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GHS Purple Book

03 February, 2023

The GHS is described in a book, informally known as the "Purple Book." This book contains information on all the classification classes and categories, and outlines all the requirements for hazard communication. In a series of annexes, it also provides guidance on applying the GHS, including guidance on the preparation of SDSs, consumer product labeling based on the likelihood of injury, and comprehensibility testing methodology. This document is the primary information source on the GHS, but other technical assistance tools have been and will be developed to assist and promote implementation. The Purple Book is revised on a regular basis to incorporate agreed changes and updates to the GHS.

What is the GHS Purple Book?

The United Nations Globally Harmonized System of Classification and Labeling of Chemicals, also known as GHS, is neither a regulation nor a standard. It is a system that provides the basis for individual countries to create GHS regulations that are compatible on a worldwide basis. The GHS Purple Book, sometimes called the GHS Document, provides GHS hazard classification and communication provisions, with explanatory information on how to apply the system. The objective is to provide a system that ensures the safe use of chemicals as they move through their product life cycle from "cradle to grave."

The GHS Purple Book provides the information governments need to create a hazard communication system that is compatible with basic international requirements. This includes providing standard mechanisms for determining whether a chemical or product is hazardous, establishing standard definitions and pictograms, and setting the criteria for creating GHS labels and Safety Data Sheets. You could say the GHS Purple Book provides the regulatory building blocks that are the foundation of national programs that address the classification of chemical hazards and the transmittal of information about those hazards and their associated protective measures.

Where Did GHS Come From?

In 1992 the United Nations Conference on Environment and Development (UNCED), often called the "Earth Summit," adopted a mandate that stated:

"A globally harmonized hazard classification and compatible labelling system, including material safety data sheets and easily understandable symbols, should be available, if feasible, by the year 2000."

The purpose was to strengthen international efforts for environmentally sound management of chemicals. It was recognized that an internationally harmonized approach to classification and labeling would provide the foundation for all countries to develop compatible, comprehensive national programs to ensure the safe use of chemicals.

The International Labor Organization (ILO) studied the tasks required to achieve harmonization. The ILO concluded that there were four major existing systems that needed to be harmonized to achieve a global approach. These were:

  • UN Transport Recommendations
  • U.S. Requirements for Workplace, Consumer, and Pesticides
  • European Union Dangerous Substance and Preparations Directives
  • Canadian Requirements for Workplace, Consumers and Pesticides

The GHS Purple Book was the result. It provides the standards used to harmonize hazard classification and communication around the world. The provisions in the GHS Purple Book were adopted by the UN GHS Sub-Committee in December 2002, and endorsed by the UN Economic and Social Council in July 2003.

To keep the GHS Purple Book up-to-date a Sub-Committee of Experts on the Globally Harmonized System of Classification has been established. They are required to:

  • Act as custodian of the GHS, managing and giving direction to the harmonization process;
  • Keep the GHS up-to-date, including consideration of the need to introduce changes or updates to ensure its continued relevance;
  • Update the GHS Purple Book as needed;
  • Promote understanding and use of the system and encourage feedback;
  • Make the system available for worldwide use;
  • Make guidance available on the application of the system, and on the interpretation and use of technical criteria to support consistency of application;
  • Prepare work programs and submit recommendations to the UNCETDG/GHS.

What is Globally Harmonized System (GHS)?

The GHS is a system for standardizing and harmonizing the classification and labeling of chemicals. Its purpose is to provide a logical and comprehensive approach to:

  • Establishing consistent definitions of the physical, health, and environmental hazards of chemicals.
  • Create classification processes that use existing chemical data to classify chemicals based on standard hazard criteria.
  • Communicate chemical hazard information, as well as protective measures, on labels and Safety Data Sheets (SDS) using a consistent and uniform format, terminology, descriptions, and pictograms.

The first step is to classify chemicals and substances based on the standard hazard definitions in the GHS Purple Book. This involves assigning the chemical to a hazard/danger category. Because in most cases the GHS Purple Book categories are clear and distinct, users can self-classify most chemicals. A decision tree approach, provided in the GHS Purple Book, helps to simplify the classification process. However, in a few cases, the GHS criteria are semi-quantitative, or even qualitative. That means that classifying chemicals that are in those categories generally will require an expert understands the hazards and can interpret the data.

The normal classification process involves looking at the intrinsic hazardous properties of substances and mixtures, and following three steps:

  1. Identification of the relevant data regarding the substance's hazards. The data may be obtained from tests, literature, and practical experience.
  2. Based on the data, identify the hazards that are associated with the chemical or substance.
  3. Determine whether the substance will be classified as a hazardous substance. If so, compare the data with the GHS Purple Book hazard classification criteria to determine the nature of and severity of the hazard.

GHS Purple Book - Bridging Principles

Bridging principles are used to determine the types of hazards that are associated with a substance when sufficient data is not available. Bridging principles use common sense assumptions structured to minimize risk. The following are the six bridging principles in the GHS Purple Book:

  • Dilution: If a mixture is diluted with a diluent that has an equivalent or lower toxicity, then the hazards of the new mixture are assumed to be equivalent to the original.
  • Batching: If a batch of a complex substance is produced under a controlled process, then the hazards of the new batch are assumed to be equivalent to the previous batches.
  • Concentration of Highly Toxic Mixtures: If a mixture is severely hazardous, then a concentrated mixture is also assumed to be severely hazardous.
  • Interpolation within One Toxic Category: Mixtures having component concentrations within a range where the hazards are known are assumed to have those known hazards.
  • Substantially Similar Mixtures: Slight changes in the concentrations of components are not expected to change the hazards of a mixture and substitutions involving toxicologically similar components are not expected to change the hazards of a mixture.
  • Aerosols: An aerosol form of a mixture is assumed to have the same hazards as the tested, non-aerosolized form of the mixture unless the propellant affects the hazards upon spraying.

The appropriate bridging principle must be used for the substance being evaluated. Usually, only one principle applies, but in some cases, two bridging principles may apply to a substance. However, in a few cases, none of the bridging principles apply. When that happens the GHS Purple Book has a procedure for estimating the hazards based on information about the components of the substance.