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Calculating Acute Toxicity Estimates and Determining Hazard Classification

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The contents presented in this article form part of our larger and more comprehensive Acute Toxicity Evaluation Guide, which covers everything from examples of GHS classification for mixtures to the specifics of OSHA’s acute toxicity categorization. We recommend reading the entire guide before performing ATE calculations on your own.

There are often multiple hazardous chemicals at play within a single product, not to mention countless products at almost any given location in your facility. Sometimes it takes rolling up your own sleeves and busting out the calculator to ensure you and your employees are wearing the appropriate personal protective equipment (PPE) to avert dermal, oral, or inhalation exposure to a harmful (if not toxic or lethal) substance.

This article will briefly touch on the definition of acute toxicity estimate (ATE), present the acute toxicity categories, and provide you with an example calculation.

What is the Acute Toxicity Estimate (ATE)?

The regulatory bodies governing worker safety—the HazCom, WHMIS, OSHA, CLP, and GHS—must have access to some sort of concrete data in order to attest to a substance’s capacity to cause harm to a person. Ideally, there aren’t enough cases of accidental human exposure for these regulatory bodies to use as a yardstick, so they turn to in vivo or in vitro experimental testing instead. These tests seek to establish the dose (for oral and dermal exposure) or concentration (for exposure via inhalation) at which 50% of the exposed population dies (after a set amount of time): the LD50 or LC50.

Determining this value allows regulatory scientists to compare the relative toxicity levels of different substances and extrapolate these values to human sizes by scaling up body mass. When the adverse effects (lethal or otherwise) manifest within 14 days of the first instance of exposure, the calculated toxicity level is said to be acute.

By definition, the acute toxicity estimate (ATE) is used to estimate the LD50 or LC50 values of mixtures. That is, the ATE formula is a way of categorizing a mixture’s toxicity within one of several toxicity level boundaries (for humans) predetermined by these experimental tests.

The Globally Harmonized System of Classification and Labelling of Chemicals (GHS), the most standard and internationally recognized classification system, sets the following boundaries:

Acute Toxicity Classification Boundaries GHSSource: United Nations Purple Book – Globally Harmonized System of Classification and Labelling of Chemicals (GHS), eight revised edition, page 117

Whether LD50/LC50 values are available or not, you must calculate the ATE of your mixture to determine if it is fatal, toxic, or harmful to a person that is exposed to it dermally, orally, or via inhalation. You should always check the UN’s Purple Book before settling on a hazard pictogram and statement, as these sometimes differ between exposure routes, but a good rule of thumb is to think of acute toxicity categories 1 and 2 as “lethal”, acute toxicity category 3 as “toxic”, and acute toxicity category 4 as “harmful”. Category 5 is only utilized in specific cases of relatively low acute toxicity hazard.

Acute Toxicity Estimate (ATE) Classification for Mixtures and Blends

The ATE formula below may look daunting, but it is actually quite simple to use. Take the first ingredient in your mixture and divide its concentration (keep the percentage as a number out of 100) by its LD50 or LC50 value. Do that for every ingredient, add those results together into a single fraction, and divide 100 by this value.ATE formula known LD50

Let’s go through an example step by step.

Assume your mixture is composed of five chemicals. You know the LD50 and acute toxicity classification for four of them, and that the fifth one has no known toxicity. To calculate the ATE for, say, dermal exposure, your equation would look like this:

ATE formula four ingredients

Since this is an example, we can give each ingredient a random concentration and ATE value.

ATE formula random concentrations

The next step would be to find the least common denominator for your fractions and convert them all into their numerical equivalents. This is (conveniently) an easy example to convert; the denominators for the first two and last ingredients are factors of the third’s denominator.

ATE formula random concentrations least common denominator

ATE formula calculation

Finally, divide 100 by your fraction.

Dermal ATE formula calculation final

Your mixture would have a dermal ATE of 139 mg/kg of body weight. Since the value lies between 50 mg/kg and 200 mg/kg, you should classify the mixture as acute dermal toxicity category 2.

Acute Toxicity Estimate (ATE) Calculator for Ingredients Without LD50 Values

You will not always be fortunate enough to use products or concoct mixtures composed entirely of ingredients with known LD50 or ATE values. In situations wherein certain substances have an unknown toxicity, you might have to use a different formula (with the same variables):

ATE formula unknown LD50Source: United Nations Purple Book – Globally Harmonized System of Classification and Labelling of Chemicals (GHS), eight revised edition, page 124

For a more detailed explanation of when to use this formula, a thorough example, and general hazard toxicity guidance, download our FREE Acute Toxicity eBook.

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And if you would like to learn more about ERA’s SDS Authoring and SDS Management solutions and how they can classify your chemicals into neat hazard classes automatically, schedule a complimentary consultation call with a project analyst below.

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NOTE: Our Acute Toxicity Evaluation Guide is the first in a series of eBooks on the health, physical, and environmental hazard requirements listed in the GHS Purple Book. We will be writing on all of these subjects, so stay tuned for the rest!