Flammable solids: examples, GHS criteria and storage

What is a flammable solid?

OSHA’s classification criteria define the class in two clauses (29 CFR 1910.1200 Appendix B, B.7.1):

“Flammable solid means a solid which is a readily combustible solid, or which may cause or contribute to fire through friction.”

“Readily combustible solids are powdered, granular, or pasty chemicals which are dangerous if they can be easily ignited by brief contact with an ignition source, such as a burning match, and if the flame spreads rapidly.”

The second clause is the useful one. It contains two separate ideas — ignites from brief contact, and the flame spreads rapidly — and a material has to do both. That is why sawdust and cardboard, which are unquestionably combustible and are a real fire load in a warehouse, are not flammable solids in the regulatory sense, while a fine organic powder in a drum is.

The test decides, not an opinion

Classification runs on the UN burning-rate test, Method N.1 (B.7.2.1):

“Powdered, granular or pasty chemicals shall be classified as flammable solids when the time of burning of one or more of the test runs … is less than 45 s or the rate of burning is more than 2.2 mm/s (0.0866 in/s).”

A strip of the powder is laid out, ignited at one end, and timed. Then comes the part that separates the two categories: a wetted zone is placed across the path of the fire, and the question is whether the fire crosses it.

Category	Chemicals other than metal powders	Metal powders	Signal word
1	Wetted zone does not stop the fire, and burning time < 45 s or burning rate > 2.2 mm/s	Burning time ≤ 5 min	Danger
2	Wetted zone stops the fire for at least 4 min, and burning time < 45 s or burning rate > 2.2 mm/s	Burning time > 5 min and ≤ 10 min	Warning

(Table B.7.1; label elements from Appendix C.) Both categories carry the same hazard statement, H228 — Flammable solid, and the same GHS02 flame pictogram. Only the signal word changes.

Two further clauses in the criteria are worth carrying. Metal powders are classified when “they can be ignited and the reaction spreads over the whole length of the sample in 10 min or less” (B.7.2.2) — a separate, slower yardstick, because a burning metal behaves nothing like a burning organic powder. And solids that cause fire through friction are handled by analogy rather than by test: “Solids which may cause fire through friction shall be classified in this class by analogy with existing entries (e.g., matches) until definitive criteria are established” (B.7.2.3). That clause is the reason match-head chemistry sits in this class at all.

How we classify this page’s substances

This is the cleanest class on the site. All 21 substances are anchored by a single hazard statement — H228 — and nothing else. There is no chemical-family path, no documented-reactivity supplement, no judgement call: if the EU harmonised list assigns H228, the substance is here, and if it does not, it is not.

That precision comes at a price, and the page should say so. Coverage is exactly as complete as CLP Annex VI is, and Annex VI is a harmonised list rather than an inventory of everything hazardous. Sulphur, magnesium ribbon, naphthalene flakes and a great many combustible powders behave like flammable solids in a store without carrying a harmonised H228 entry. Absence from this list is not evidence that a powder is safe.

Two other numbers are worth stating plainly:

• 19 of 21 carry Danger, 2 carry Warning — this class is overwhelmingly Category 1, the one where a wetted strip does not stop the fire;
• all 21 carry the GHS02 flame, but 9 also carry GHS08 (health hazard) and 9 carry GHS09 (environmental). For roughly two-thirds of this class, fire is not the only reason it is controlled, and for some of them it is not the main one.

What is actually in this class

The recognisable half of the list is smaller than the total suggests, and more domestic:

Where you meet it	Substance
Match heads, safety matches	tetraphosphorus trisulphide, red phosphorus
Slug and snail pellets	metaldehyde
Camping fuel tablets, resin curing	methenamine (hexamine)
Lubricant and oil additives, pesticides	diphosphorus pentasulphide
Plastics, resin feedstock	1,3,5-trioxane
Catalysts and organometallic intermediates	aluminium isopropoxide, sodium 2-ethylhexanolate, titanium and zinc complexes
Veterinary antibacterial	carbadox

The rest are organometallic catalysts and speciality intermediates with names longer than this sentence — real substances, but not things that arrive on a pallet at a general chemical store.

