Classifying Hazmat Mixtures (On the Dark Side)

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.

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).
 
  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
• T_M = the oral LD₅₀ value of the mixture

When we plug the numbers of Darth Vader’s mixture into the formula, it looks like this:
 
The math works out as follows:

0.4 + 0.6 + 4.0 = 100/T_M

5.0 = 100/T_M

5.0 x T_M = 100

T_M = 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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