Subpart Z
Hazard classification means you consider only the intrinsic hazardous properties of the chemical itself.
The three basic steps are: identify relevant hazard data; review that data to determine hazards; and decide whether and how the chemical should be classified.
Expert judgment is sometimes required because many hazard classes use semi-quantitative or qualitative criteria that need interpretation.
No—there is no requirement to perform new testing to classify a chemical.
Yes—any test method that is scientifically validated and yields reliable, relevant results may be used for classification.
Yes—if conclusive experimental data from scientifically validated methods show the chemical is not biologically available, it need not be classified.
Yes—epidemiological and occupational experience data must be considered when evaluating human health hazards.
Conflicting data must be evaluated by weighing quality, consistency, and relevance; reliable human data generally take precedence but robust animal data can justify classification when human data are limited.
Yes—if mechanistic or metabolism studies show an effect is not relevant to humans, the classifier may lower the classification or not classify the chemical.
Yes—a single well-designed, scientifically sound animal study with statistically and biologically significant results can be sufficient to justify classification.
Classify a mixture based on (1) test data for the mixture if available; (2) bridging principles for similar mixtures if mixture data are not available; or (3) ingredient-based estimation methods (e.g., cut-off values) if neither mixture data nor bridging are available.
Yes—Carcinogenicity, Germ Cell Mutagenicity, and Reproductive Toxicity require classification based on ingredient information unless a case-by-case justification supports classifying the whole mixture.
Yes—if you have information showing the ingredient’s hazard is present below the cut-off, you must classify the mixture accordingly.
Yes—in exceptional cases conclusive data can demonstrate that an ingredient above the cut-off does not present a hazard in the mixture, allowing classification based on those data.
Antagonistic effects can be used to lower classification only if supported by sufficient data demonstrating reduced hazard in the mixture.
The diluted mixture may be classified as equivalent to the tested mixture if the diluent has equal or lower toxicity and does not affect other ingredients’ toxicity; for acute toxicity you may alternatively use the additivity formula in A.1.3.6.
It is acceptable when the untested batch is produced by or under the control of the same chemical manufacturer and there is no reason to believe significant variation has changed toxicity.
If a tested mixture is Category 1 and the concentration of Category 1 ingredients increases, the untested mixture must be Category 1; if an untested mixture has ingredient concentrations between two tested mixtures that are in the same hazard category, you may interpolate and assign the same category.
Substantially similar mixtures are those that share ingredients with essentially the same concentrations and where the available toxicity data for differing ingredients are substantially equivalent; if one mixture of such a pair is classified based on test data, the other can be assigned the same hazard category.
An aerosol form shall be classified in the same hazard category as the tested non-aerosol form provided the added propellant does not affect toxicity when spraying.
Acute toxicity refers to serious adverse effects (including lethality) after a single or short-term exposure, and substances are placed into four numeric hazard categories for oral, dermal, or inhalation routes using the cut-off criteria in Table A.1.1.
Category limits for oral acute toxicity are based on the acute toxicity estimate (ATE) in mg/kg bodyweight: Category 1 ATE ≤ 5; Category 2 > 5 and ≤ 50; Category 3 > 50 and ≤ 300; Category 4 > 300 and ≤ 2000. See Table A.1.1 in 1910.1200AppA for the official values and footnotes.
Each exposure route has its own ATE ranges for Categories 1–4: for dermal (mg/kg) Category 1 ATE ≤ 50; Category 2 > 50 ≤ 200; Category 3 > 200 ≤ 1000; Category 4 > 1000 ≤ 2000. For inhalation gases (ppmV) Category 1 ATE ≤ 100; Category 2 > 100 ≤ 500; Category 3 > 500 ≤ 2500; Category 4 > 2500 ≤ 20000. For inhalation vapors (mg/L) Category 1 ATE ≤ 0.5; Category 2 > 0.5 ≤ 2.0; Category 3 > 2.0 ≤ 10.0; Category 4 > 10.0 ≤ 20.0. For inhalation dusts and mists (mg/L) Category 1 ATE ≤ 0.05; Category 2 > 0.05 ≤ 0.5; Category 3 > 0.5 ≤ 1.0; Category 4 > 1.0 ≤ 5.0. These ranges are listed in Table A.1.1 in 1910.1200AppA.
Convert 1‑hour inhalation test results to 4‑hour equivalents by dividing by the appropriate factor: divide by 2 for gases and vapors, and divide by 4 for dusts and mists. This conversion method is specified in 1910.1200AppA where inhalation conversion factors are discussed.
For classification purposes: “dust” means solid particles suspended in a gas (usually air); “mist” means liquid droplets suspended in a gas (usually air); and “vapor” means the gaseous form of a substance released from its liquid or solid state. These definitions appear in 1910.1200AppA in the discussion of inhalation routes.
The preferred species are the rat for oral and inhalation acute toxicity testing, and the rat or rabbit for acute dermal toxicity testing. [1910.1200AppA] (https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200#1910.1200AppA) states these preferences and indicates that when multiple species data exist, scientific judgment should be used to select the most appropriate LD50 or LC50.
Yes — human experience (occupational data, accident databases, clinical reports, epidemiology) should be considered as part of a weight‑of‑evidence approach when available. 1910.1200AppA specifically directs classifiers to consider human data along with animal and in vitro evidence.
