How to Classify Chemical Hazards: The Bridging Principle

If you’ve looked into GHS SDS conversion beyond surface level, you already know there is much more to it than a simple format change. While the UN GHS standard does include strict guidelines for formatting that may be very different from the way your MSDSs have been created in the past, this is the easiest of the changes to your safety data sheets to make.


Some of the most complex changes to the overall GHS SDSs and OSHA SDSs, among others, come from the hazard classifications every single hazardous chemical requires. Each chemical will have at least one hazard classification, but there is no limit on the number of categories it can potentially fit into except when the maximum number of hazard classes that already exist. With each of these hazard classifications comes a corresponding hazard class pictogram, safety instructions, and precautionary hazard statements. These elements are most important on the safety data sheet’s corresponding chemical label that is applied directly to the outermost container of however the hazardous chemical is stored.


The exact hazard classifications and an overall explanation of the hazardous chemical UN GHS hazard classification process can be found in another of our blog posts here.  But how do we arrive at these classes?


Since there is no testing requirement associated with the new GHS hazard classification system, the most direct way to understand a chemical’s hazards is by using data from past tests. This is relatively straightforward and simple; all we need to do to classify a pure substance is a little research before we find out its hazards and to which classifications it belongs. But mixtures are more complex. Their hazards and toxicities cannot be so easily researched, and so we must rely on what is called a “bridging principle.”


Simply put, bridging principles are the difference between a mixture’s components and its classifications. The GHS specifies six separate bridging principles: Dilution, Batching, Concentration of Highly Toxic Mixtures, Interpolation Within One Toxic Category: Mixtures, Substantially Similar Mixtures, and Aerosols. These bridging principles are combined in different ways and then weighed to calculate if a mixture fits into each hazard classification, or as the United States’ Occupational Health and Safety Administration refers to it in their literature here, into each “health and environmental endpoint,” or not.


As you will see if you reference the link above, the list of hazards is long and the number of bridging principles included in deciding if a mixture fits into a hazard class or not varies incredibly. Done by hand, the process of deciding hazard classifications will be incredibly lengthy. No category can be ruled out, so every mixture must be tested with the corresponding bridging principles for every single hazard class, and precision is important. The process must be completely accurate, because any missteps may lead to overclassification or underclassification, both of which can lead to accidents or injuries when the chemical is in use. Overall, since the exactness of the use of the bridging principles in every case is essential, the conversion process is best left to either professionals or to software, as no other method is guaranteed to be accurate.


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