You do not always need to begin with ingredient calculations when classifying an untested chemical mixture.
If you have reliable data from a similar tested mixture, a GHS bridging principle may allow you to use that evidence for the new mixture. However, bridging does not exempt you from classification rules. Each principle has specific requirements, and the appropriate one depends on the hazard class and applicable regulations.
This distinction is important because classification methods must be applied in a specific order.
Under OSHA HazCom, for most health hazard classes the sequence is clear: use complete-mixture test data when available; if the mixture itself is untested, consider the bridging principles designated for that hazard class; and if bridging cannot be applied, use the hazard-specific ingredient method, such as concentration limits or an additivity formula. OSHA makes an exception to that order for carcinogenicity, germ cell mutagenicity, and reproductive toxicity, which are primarily classified from ingredient information unless a case-by-case justification supports whole-mixture data.
OSHA Appendix A sets this order, so it is not just a general best practice.
The key question is not whether you can skip calculations, but whether you have sufficient evidence to apply a specific bridging principle for the hazard class under your market's regulations.
What are GHS bridging principles?
Bridging principles help classify certain untested mixtures by using data from similar tested mixtures and their ingredients. The aim is to use existing evidence rather than treat every new formulation as entirely unknown.
The concept comes from the United Nations Globally Harmonized System of Classification and Labelling of Chemicals. The current UN reference is GHS Rev. 11 (2025). The GHS provides a harmonized framework, but jurisdictions adopt and implement that framework through their own laws and regulations.
Therefore, companies operating in multiple markets need a global GHS compliance strategy that reviews the rules in each target country, rather than assuming a universal process applies.
Where do bridging principles fit in mixture classification?
For most OSHA health hazard classes, bridging is considered after using whole-mixture test data and before ingredient-based estimation.
For example, if your team needs to classify untested Formula B, and you have reliable ingredient data, test results for Formula A, and sufficient details to assess a bridging condition, bridging should be evaluated.
If there is no relevant tested mixture or the evidence does not meet the conditions of an applicable principle, proceed to the ingredient-based method for that hazard class.
For acute toxicity under OSHA Appendix A, this may involve calculating an acute toxicity estimate using the additivity formula. For other health effects, use ingredient classifications and applicable cut-off values or concentration limits.
The six main GHS bridging principles
The six main GHS bridging principles are dilution, batching, concentration, interpolation, substantially similar mixtures, and aerosols. Not all are available for every hazard endpoint.
The following table summarizes these principles using OSHA HCS terminology for practical reference.
| Bridging principle | Core idea | Important condition |
|---|---|---|
| Dilution | Use information from a tested mixture when it is diluted. | The diluent must meet the hazard-specific conditions and must not be expected to change the relevant hazard of the other ingredients. |
| Batching | Use a tested production batch to support another batch of the same mixture. | Production must be sufficiently consistent; if a significant variation could change the hazard, a new classification is needed. |
| Concentration of mixtures | Carry the highest hazard category to a more concentrated version. | Under OSHA this is limited to designated health hazard classes and requires the tested mixture to be in the specified highest category. |
| Interpolation | Classify an intermediate formulation using two tested formulations. | The mixtures must meet the identical-ingredient, same-category, and intermediate-concentration conditions for the applicable hazard class. |
| Substantially similar mixtures | Transfer a classification between two closely comparable mixtures. | The ingredient concentrations and hazard data must satisfy the rule's specific similarity conditions. |
| Aerosols | Use the classification of a tested non-aerosolized mixture for its aerosol form. | The propellant must not affect the relevant hazard, and aerosol bridging must be allowed for that hazard class. |
Dilution: Adding a less hazardous diluent does not necessarily result in a lower classification.
Dilution is often misunderstood. Under OSHA's general bridging provision, a tested mixture may be diluted with a material that has an equivalent or lower toxicity classification than the least toxic original ingredient, provided the diluent is not expected to affect the toxicity of other ingredients. The new diluted mixture is then classified as equivalent to the original tested mixture, or, for acute toxicity, the additivity formula is applied.
That is why "dilution bridging" does not always eliminate the need for calculation. See the dilution provision in OSHA Appendix A.
Simply adding water, a solvent, a carrier, or another diluent is not sufficient to justify bridging. You must still consider how the diluent affects the relevant hazard.
Batching: The formulation and production process must remain meaningfully consistent.
Batching applies when a tested batch and an untested batch represent the same commercial mixture produced under controlled conditions.
Under OSHA, the toxicity of a tested production batch can be assumed to be substantially equivalent to another untested production batch of the same mixture when both are produced by, or under the control of, the same chemical manufacturer, unless there is reason to believe significant variation has changed the toxicity. Canada's Hazardous Products Regulations use the same logic and define a production batch as one from a consistent production process using fixed physico-chemical parameters.
Having the same product name is not sufficient. The classifier must ensure the untested batch is still representative of the tested material.
Concentration: This rule applies only to certain hazard classes and is not a general assumption.
The concentration principle is more limited than assuming that increasing a hazardous ingredient always results in the same classification.
Under OSHA, concentration of mixtures applies only to designated health hazard classes. When a tested mixture is already in Category 1 for an applicable class and the concentration of the Category 1 ingredients is increased, the resulting untested mixture remains Category 1. OSHA Appendix A specifies exactly where this principle is available.
For that hazard, the tested mixture is already in the highest category allowed by the rule. However, the classifier must confirm that the concentration principle is permitted for that hazard class.
Interpolation: The untested formula must fall between two tested formulations in terms of composition and hazard category.
Interpolation is especially useful for product families with controlled formulation ranges.
Under OSHA, the classic case involves three mixtures (A, B, and C) with identical ingredients. Mixtures A and B have been tested and are in the same hazard category. Untested mixture C contains the same toxicologically active ingredients at concentrations that are intermediate to those in A and B. When the hazard-class rules permit interpolation, C can be assigned the same category.