If available data indicate inhalation effects are due to corrosivity, the classifier must consider whether the chemical is corrosive to the respiratory tract; if the corrosive effect can cause lethality, label the chemical with the "corrosive to the respiratory tract" hazard statement and use the corrosive pictogram. 1910.1200AppA explains how corrosivity is evaluated and how corrosive respiratory effects must be communicated.
Evaluation of respiratory corrosivity may be based on expert judgment using human and animal experience, in vitro data, pH values, data from similar substances, or any other pertinent information. Guidance for this evaluation and its labeling consequences are found in 1910.1200AppA.
Relevant ingredients are those present at concentrations ≥ 1% (weight/weight for solids, liquids, dusts, mists and vapors; volume/volume for gases). An ingredient below 1% must still be considered if there is reason to suspect it will affect the mixture’s acute toxicity. This definition is in 1910.1200AppA A.1.3.3.
No — classification of a mixture for acute toxicity must be carried out for each route of exposure only when relevant evidence indicates toxicity by multiple routes; otherwise, one route may be used provided that route (estimated or tested) is followed for all ingredients. 1910.1200AppA A.1.3.2 describes this requirement and states the pictogram/signal word must reflect the most severe hazard identified.
If acute toxicity test data exist for the complete mixture, classify the mixture using the same criteria as for substances (Table A.1.1). 1910.1200AppA A.1.3.4 directs that tested mixtures be classified according to Table A.1.1.
Calculate the mixture ATE using the additivity formula: 100 / ATE_mix = sum over ingredients of (C_i / ATE_i), where C_i is the concentration (%) of ingredient i and ATE_i is its acute toxicity estimate. The formula and explanation are given in 1910.1200AppA A.1.3.6.1 and the associated figure.
If an ingredient lacks an ATE, first see if a reliable derived conversion value can be obtained (e.g., from extrapolation, human evidence, other assays, or structure‑activity analogs) and then apply the additivity formula; if reliable estimation is not possible and an ingredient ≥1% has unknown acute toxicity, you cannot assign a definitive ATE for the mixture and must follow the provisions in A.1.3.6.2.4. 1910.1200AppA A.1.3.6.2 explains these steps and the need for expert technical information when deriving estimates.
If converted acute toxicity point estimates for all ingredients are within the same category, then the mixture should be classified in that same category. This rule is stated in 1910.1200AppA A.1.3.3(c).
When only range (e.g., LD50 range) or hazard‑category data are available for ingredients, they may be converted to point estimates using the conversion guidance in Table A.1.2 of 1910.1200AppA, and those point estimates can then be used in the additivity calculation described in A.1.3.6.1.
Classify the mixture based only on the known ingredients, but you must use the formula in A.1.3.6.1 when the total of relevant unknown-toxicity ingredients is ≤ 10%.
When unknown-acute-toxicity ingredients total more than 10% of the mixture, adjust the A.1.3.6.1 formula to correct for the percent unknown as shown in the appendix and used to calculate the mixture's acute toxicity estimate.
Use the point estimates given in Table A.1.2 to convert experimentally obtained acute toxicity ranges or hazard categories into Acute Toxicity Estimate (ATE) point values for the classification formulas.
The tiered approach organizes available skin corrosion/irritation information into sequential levels so you can classify based on the highest-quality consistent data or use weight-of-evidence when results conflict.
Yes — reliable, good-quality human data should be given high weight and can be used to classify skin corrosion or irritation.
Subcategories are based on the speed and extent of corrosive responses in validated animal tests (OECD TG 404 criteria summarized in Table A.2.1).
A substance is Category 2 (irritant) when animal test results meet the specific mean-score or persistence criteria described in Table A.2.2.
Yes — validated in vitro/ex vivo methods can be used for classification when the test method is internationally accepted and the substance is within the method's applicability domain; sometimes more than one method is needed.
Yes — observations of skin corrosion/irritation made during acute toxicity studies can be used for classification provided the study conditions (dilutions and species) are relevant.
Yes — in the absence of other information, substances with extreme pH are presumed corrosive: pH ≤ 2 or pH ≥ 11.5 normally leads to Skin Category 1 unless buffering/acid–alkaline reserve indicates otherwise.
Yes — non-test methods can be used on a case-by-case basis if their reliability and applicability are considered carefully.
If in vitro/ex vivo data cannot distinguish between subcategories 1B and 1C, use additional information to differentiate; if no sufficient additional information exists, classify the substance as Category 1 (uncategorized).
You may use read-across when you have sufficiently reliable test data on similar substance(s) and you can justify that the tested substance(s) are similar enough to the untested substance to support the classification—when that justification is adequate, read‑across generally carries more weight than (Q)SAR predictions.
For more on applying initial information in a structured way, consider the tiered approach described in A.2.2.7 which emphasizes human/animal data first, then in vitro/ex vivo data, and then non‑test methods like read‑across.
Use the additivity (ingredient-based) approach: identify the "relevant ingredients" (usually those ≥1%), apply the prescribed weighting rules (10× for some corrosive ingredients below the Category 1 limit), and compare the weighted sums to the cut‑off values in Table A.2.3; if the additivity approach cannot be applied (e.g., strong acids/bases or certain chemistries), use the special rules in Table A.2.4 (including pH criteria).
For the full decision logic and examples, consult the mixture classification section in A.2.3 of the Appendix to 1910.1200.