll eat you alive. Just ask the colleagues I ate lunch with yesterday... it was so bad we finally busted out lauging about LULZ HOW MANY Ph.Ds DOES IT TAKE TO SPLIT A LUNCH TAB, durrr! Feh, our usual culprit from Finance was out sick.

There's nothing mathematical about it. It's a purely linguistic problem. You have per cent (per hundred) and you want to state it in per million. You're not doing anything to the numbers. How many hundreds in a million? About ten thousand. If you want to get numerical, you can write out 1,000,000 (easy enough) and then cross out the number of zeroes in 100 (two). 10,000 has four zeroes, so move the decimal four places to the right: 00.013000 -> 000.13000 -> 0001.3000 -> 00013.000 -> 000130.00 (you get as many free zeroes on either end as you need to hold your place when you're moving decimal points around.

As lfr says, it's the arithmetic that will kill you. No calculation needed here, so you're safe.

The Unix units command (available with Cygwin for Windows) is great for conversions:

$ units
1989 units, 71 prefixes, 32 nonlinear units

You have: 0.013 %
You want: ppm
0.013 % = 130 ppm
0.013 % = (1 / 0.0076923077) ppm

It's probably not wise to argue with an engineer over something like this, but how do you get that "ppm = 1 million * mc/ms", muddgirl? Isn't it 1 million times the volume of the contaminant / volume of the solution? That is, isn't the weight (mass really) taken into account in % by weight but ignored in ppm? When talking about lead (and a presumably much lighter solution) this would make a fair bit of difference.

Bah. I know you got it from the link you posted. I meant, is that web site right to claim that ppm is a mass ratio? I always thought it was volume but I couldn't say why.

Well, PPM is actually a dimensionless quantity. It can be ppm by volume, by mass, by length (used by physicists a lot), or whatever.

I assumed in this case that it was mass, but your right, it could very well need to be ppm by volume. If that's the case then danf would need to divide by the densities.

If I think about it, 1 ppm by volume of iron would be more than 1 ppm by mass (because iron is more dense than paint), so that would actually be a looser requirement? Right?