Physical hazard criteria

An explosive chemical is a solid or liquid chemical capable, by chemical reaction, of producing gas at such temperature, pressure and speed as to cause damage to the surroundings, and this definition does include pyrotechnic chemicals. See Appendix B to 1910.1200—Physical Hazard Criteria (B.1.1.1) for the definition and the explicit inclusion of pyrotechnic chemicals.

Explosives are assigned either to "unstable explosives" or to one of six divisions (1.1 through 1.6) based on the type of hazard they present (e.g., mass explosion, projection, fire hazard or minimal hazard). See B.1.2 in Appendix B to 1910.1200 for the division descriptions and criteria.

Explosives must be classified using the three-step procedure in Part I of UN ST/SG/AC.10 (Test Series 1–7): (1) screening to see if explosive effects exist (Test Series 1), (2) acceptance tests (Test Series 2–4), and (3) assignment to a hazard division (Test Series 5–7). Appendix B requires use of this UN test scheme as incorporated by reference; see B.1.3.1 in Appendix B to 1910.1200 and the incorporation by reference at 1910.6.

Yes — classification of a solid chemical must be based on tests performed on the chemical in the physical form that will be supplied or transported; if the physical form is changed and that change could materially affect test performance, the chemical must be re-tested in the new form. See Note 1 in B.1.3.1 of Appendix B to 1910.1200.

Chemicals that have been wetted with water or alcohols, or diluted/dissolved to reduce explosive properties are classified as desensitized explosives and handled accordingly (see Chapter B.17 reference in the appendix). See Note 2 in B.1.3.1 of Appendix B to 1910.1200.

If the exothermic decomposition energy of an organic material is less than 800 J/g, neither a Series 1 propagation-of-detonation test nor a Series 2 sensitivity-to-detonative-shock test is required. See B.1.3.2 in Appendix B to 1910.1200.

Oxygen balance is calculated using the formula oxygen balance = −1600 [2x + (y/2) − z] / molecular weight for the reaction CxHyOz + [x + (y/4) − (z/2)] O2 → x CO2 + (y/2) H2O, and an organic substance whose calculated oxygen balance is less than −200 is not classified as explosive on that basis. See B.1.3.4(b) in Appendix B to 1910.1200.

An organic substance (or a homogeneous mixture of organic substances) that contains groups associated with explosive properties can still be excluded from classification if its exothermic decomposition energy is less than 500 J/g and the onset of exothermic decomposition is below 500 °C. See B.1.3.2(c) in Appendix B to 1910.1200.

If a mixture contains any known explosives, the acceptance procedure (the UN acceptance tests referenced in Part I of UN ST/SG/AC.10) is required for proper classification. See B.1.3.3 in Appendix B to 1910.1200 and the incorporation by reference at 1910.6.

Explosive properties must be communicated on the SDS in Section 2 (Hazard identification) and Section 9 (Physical and chemical properties), as appropriate. See B.1.3.5 in Appendix B to 1910.1200.

A flammable gas is a gas that has a flammable range with air at 20 °C (68 °F) and 101.3 kPa (14.7 psi). See B.2.1 in Appendix B to 1910.1200 for the formal definition.

Category 1A flammable gases are those that at 20 °C and 101.3 kPa are ignitable at 13% by volume or less in air, or have a flammable range with air of at least 12 percentage points. See Table B.2.1 (Criteria for Flammable Gases) in Appendix B to 1910.1200.

Category 1B gases meet the flammability criteria for 1A but are not pyrophoric or chemically unstable and must also have either a lower flammability limit (LFL) greater than 6% by volume in air or a fundamental burning velocity less than 10 cm/s. See Table B.2.1 in Appendix B to 1910.1200.

No — aerosols should not be classified as flammable gases. See Note 1 under B.2.2 in Appendix B to 1910.1200.

In the absence of data allowing classification into Category 1B, a flammable gas that otherwise meets Category 1A criteria shall by default be classified as Category 1A. See Note 2 in B.2.2 and B.2.3.1 in Appendix B to 1910.1200.

Pyrophoricity should be determined at 54 °C (130 °F) using IEC 60079-20-1 or DIN 51794:2003, and chemical instability should be determined per the UN test methods in Part III of UN ST/SG/AC.10; these test methods are incorporated by reference through 1910.6. See B.2.3.2 and B.2.3.4 in Appendix B to 1910.1200.

