GHS 101: Classification of Substances and Mixtures

GHS 101: Classification

[For OSHA specific guidance on classifying, visit health hazards and physical hazards]

As we learned in GHS 101: An Overview, the original 1992 Earth Summit mandate had two primary goals related to a harmonized system: 1) harmonized criteria for classifying substances and mixtures according to their health environmental and physical hazards; and 2) harmonized hazard communication elements.

The IOMC divided work on the scope of these two elements into three parts, with each part developed by a different committee or organization:

  1. Classification criteria for health and environmental hazards - Developed by the Organization for Economic Cooperation and Development (OECD)
  2. Classification criteria for physical hazards - Developed by the United Nations Subcommittee of Experts on the Transport of Dangerous Goods (UNSCETDG) and the International Labour Organization (ILO) working group
  3. Hazard communication elements (including SDSs and labels) - Developed by the ILO

Three primary tasks for teams working on harmonizing classification criteria:

  1. Compare major classification systems and identify similar or identical elements as well as develop consensus on elements that were dissimilar
  2. Examine scientific basis for criteria which define hazard class of concern (e.g. acute toxicity, carcinogenicity) and gain expert consensus on the test methods, data interpretation and level of concern and consensus on the criteria
  3. Develop consensus on the process or the scheme for using criteria when there is a decision tree approach or where there is a dependant criteria in the classification scheme

As you can see, classification is the starting point for hazard communication.

What Exactly Does the GHS Cover?

For classification, the GHS applies to pure substances and their dilute solutions and to mixtures. It does not apply to “articles” as defined by OSHA.

Substances means chemical elements and their compounds in the natural state or obtained by any production process, including any additive necessary to preserve the stability of the product and any impurities derived from the process used, but excluding any solvent, which may be separated without affecting the stability of the substance or changing its composition.

Mixtures means a mixture or solution composed of two or more substances in which they do not react.

"Article" means a manufactured item other than a fluid or particle: (i) which is formed to a specific shape or design during manufacture; (ii) which has end use function(s) dependent in whole or in part upon its shape or design during end use; and (iii) which under normal conditions of use does not release more than very small quantities, e.g., minute or trace amounts of a hazardous chemical (as determined under paragraph (d) of this section), and does not pose a physical hazard or health risk to employees.

Alloy means a metallic material, homogeneous on a macroscopic scale, consisting of two or more elements so combined that they cannot be readily separated by mechanical means. Alloys are considered to be mixtures for the purposes of classification under the GHS.

The GHS is meant to be simple and transparent with a clear distinction between classes and categories in order to allow for self-classification.

When classifying hazards, only the intrinsic hazardous properties of substances or mixtures are considered. The data used for classification may be obtained from tests, literature and practical experience.

The process starts by identifying relevant information regarding the hazards of a substance or mixture; that information is reviewed, and a decision is made whether the substance or mixture should be classified as hazardous and the degree of the hazard.

How to Classify Mixtures for the GHS:

  • Test data for that mixture is used when available
  • If no test data exists, bridging principles can be applied
    • Bridging principles work by taking the available test data for the substances and/or ingredients that make up the mixture, and using it to classify the mixture
  • If no test data exists, and bridging principles will not work, then each hazard in the official GHS book has information on estimating the hazard of a mixture

Additional Testing Considerations

The GHS does not include requirements for testing substances. Tests that are carried out should be done so in a scientific and reproducible manner.

The criteria for determining health and environmental hazards are test method neutral – allowing for different approaches as long as they are scientifically sound.

Test methods for determining physical hazards are generally more clear-cut and are specified in the GHS.

One goal of the GHS is to reduce duplicative testing and animal testing, and it allows test data already generated for the classification of chemicals under existing systems to be used when classifying the same chemicals for GHS.

When this creates conflict between an existing system and GHS, expert judgment is needed.

Furthermore, tests that do not require the use of live animals are preferred to those using sentient live experimental animals.

Testing on humans solely for hazard identification purposes is generally not acceptable; nevertheless, epidemiological data and experience on the effects of chemicals on humans should be taken into account in the evaluation of human health hazards of chemicals.

The quality and consistency of the data are important. For some hazard classes, classification results directly when the data satisfy the criteria.

For others, classification of a substance or a mixture is made on the basis of the total weight of evidence.

There are specific considerations for the classification of mixtures.

Some substances react slowly with atmospheric gases, e.g. oxygen, carbon dioxide, water vapour, to form different substances, or react very slowly with other ingredients of a mixture or self polymerise; however, substances produced by such reactions are typically sufficiently low that they do not affect classification of the mixture.

Cut-off Values and Concentration Limits

There may be cases where the cut-off values or concentration limits used for untested mixtures based on hazards of its ingredients do not adequately convey the identifiable hazard the mixture poses. If a classifier has information that the hazard of an ingredient will be evident below the cut-off value or concentration limit, then the mixture should be classified accordingly.

Conversely, if data shows hazard of an ingredient will not be present at level above GHS cut-off, mixture could be classified according to those data. In such cases, data should be retained and made available for the use of values other than the GHS cut-off and concentration limits.