Acutely toxic chemicals: GHS categories, LD50 cut-offs, and 565 classified substances

What “acutely toxic” means under GHS

OSHA’s health hazard criteria define acute toxicity as serious adverse health effects — that is, lethality — occurring after a single or short-term oral, dermal or inhalation exposure (29 CFR 1910.1200 Appendix A, A.1.1). Every word of that is doing work.

Single or short-term is what separates this class from the rest of the health hazards. Carcinogenicity, reproductive toxicity and specific target organ toxicity after repeated exposure are all about accumulation over time. Acute toxicity is about one exposure. It is the class that answers the question an emergency responder asks first.

Lethality is the endpoint, and it is why this class is the only one in GHS defined by a numeric threshold rather than a described behaviour. An oxidizer is classified by what it does to a fuel; a water-reactive by what it does with water. An acute toxicant is classified because a measured dose killed half of a test population and that figure fell below a published cut-off. There is no chemistry in the definition at all — which is why this class contains hydrogen cyanide and nicotine and mercury and methanol, substances with nothing else in common.

Substances are allocated to one of four categories by route of exposure. The label elements split the four into two groups, and the split governs almost everything downstream:

Categories	Pictogram	Signal word	Treated as
1, 2 and 3	GHS06 — skull and crossbones	Danger	Acute toxics
4	GHS07 — exclamation mark	Warning	Harmful; handled with irritants

This page, and the storage class behind it, covers Categories 1 to 3. A substance classified only in Category 4 — carrying H302, H312 or H332 — is genuinely hazardous and belongs on a label, but it does not carry the skull, and no storage scheme we are aware of segregates it as a poison. The boundary is an oral acute toxicity estimate of 300 mg/kg bodyweight, and it is worth knowing precisely because it is where a great many industrial substances sit.

The six H-codes: oral, dermal and inhalation

Three routes, two severity bands, six statements. The statements are exact regulatory text and should never be paraphrased on a label:

Code	Statement	Route	Categories
H300	Fatal if swallowed	Oral	1, 2
H301	Toxic if swallowed	Oral	3
H310	Fatal in contact with skin	Dermal	1, 2
H311	Toxic in contact with skin	Dermal	3
H330	Fatal if inhaled	Inhalation	1, 2
H331	Toxic if inhaled	Inhalation	3

A substance is classified separately for each route for which data exist, so a single entry can carry several of these at once. Hydrogen fluoride carries H330, H300 and H310 — fatal by all three routes. Methanol carries H301, H311 and H331 — toxic by all three, fatal by none.

Across the 565 substances in this class, the distribution is not what a first guess suggests:

Statement	Substances carrying it
H301 — Toxic if swallowed	299
H331 — Toxic if inhaled	176
H311 — Toxic in contact with skin	166
H330 — Fatal if inhaled	158
H300 — Fatal if swallowed	138
H310 — Fatal in contact with skin	72

Those figures overlap and do not sum to 565, because most substances carry more than one. The clean split is by severity: 227 of the 565 are Category 1 or 2 by at least one route — the “fatal” band — and the remaining 338 are Category 3 only. The dermal route is the rarest at the fatal end, which reflects how much harder it is for a substance to cross intact skin in a lethal quantity than to be inhaled or swallowed.

How we classify this page’s substances

This class is anchored entirely by GHS hazard statements. Every one of the 565 substances is here because its harmonised classification carries H300, H301, H310, H311, H330 or H331 — none are included on the strength of reactivity data or documented handling experience.

That is a different provenance from most classes on this site, and it cuts both ways. It makes the list unusually defensible: there is no judgement call in the membership, and nothing to re-verify if a source database changes. But it also makes the list complete only with respect to the harmonised list it draws on. A substance that is genuinely acutely toxic but has no harmonised CLP entry will not appear here. Absence from this page is not evidence of safety, and for an unlisted substance the safety data sheet is the only authority.

Two further limits are worth stating plainly. Several harmonised entries are concentration-qualified — formaldehyde and nitric acid both appear with a concentration condition attached, because the classification depends on how strong the solution is. A class list carries the substance; only the SDS carries the concentration you actually have. And a harmonised classification for acute toxicity is a minimum classification: a supplier may hold data supporting a stricter one. Where the label in your hand disagrees with a class list, the label wins.

LD50, LC50 and the ATE cut-offs

The four categories are drawn on measured numbers, and those numbers are public. An acute toxicity estimate is derived from the LD50 for oral and dermal routes, or the LC50 for inhalation, wherever such a value is available. The cut-offs below are the harmonised GHS values, used identically by OSHA HCS and EU CLP (Appendix A, Table A.1.1):

Exposure route	Category 1	Category 2	Category 3	Category 4
Oral (mg/kg bodyweight)	≤ 5	> 5 – 50	> 50 – 300	> 300 – 2000
Dermal (mg/kg bodyweight)	≤ 50	> 50 – 200	> 200 – 1000	> 1000 – 2000
Inhalation — gases (ppmV)	≤ 100	> 100 – 500	> 500 – 2500	> 2500 – 20 000
Inhalation — vapours (mg/l)	≤ 0.5	> 0.5 – 2.0	> 2.0 – 10.0	> 10.0 – 20.0
Inhalation — dusts and mists (mg/l)	≤ 0.05	> 0.05 – 0.5	> 0.5 – 1.0	> 1.0 – 5.0

Two footnotes to this table are routinely missed and both change the answer.

The inhalation cut-offs assume a four-hour exposure. Older data generated over one hour must be converted before it is compared — divided by two for gases and vapours, by four for dusts and mists. Comparing a one-hour LC50 directly against this table will place a substance a category too severe.