If a flammable gas mixture contains more than 1% (by volume) pyrophoric component(s) and lacks data to the contrary, it should be classified as a pyrophoric gas; for mixtures with 1% or less pyrophoric components, expert judgment may be used and testing considered only if that judgment indicates a need. See Note 4 in B.2.2 and B.2.3.3–B.2.3.4 in Appendix B to 1910.1200.

For mixtures of inorganic oxidizing substances with organic material(s), explosive classification considerations allow that the inorganic oxidizer concentration be less than 15% by mass if the oxidizer is Category 1 or 2, and less than 30% by mass if the oxidizer is Category 3, to avoid explosive classification. See B.1.3.4(d) in Appendix B to 1910.1200.

Flammability is to be determined by tests or calculation in accordance with ISO 10156:1996 or ISO 10156:2017, and if fundamental burning velocity is used for Category 1B, use Annex C of ISO 817:2014(E) (or equivalent validated methods if data are insufficient). These ISO standards are incorporated by reference as described in B.2.3.1 of Appendix B to 1910.1200 and through the incorporation mechanism in 1910.6.

Yes — expert judgment may be used to assess the need for classification or additional testing for flammable gas mixtures containing one percent or less pyrophoric components; testing is required only if expert judgment indicates it is necessary. See B.2.3.4 in Appendix B to 1910.1200.

Yes — a Category 1 flammable gas meeting the Hazard Communication definition can be a Process Safety Management (PSM) covered chemical, and when the aggregate on-site quantity in one location meets or exceeds the PSM threshold quantity (10,000 lb), PSM can apply to activities like storage and on-site movement; see Appendix B to 1910.1200 regarding Category 1 flammable gases and the PSM interpretation at https://www.osha.gov/laws-regs/standardinterpretations/2024-06-06 for aggregation and PSM coverage guidance.

An aerosol is a non-refillable receptacle containing a gas compressed, liquefied, or dissolved under pressure, fitted with a release device that ejects the contents as particles in a gas, or as a foam, paste, powder, liquid or gas. See the Aerosol definition in 1910.1200AppB for the exact wording.

An aerosol must be considered for classification in Category 1 or 2 if it contains more than 1% (by mass) components that are classified as flammable (flammable gases, liquids, or solids) or if its chemical heat of combustion is at least 20 kJ/g. See the Aerosol classification criteria in 1910.1200AppB.

An aerosol is Category 1 if it either contains ≥85% flammable components and has a chemical heat of combustion ≥30 kJ/g, or (for spray aerosols) ignition in the ignition distance test occurs at ≥75 cm, or (for foam aerosols) it meets the aerosol foam flammability test criteria (for example, flame height ≥20 cm and duration ≥2 s). See Table B.3.1 in 1910.1200AppB for the detailed criteria.

Chemicals under pressure are liquids or solids pressurized with a gas at ≥200 kPa (gauge) at 20 °C in receptacles other than aerosol dispensers, and they typically contain 50% or more by mass of liquids or solids (whereas mixtures with >50% gases are usually gases under pressure). See the definition of Chemicals under pressure in 1910.1200AppB.

A chemical under pressure is Category 1 if it contains ≥85% flammable components (by mass) and has a heat of combustion of ≥20 kJ/g. See Table B.3.2 in 1910.1200AppB for the exact criterion.

The chemical heat of combustion (ΔHc) for a product is the sum of the heats of combustion of each component weighted by their mass fractions: ΔHc(product) = Σ [w(i) × ΔHc(i)], where w(i) is the mass fraction of component i. Literature values, calculations, or tests (for example, ASTM D240) may be used to obtain component heats of combustion. See B.3.3.2–B.3.3.3 in 1910.1200AppB.

The Ignition Distance Test, Enclosed Space Ignition Test, and Aerosol Foam Flammability Test must be performed in accordance with sub-sections 31.4, 31.5, and 31.6 of the UN ST/SG/AC.10 test protocols (incorporated by reference). See B.3.3.4 in 1910.1200AppB for the reference to the UN test methods.

If such an aerosol is not submitted to the flammability classification procedures in Appendix B, it shall be classified as Category 1. See Note 3 in 1910.1200AppB B.3 for this presumption.