Red phosphorus is not white phosphorus

The clearest lesson in this class is that the same element appears in two hazard classes, and the storage regimes are opposite.

Red phosphorus (CAS 7723-14-0) carries H228 and H412. CAMEO Chemicals describes it as “not white phosphorus but rather one of the numerous less reactive polymorphic forms of the element”, “much less reactive than the allotropic white phosphorus”, and notes it “catches fire when heated in air to approximately 500 °F”.

White (yellow) phosphorus is pyrophoric. NIOSH records that it “ignites SPONTANEOUSLY in moist air”; CAMEO puts the ignition point at about 30 °C in moist air and notes that “storage under water prevents this reaction”. It belongs with the water-reactives and pyrophorics, not here.

If a delivery note says only “phosphorus”, that is not enough information to store it.

Three of them are also water-reactive

Red phosphorus’s two sulphide relatives — diphosphorus pentasulphide and tetraphosphorus trisulphide — carry H260 alongside H228: in contact with water they release a flammable gas. Diphosphorus pentasulphide also carries EUH029, contact with water liberates toxic gas.

That combination is worth pausing on, because the two hazards pull storage in opposite directions. A flammable solid wants a sprinklered store. A water-reactive solid must not get wet. Where a substance carries both, the water-reactive hazard governs the decision and the fire strategy has to be built around it. Check the individual substance before assuming the class-level answer applies.

Segregation: three hard stops, and they are all the same stop

Of the twelve other storage classes, only three are marked never store with: oxidizers, oxidizing acids and organic peroxides.

They are one prohibition wearing three labels. Each of those classes carries oxygen chemically rather than drawing it from the air, so a fire they feed cannot be starved by smothering — and a flammable solid is the ideal partner for them, because the property that defines it is a large surface area of fuel in a form that ignites from a match. Mixed intimately, an oxidizer and a finely divided fuel stop behaving like a fire and start behaving like a deflagration.

Eight classes sit at keep separate, which is a distance-and-containment answer rather than a prohibition, and one class — flammable liquids — is marked generally compatible, because the control set is identical: exclude ignition sources, exclude oxidizers, contain the spill.

What the law actually gives you, and what it does not

Less than most people assume.

There is no general OSHA storage standard for flammable solids. The standard everyone reaches for, 29 CFR 1910.106, is titled “Flammable liquids” and is bounded by its own definition at (a)(19): “Flammable liquid means any liquid having a flashpoint at or below 199.4 °F (93 °C).” Its paragraphs cover tank storage, piping, container and portable-tank storage, industrial plants, bulk plants, refineries and service stations. No paragraph of it addresses solids. Storage-cabinet quantities, the 60-gallon container limits, the fire-resistance ratings — none of that is written for a drum of powder.

Where a flammable solid is handled as a dust, a different body of enforcement applies. OSHA’s Combustible Dust National Emphasis Program (CPL 03-00-008) defines combustible dust as “a finely divided combustible particulate solid that presents a flash-fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations”, and names metal dusts including aluminium and magnesium among the materials in scope. There is no single combustible-dust standard; the NEP works through the standards that do apply and, where none does, the general duty clause.

The practical consequence is that the controls for this class are mostly derived rather than prescribed: keep the material dry and cool, keep ignition sources and friction away from it, keep the oxidizer classes out of the room, and treat any operation that puts the solid into the air as a separate and more serious hazard than the drum on the shelf.

The flammable pictogram — the flame — is GHS02. Class-level segregation is a starting point rather than a verdict on any individual substance: check SDS sections 7 and 10, or look the substance up in the storage compatibility matrix. The three prohibited classes have their own pages — oxidizers, oxidizing acids and organic peroxides and self-reactives — and the pyrophoric half of the phosphorus story is on water-reactives.