The physical form decides which row applies. Gases are judged in ppmV, vapours and dusts in mg/l, and the thresholds differ by orders of magnitude between them. A substance tested as a dust and a substance tested as a vapour are not being measured on the same scale, and the same numeric LC50 means very different things depending on which row it belongs in.

The spread across the table is worth pausing on. An oral Category 1 substance is one where five milligrams per kilogram of bodyweight is enough — for the whole four-category range to span from that to 2000 mg/kg is a factor of four hundred. “Acutely toxic” is not one hazard level. It is a scale, and the category is the part of the classification that tells you where on it you are standing.

Acutely toxic substances and where you meet them

The class is defined by a number, not a chemistry, so the membership is strikingly varied — laboratory reagents alongside bulk industrial feedstocks alongside agricultural products:

Where you meet it	Substance
Water treatment, bleaching	chlorine
Oil and gas, sewers, pulp mills	hydrogen sulphide
Isocyanate and polycarbonate manufacture	phosgene
Metal plating, mining, fumigation	hydrogen cyanide
Glass etching, pickling, semiconductors	hydrogen fluoride
Solvent, fuel, antifreeze, biodiesel	methanol
Resins, disinfectants, laboratory	phenol
Dyes, rubber chemicals, pharmaceuticals	aniline
Epoxy resins, water treatment polymers	epichlorohydrin
Preservative, tissue fixation, resins	formaldehyde
Laboratory reagent, airbag propellant	sodium azide
Wood preservative, glass, alloys	arsenic trioxide
Instrumentation, dentistry, lamps	mercury
Pesticides and biocides	parathion, rotenone, TCMTB, strychnine
Tobacco products, insecticide	nicotine

Two things stand out from this list and both matter operationally.

Many of the worst are gases. Chlorine, hydrogen sulphide, phosgene and hydrogen cyanide are all acute toxics that arrive in cylinders, which means the storage question is really a compressed-gas question — cylinder security, valve protection, ventilation and detection — layered on top of the toxicity.

Several are things nobody files under “poison”. Methanol is a bulk solvent sold by the drum. Phenol is in disinfectants. Formaldehyde is in laboratories everywhere. These are Category 3 substances, so they carry the skull and the word Danger, but they are handled with the familiarity of ordinary chemicals — and familiarity, not ignorance, is what usually precedes an exposure.

Storage: exposure control, not reactivity

Acute toxics have the most unusual compatibility profile of any class on this site, and it is worth reading carefully because it is easy to misinterpret.

No class is marked “never store with”. Eleven of the twelve other classes are marked “keep separate”, and the twelfth — cyanides and sulfides — is compatible, for the plain reason that cyanides and sulfides are themselves acute toxics and belong in the same secured space.

That is not a lenient matrix. It reflects a real difference in what the hazard is. An oxidizer beside a flammable is dangerous because of what the two chemicals do to each other. An acutely toxic substance is dangerous because of what it does to a person, and it does not become more or less toxic for having a base or a mineral acid on the next shelf. The controlling hazard is exposure, not reaction — so the controls that matter are different ones.

In practice that means the priorities for this class are access control, an inventory that matches what is actually on the shelf, ventilation appropriate to the substance and its physical form, secondary containment sized to the largest container, and a documented emergency plan — rather than the fire-separation distances and firewall ratings that dominate storage planning for oxidizers and flammables.

The eleven “keep separate” markings still earn their place, for three reasons that have nothing to do with the toxicity itself. First, many acute toxics carry a second hazard which does react — that is the next section. Second, a spill of something else onto a toxic container is how a sealed hazard becomes an airborne one. Third, segregation makes the inventory legible: a locked cabinet that contains only acute toxics can be audited at a glance, and one that also contains solvents cannot.

At site scale, the class has its own regulatory threshold. Under the Seveso III Directive and its UK transposition in the COMAH Regulations, acute toxicity is named as a qualifying hazard category with quantities that trigger major-accident duties: Category 1 by any route qualifies at 5 tonnes (lower tier) and 20 tonnes (upper tier), and Category 2 by any route, together with Category 3 by the inhalation route, qualifies at 50 and 200 tonnes. Those are among the lowest thresholds in the whole schedule. If your site holds acute toxics in bulk, the classification is not only a labelling question — it may determine which regulatory regime the site sits under.

Toxics that are also something else

The most consequential acute toxics are rarely only acutely toxic, and the second hazard is often the one that dictates where the container physically goes:

• Hydrogen cyanide is acutely toxic by inhalation and also flammable, so it collides with the flammables segregation rules as well as the toxics ones. It is also released by cyanide salts on contact with acid, which is why cyanides and sulfides are kept away from every acid class.
• Hydrogen fluoride is fatal by all three routes and severely corrosive, with a well-documented delayed injury pattern that makes first-aid planning specific rather than generic.
• Chlorine and bromine are acute toxics and oxidizing, which puts them under the oxidizer segregation rules as well.
• Methanol is a Category 3 toxicant by all three routes and a flammable liquid — in most workplaces it is stored as a flammable, and the toxicity is the hazard that gets forgotten.
• Nitric acid above 70% is fatal if inhaled and a strong oxidizing acid, a combination that governs both its storage and its spill response.

Where a substance carries hazards from two classes, the segregation requirement is the stricter of the two, not the more convenient one. That is the case for looking substances up individually rather than filing them by the first pictogram on the drum.

The acute toxicity pictogram for Categories 1 to 3 is GHS06, the skull and crossbones; Category 4 uses GHS07. Class-level segregation is a starting point rather than a verdict on any individual substance — check sections 7 and 8 of the safety data sheet for handling, storage and exposure controls, or look the substance up in the storage compatibility matrix to see its own verdict, its second hazards and its transport class.