An oxidizing gas is any gas that may, generally by providing oxygen, cause or contribute to combustion more than air does (typically with oxidizing power greater than 23.5% as determined by ISO 10156 methods). Such gases are classified in a single category (Category 1) for this hazard class. See the Oxidizing gas definition and classification in 1910.1200AppB and the incorporation of ISO methods via 1910.6.

Gases under pressure are gases contained in a receptacle at a pressure of 200 kPa (29 psi) gauge or more at 20 °C, or gases that are liquefied or refrigerated; they include compressed, liquefied, refrigerated liquefied, and dissolved gases. See the Gases under pressure definition in 1910.1200AppB.

Flammable liquids are placed into four categories: Category 1 — flash point <23 °C and initial boiling point ≤35 °C; Category 2 — flash point <23 °C and initial boiling point >35 °C; Category 3 — flash point ≥23 °C and ≤60 °C; Category 4 — flash point >60 °C and ≤93 °C. See Table B.6.1 in 1910.1200AppB for the full table and 1910.106(a)(14) for flash point test method references.

The flash point shall be determined using accepted methods such as ASTM D56-05, ASTM D3278, ASTM D3828, ASTM D93-08, or any method specified in [29 CFR 1910.106(a)(14)], or methods in GHS Revision 7, Chapter 2.6. See B.6.3 in 1910.1200AppB and 1910.106(a)(14).

Powdered, granular, or pasty chemicals are classified as flammable solids when the burning time in the specified UN test is less than 45 seconds or the burning rate is more than 2.2 mm/s; metal powders are flammable solids when they ignite and the reaction spreads over the whole sample within 10 minutes. See B.7.2.1–B.7.2.2 in 1910.1200AppB for the test thresholds.

If the chemical will be presented in a physical form different from the one tested and that change is likely to materially alter its performance in classification tests, you must base classification on testing of the chemical in the new form. See Note 1 in B.7 of 1910.1200AppB.

A chemical is not classified as self-reactive if it is already classified as an explosive, an oxidizing liquid or solid (except certain mixtures), an organic peroxide, if its heat of decomposition is less than 300 J/g, or if its SADT for a 50 kg package is greater than 75 °C. See B.8.2.1 in 1910.1200AppB for the full exclusion list.

No—Appendix B states that aerosols should not be classified as flammable liquids, flammable solids, or gases under pressure; aerosols have their own classification rules in section B.3. See B.3 and the notes in 1910.1200AppB.

Flammable components are those components that themselves meet the GHS criteria for flammable gases, flammable liquids, or flammable solids (as detailed in sections B.2, B.6, and B.7). See B.3.2.2.2 in 1910.1200AppB for the specific cross-references.

Yes—if calculations performed in accordance with ISO 10156:1996 or ISO 10156:2017 (incorporated by reference via 1910.6) show a gas mixture is not flammable, no additional testing is required for determining chemical instability for classification purposes. See the opening statement of 1910.1200AppB.

Yes—storing pre-charged appliances that contain a Category 1 flammable gas on site in one location in aggregate quantities at or above 10,000 pounds is a covered "process" and can make the site subject to the PSM standard, 29 CFR 1910.119. OSHA explained this application in its interpretation on PSM flammable gas aggregation (June 6, 2024). See the OSHA interpretation on PSM flammable gas aggregation (June 6, 2024) and 1910.1200AppB for the Category 1 flammable gas definition.

Under 1910.1200 App B B.8.2.3, self-reactive chemicals are sorted into Types A–G based on how violently they react in package or during lab tests. Broadly:

• Type A: Chemicals that can detonate or rapidly deflagrate as packaged.
• Type B: Chemicals with explosive properties that won’t detonate/deflagrate rapidly as packaged but could undergo a thermal explosion in their package.
• Type C: Chemicals with explosive properties that, as packaged, cannot detonate, deflagrate rapidly, or undergo a thermal explosion.
• Type D: Chemicals meeting one of three lab patterns—partial detonation with no violent heating effect, slow deflagration with no violent heating effect, or no detonation/deflagration but a medium heating effect under confinement.
• Type E: Chemicals that neither detonate nor deflagrate and show low or no effect when heated under confinement.
• Type F: Chemicals that do not detonate (even when cavitated), do not deflagrate, show low/no heating effect under confinement, and have low/no explosive power.
• Type G: Chemicals that show no detonation/deflagration, no heating effect under confinement, no explosive power, are thermally stable (SADT 60–75 °C for a 50 kg package), and, for liquid mixtures, are desensitized with a diluent boiling ≥150 °C.

