Getting to know your toxic end points

How well do you know your RMP chemicals?  I mean really know them.  One of the top questions we have received in the past several years is asking about the RMP chemicals’ toxic endpoints and why EPA uses mg/L instead of ppm.  PPM is a volume-to-volume or mass-to-mass ratio, whereas mg/l is a mass-to-volume relationship.  We can convert from mg/L to ppm pretty easily, using the following equation and knowing the molecular weight of the chemical:

Endpoint (ppm) = [Endpoint (mg/L) x 1000 x 24.5] / [Molecular Weight]

Working Example using Anhydrous Ammonia (NH3):

NH3 has an end-point of 0.14 mg/L and a molecular weight of 17.03.  We can look up the molecular weight in reference tables, but for the safety/chemical geeks, here is a refresher for establishing the molecular weight.  Using the periodic table, we find the molecular weights for Nitrogen and Hydrogen.  We have one (1) Nitorgen and three (3) hydrogens in NH3, so it looks like this:

N = 14.007

H = 1.008 (times 3)= 3.024

14.007 + 3.024 = 17.03

Using the molecular weight of 17.03 in the formula it looks like this

↓

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201.409278

Using this equation we get 201.4 ppm and EPA used rounding to the most significant digit to get 200 ppm.

We also get asked how did EPA come up with these endpoints?

Most end-points use the Emergency Response Planning Guide (ERPG) Tier 2 concentrations, but not all endpoints are based on this.  ERPG-2, established by the American Industrial Hygiene Association (AIHA), is defined as:

the maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual’s ability to take protective action.

See NOAA’s page of Public Exposure Guidelines

Some endpoints are based on the chemical’s Toxic Levels of Concern (LOC)-IDLH, which is used when no public exposure guidelines are defined for a given chemical. As we know, IDLH is a workplace exposure limit that is used primarily for making decisions regarding respirator use.

Some endpoints are based on a LOC-equivalent value that was estimated from 1/10^th of the IDLH.