The key is not simply finding a midpoint. You need comparable composition, matching tested categories, and intermediate concentrations for hazardous ingredients.
Canada's Hazardous Products Regulations make this explicit: mixture C must have the same hazardous ingredients as tested mixtures A and B, at concentrations intermediate to those in A and B.
Substantially similar mixtures: The term "similar" has a specific technical definition.
This principle is another area where imprecise wording can create risk.
In OSHA's formulation, the rule compares two mixtures represented as A + B and C + B. Ingredient B must be present at essentially the same concentration, A and C must be present at equivalent concentrations, and toxicity data for A and C must be substantially equivalent. If one mixture is already classified from test data and those conditions are satisfied, the other can receive the same hazard category.
Describing formulas as "pretty close" is not sufficient for bridging. The similarity must meet the exact conditions specified in the rule.
Aerosols: Adding a propellant does not necessarily change the health-hazard classification.
For specified OSHA health hazard classes, an aerosol form of a mixture is classified in the same hazard category as the tested, non-aerosolized form, provided the added propellant does not affect the toxicity of the mixture when spraying.
The principle is not universal. Under OSHA it applies to acute toxicity, skin corrosion/irritation, serious eye damage/eye irritation, respiratory or skin sensitization, and specific target organ toxicity, but not aspiration hazard.
Again, the hazard class determines whether this principle can be applied.
Not all bridging principles apply to every hazard class.
This is likely the most important practical consideration on this topic.
Do not begin by selecting a bridging principle and trying to make it fit. Start with the hazard endpoint, then determine which classification methods are permitted for that endpoint.
Under OSHA Appendix A, for example:
Acute toxicity uses all six bridging principles.
Skin corrosion/irritation and serious eye damage/eye irritation use all six.
Respiratory or skin sensitization uses all six.
Specific target organ toxicity (single and repeated exposure) uses all six.
Germ cell mutagenicity, carcinogenicity, and reproductive toxicity use dilution, batching, and substantially similar mixtures only.
Aspiration hazard uses dilution, batching, concentration, interpolation, and substantially similar mixtures, but not aerosol bridging.
Canada's Hazardous Products Regulations also specify which bridging principles apply to each health hazard class (s. 2.3).
When should you stop and use the ingredient-based method instead?
You should stop using bridging when the evidence no longer supports its application. Common reasons include the following.
There is no suitable tested mixture
Ingredient data alone does not automatically justify bridging. If the rule requires data from a similar tested mixture and you do not have it, proceed to the next classification method for that hazard class.
The formulation changed in a way that breaks the principle
A new ingredient, significantly different concentration, different process, or changed hazard profile can make the tested formulation unsuitable for bridging.
The reference mixtures do not meet the same-category or composition conditions
Interpolation requires more than two similar formulas. The tested mixtures must meet the rule's composition and category requirements, and the untested mixture must fit between them as specified.
The diluent or propellant may change the relevant hazard
Adding a less hazardous diluent or a propellant does not automatically maintain the same classification. The bridging rule still requires scientific justification that the hazard will not change.
The hazard class does not permit that principle
The general list of six principles does not override the specific rules for each hazard endpoint. If a hazard class does not permit a principle, it cannot be used solely based on formula similarity.
Other evidence points to interaction effects
OSHA requires classifiers to consider all available information about possible synergistic effects between mixture ingredients. A less hazardous classification based on antagonistic effects is allowed only with sufficient supporting data. OSHA Appendix A should be used as a decision framework, not as a list of shortcuts.
GHS does not mean there is one identical rulebook worldwide.
The UN GHS is the international framework, but chemical suppliers classify under the legislation that applies in the target jurisdiction.
In the United States, OSHA's HCS provides mandatory health-hazard mixture rules. In Canada, the Hazardous Products Regulations use a tiered mixture-classification process and identify which bridging principles apply to each health hazard class.
The EU CLP Regulation also requires bridging principles to be considered where test data for a mixture are inadequate or unavailable. Current CLP provisions also require a weight-of-evidence determination using expert judgement when more than one similar tested mixture is available for the bridging assessment (CLP Article 9(4), second subparagraph, as amended by Regulation (EU) 2024/2865).
Therefore, do not transfer a classification rationale from one market to another without reviewing local laws and hazard-specific rules.
A practical bridging-principle review
Before applying bridging, ensure you can clearly answer these six questions:
Which hazard class are we classifying?
Does the applicable regulation allow this bridging principle for that hazard class?
What tested mixture or tested batch are we relying on?
Which factual conditions of the bridging rule are satisfied?
Has anything in the formulation, concentration, manufacturing process, or available hazard evidence changed the assumption?
Can another reviewer reproduce the reasoning from the available data?
If any of these answers are unclear, you should stop before proceeding with the classification.
A good classification record should clearly document the evidence and reasoning: the reference formulation, test data, hazard endpoint, chosen bridging rule, comparison with the new mixture, and the final decision. This is much more effective than simply stating, "Bridging applied."
Bridging principles are rules for using evidence, not shortcuts.
GHS bridging principles help classifiers use reliable existing information. They do not eliminate the need for proper classification work.
They shift the question from "What does the ingredient calculation show?" to "Do we already have relevant tested evidence, and does the rule allow us to use it?"
If the answer is yes, bridging can help avoid unnecessary repetition while still supporting a sound classification. If the conditions are not met, proceed to the next method rather than extending a bridging principle beyond what the rules allow.
For chemical teams managing many formulations, the main benefit is not just faster categorization, but the ability to demonstrate which evidence and rules informed the decision. This principle of clear, reviewable logic is also central to how Valenc designs GHS classification software and SDS authoring.