These categorization rules and the laboratory test basis are set out in the classification principles in 1910.1200AppB.

A self-reactive mixture is classified as Type F instead of Type G when it is not thermally stable or the diluent used for desensitization has a boiling point below 150 °C (302 °F).

• Type G requires the mixture to be thermally stable (SADT 60–75 °C for a 50 kg package) and, for liquid mixtures, a desensitizing diluent with boiling point ≥150 °C.
• If those conditions are not met (SADT lower than required or diluent BP <150 °C), the mixture must be classified as Type F.

See the Type F/Type G distinction in 1910.1200AppB.

You may skip the full self-reactive classification procedure when either (a) the chemical lacks molecular groups associated with explosive or self-reactive behavior, or (b) for a single organic substance or homogeneous organic mixture the estimated SADT is >75 °C (167 °F) or the exothermic decomposition energy is <300 J/g.

• Examples of excluded molecular groups are listed in Tables A6.1 and A6.2 of Appendix 6 of the UN recommendations (these are referenced by the OSHA appendix).
• The SADT and energy thresholds are used to screen out materials that are not likely to present self-reactive hazards.

These exceptions are described in 1910.1200AppB.

Yes — if presenting the chemical in a different physical form (for supply or transport) is likely to materially change its performance in classification tests, you must base classification on testing of the new form.

• The appendix explicitly requires re-testing when a change in physical form could alter test performance (for example, changing particle size, concentration, or phase).
• This rule applies to pyrophoric solids/liquids, self-heating materials, and other physical-hazard classes where form affects behavior.

See the requirement to classify based on testing of the chemical in the new form in 1910.1200AppB.

A pyrophoric liquid is one that can ignite within five minutes after contact with air; a liquid meets Category 1 if it ignites within 5 minutes when added to an inert carrier exposed to air or chars/ignites filter paper on contact with air within 5 minutes.

• The classification test is test N.3 from the UN recommendations as referenced in 1910.1200AppB.
• You may omit the formal pyrophoric-liquid test procedure if operational experience in production or handling clearly shows the chemical does not ignite spontaneously on contact with air at normal temperatures (i.e., is stable at room temperature for prolonged periods).

See the definition, test basis, and the experience-based exception in 1910.1200AppB.

Classify a self-heating chemical as Category 1 if the standardized test (N.4) gives a positive result on a 25 mm cube sample at 140 °C (284 °F). Classify it as Category 2 if the 25 mm cube is negative at 140 °C but a 100 mm cube is positive at 140 °C and either the unit volume exceeds 3 m³ OR the 100 mm cube is positive at lower temperatures under specified volume conditions.

Key points from the appendix:

• Category 1: positive result with a 25 mm cube at 140 °C.
• Category 2: negative with 25 mm at 140 °C but positive with 100 mm at 140 °C plus one of: unit volume >3 m³; OR 100 mm positive at 120 °C and unit volume >450 L; OR 100 mm positive at 100 °C.
• Materials with spontaneous-combustion temperatures >50 °C for 27 m³ (or >50 °C for 450 L when assigning Category 1) are not classified as self-heating or not in Category 1, respectively.

See the test criteria and volume/temperature thresholds in 1910.1200AppB.

Chemicals that react with water to emit flammable gases are placed in Categories 1–3 based on reaction vigor, propensity to ignite spontaneously, and the rate of gas evolution measured by test N.5.

• Category 1: Vigorous reaction at ambient temperature with tendency for the produced gas to ignite spontaneously, or a flammable-gas evolution rate ≥10 liters/kg over any one minute.
• Category 2: Readily reacts at ambient temperature with a maximum flammable-gas evolution rate ≥20 liters/kg per hour but not meeting Category 1.
• Category 3: Reacts slowly at ambient temperature with a maximum flammable-gas evolution rate >1 liter/kg per hour but not meeting Categories 1 or 2.
• Also: if spontaneous ignition occurs at any step of the test, the chemical is classified as one that emits flammable gases in contact with water.

