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Classifying Hazmat Mixtures (On the Dark Side)

Posted on 5/17/2016 by Noah Polsky

When you ship hazmat, classifying your material is a crucial first step. To package, mark, label, and document a hazmat shipment properly for handling and transport, you must know exactly what it is and what hazard(s) it poses.

The DOT Hazmat Shipper Certification Workshop will prepare you to achieve and maintain compliance with the 49 CFR Hazardous Materials Regulations. In June, join us in Houston, Dallas, Pittsburgh, Detroit, and Cincinnati. 

For potentially toxic materials, LD50 is the measurement that determines whether or not the material meets US DOT’s definition of a Division 6.1 hazmat. Often, the LD50 of a given chemical will be found on the chemical’s Safety Data Sheet (SDS). LD stands for “lethal dose,” and 50 represents 50% of a group of rats. LD50 is a measure of the dosage it takes to kill half of the rats. Simply put, the more toxic the chemical, the lower the LD50.

Classifying Toxicity—Pure Substances vs. Mixtures

When you classify a pure substance, the process is straightforward—you compare the SDS LD50 data of your material to the packing group assignment table at 49 CFR 173.133 and choose the corresponding packing group. Sodium cyanide, for example, has an oral LD50 of 5 mg/kg, making it a Division 6.1, PG I. Cadmium oxide has an oral LD50 of 72 mg/kg, so it is classified as a 6.1, PG III. But what if someone is shipping a hazmat mixture with different percentages of each component, and the components each have a unique LD50? How can the shipper determine the overall LD50 of the mixture? Let’s try an example.

Classifying Toxic Hazmat Mixtures on the Dark Side

Darth Vader needs your help! The leader of the Empire must ship some very dangerous hazmat across the galaxy (using the 49 CFR hazmat regulations, of course). Since he is a caring, concerned citizen (the guy has kids, after all), he wants to make sure his material is properly classified. He wants to ship a solid mixture that contains 40% skywalkerium, 30% soloium, 20% Binksium, and 10% other (non-hazmat).
Darth Vader ships toxic hazmat mixture
Individually, the toxicity measurements of each constituent are as follows:
Material Oral LD50 6.1 PG
Skywalkerium 100 mg/kg III
Soloium 50 mg/kg II
Binksium 5 mg/kg I

Vader isn’t sure how to classify his mixture. He thinks the mixture is most likely a 6.1 poison because 90% of its total composition are poisonous materials. Still, he has questions.
  • Should he call it a PG III because its largest component is a PG III?
  • Should he call it a PG I because the Lion instructor from class said something about choosing the most severe packing group in certain situations?
  • Why does Jar Jar Binks get an element named after him? No one even likes Jar Jar Binks.
These are all important questions. Vader knows he can determine the LD50 of his mixture by testing it on the womp rats from planet Tatooine, but this would cause the untimely demise of many womp rats. Vader’s compassionate side is a major force in his life, so he prefers to avoid killing womp rats, if possible. Uncertain how to proceed, he turns to the Hazardous Materials Regulations (HMR) for help.

Luckily for Darth Vader’s conscience, hazmat shippers can use math to calculate overall LD50 of a toxic hazmat mixture instead of testing it on rats.

According to 49 CFR 173.132(c)(3), “If reliable, accurate data [for oral LD50 of a mixture] is not available, apply the formula:

In this LD50 formula:
  • C = the % concentration of constituents A, B, …Z in the mixture
  • T = the oral LD50 values of constituents A, B, …Z
  • TM = the oral LD50 value of the mixture

When we plug the numbers of Darth Vader’s mixture into the formula, it looks like this:
40%/100mg/kg + 30%/50mg/kg + 20%/5mg/kg = 100/TM

The math works out as follows:

0.4 + 0.6 + 4.0 = 100/TM

5.0 = 100/TM

5.0 x TM = 100

TM = 20mg/kg

So, the oral LD50 of the mixture is 20 mg/kg, which makes it a Division 6.1, PG II hazardous material.

This useful formula can be applied to any potentially toxic mixture for which there is not already LD50 data, regardless of how many toxic constituents it contains.

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