These criteria are summarized in 1910.1200AppB. For how a Category 1 flammable gas can affect other standards (for example, Process Safety Management), see OSHA's interpretation discussing aggregation of Category 1 flammable gases at an installation in https://www.osha.gov/laws-regs/standardinterpretations/2024-06-06.

Oxidizing liquids are categorized using test O.2 (chemical mixed 1:1 by mass with cellulose) and comparing the mixture's mean pressure rise time against reference oxidizers: perchloric acid, aqueous sodium chlorate, and aqueous nitric acid.

• Category 1: The 1:1 mixture spontaneously ignites, or its mean pressure rise time is faster than that of a 1:1 mix of 50% perchloric acid and cellulose.
• Category 2: The mixture's mean pressure rise time is less than or equal to that of a 1:1 mix of 40% aqueous sodium chlorate and cellulose, and it does not meet Category 1.
• Category 3: The mixture's mean pressure rise time is less than or equal to that of a 1:1 mix of 65% aqueous nitric acid and cellulose, and it does not meet Categories 1 or 2.

The test method and the comparison criteria are set out in 1910.1200AppB.

Do not apply the oxidizing-solid classification procedure to an organic chemical when it either does not contain oxygen, fluorine, or chlorine, or when those elements are present but are bonded only to carbon or hydrogen. This is stated in the additional classification considerations for oxidizing solids in 1910.1200AppB.

Practical points to help compliance:

• If the organic molecule lacks O, F, and Cl entirely, you can stop—the oxidizing-solid tests in Appendix B are not applicable.
• If O, F, or Cl are present but are bonded only to carbon or hydrogen (for example, in certain organochlorine or organofluorine structures where halogens are not available to act as oxidizers), you also do not apply the oxidizer tests.
• If laboratory test results conflict with practical experience in handling or transportation (for example, field incidents or well-documented reactivity), the guidance in 1910.1200AppB says that judgments based on known experience should take precedence over test results—so document and justify any reliance on experience-based classification.
• If you plan to supply or transport the same chemical in a materially different physical form (e.g., powder vs pellets) that could affect test performance, you must base classification on testing of the chemical in that new form (see the retesting guidance in 1910.1200AppB).

Relevant authority: see the Additional Classification Considerations and related text in 1910.1200AppB and the incorporated references listed via 1910.6.

You must determine whether the material meets the organic peroxide criteria, calculate available oxygen (if relevant), determine the self-accelerating decomposition temperature (SADT) for mixtures, and then classify the material into Type A–G per the Appendix B rules. The Appendix gives specific exemption thresholds and classification rules in 1910.1200AppB.

Key compliance actions and what they mean:

• Check the exemption thresholds: an organic peroxide is generally considered for classification unless it contains not more than 1.0% available oxygen when it contains ≤1.0% hydrogen peroxide, or not more than 0.5% available oxygen when it contains >1.0% but ≤7.0% hydrogen peroxide. Use the available-oxygen formula given in 1910.1200AppB to calculate this value.
• Determine SADT for the substance or mixture: the Appendix requires that SADT be determined in accordance with the UN test methods incorporated by reference; SADT for mixtures must be measured because two stable ingredients can make a less stable mixture (SADT of the mixture controls). See the SADT requirement in 1910.1200AppB and the incorporated UN test methods via 1910.6.
• Classify into Types A–G by the material’s tested properties (detonation/deflagration behavior, thermal explosion potential, and effect when heated under confinement). For example, Type A is any organic peroxide that can detonate or deflagrate rapidly as packaged; Type G requires thermal stability (SADT ≥ 60 °C for a 50 kg package) and use of a diluent with boiling point ≥ 150 °C, per 1910.1200AppB.
• For mixtures, classify the mixture as the same type as the most dangerous ingredient when appropriate, but always verify SADT and test-series results for the final formulation (the Appendix warns that stable ingredients can form a thermally less stable mixture).
• Put the classification and key test results (including SADT and available oxygen where applicable) on the safety data sheet and use them in workplace hazard communication and handling controls.

Relevant authority: see the Organic Peroxides section and SADT/mixing guidance in 1910.1200AppB and the incorporated UN test methods via 1910.6.