Reading GHS and CLP

Every hazard figure in this course is quoted from a classification, and a classification is a document with a grammar. This page is the grammar: what the diamond on the bottle means, what the codes beside it mean, and — the part that catches everyone — what a percentage beside a hazard code on an aggregated record is actually a percentage of.

Part II’s lesson on reading a safety data sheet teaches the sheet as a document you use in a room. This page is the lookup table the rest of the course cites: the codes, the categories and the reading rules.

The system, in one paragraph

GHS, the Globally Harmonized System of Classification and Labelling of Chemicals, is a United Nations scheme that sets criteria for classifying health, physical and environmental hazards and specifies what a label and a safety data sheet must carry. It is not law anywhere by itself: jurisdictions implement it. Europe and Great Britain implement it as CLP; the United States implements it in OSHA’s Hazard Communication Standard. The course works from GHS Rev. 11 of 2025, which is the revision PubChem’s reference summary currently reflects, and dates every classification it quotes.

The nine pictograms

Each pictogram stands for a group of hazard classes rather than for a single statement, which is why one diamond can cover several very different reasons for care.

Code	Name	Hazard class	Where this course meets it
GHS01	Exploding Bomb	Explosives	Nowhere in normal practice — but silver nitrate and ammonia can make one by accident, which is on the incompatibility matrix
GHS02	Flame	Flammables	Ethanol, isopropanol, glacial acetic acid
GHS03	Flame Over Circle	Oxidisers	Silver nitrate; the dichromates and uranyl nitrate of the Level D pages
GHS04	Gas Cylinder	Compressed gases	Nowhere; the course uses no compressed gas
GHS05	Corrosion	Corrosives to skin, eyes or metals	Sodium and potassium hydroxide, glacial acetic acid, silver nitrate, sodium sulfide
GHS06	Skull and Crossbones	Acute toxicity	Nothing at Level A or B; it marks the substances taught as history only
GHS07	Exclamation Mark	Irritant	Metol, phenidone, potassium bromide, sodium carbonate, chrome alum — the commonest chip on a photographic shelf
GHS08	Health Hazard	Serious longer-term health hazards	Hydroquinone, metol, formaldehyde
GHS09	Environment	Hazardous to the aquatic environment	Silver salts, metol, hydroquinone — the reason the waste pages exist

Two of them are routinely misread, and the misreading runs in opposite directions. GHS07 is not the mild one: skin sensitisation, H317, sits under it, and sensitisation is permanent once acquired. GHS08 is not the severe version of GHS07: it marks a different kind of harm — carcinogenicity, mutagenicity, reproductive toxicity, organ toxicity on repeated exposure — none of which will hurt you today, which is exactly why it gets discounted.

Signal words

Exactly two exist. Danger marks the more severe categories, Warning the less severe. Only one appears on a label, and where both would apply it is the more severe. A category that warrants neither carries no signal word at all, which is why sodium thiosulfate’s label has none. The signal word is a property of the category, not a judgement added by the supplier: H314 is always Danger, H315 is always Warning.

Hazard statements: the code grammar

An H-code has three digits, and the first says which family the hazard belongs to.

• H2xx — physical hazards. Explosion, flammability, oxidising power, pressure, corrosivity to metals. H272, may intensify fire; oxidizer, is the one this course meets most.
• H3xx — health hazards. Acute toxicity, corrosion and irritation, sensitisation, mutagenicity, carcinogenicity, reproductive toxicity, organ toxicity, aspiration.
• H4xx — environmental hazards. Aquatic toxicity, acute and long-term, and the ozone layer.

A statement never stands alone. It belongs to a hazard class and a category within that class, and the category is what fixes the signal word and the precautionary statements. Here are the statements the course meets most often, with the rest of each row, all from the Rev. 11 summary.

Code	Statement	Class	Category	Signal word
H272	May intensify fire; oxidizer	Oxidising liquids and solids	2 / 3	Danger / Warning
H302	Harmful if swallowed	Acute toxicity, oral	4	Warning
H314	Causes severe skin burns and eye damage	Skin corrosion/irritation	1, 1A, 1B, 1C	Danger
H315	Causes skin irritation	Skin corrosion/irritation	2	Warning
H317	May cause an allergic skin reaction	Sensitisation, skin	1, 1A, 1B	Warning
H318	Causes serious eye damage	Serious eye damage/irritation	1	Danger
H319	Causes serious eye irritation	Serious eye damage/irritation	2A	Warning
H334	May cause allergy or asthma symptoms or breathing difficulties if inhaled	Sensitisation, respiratory	1, 1A, 1B	Danger
H335	May cause respiratory irritation	Specific target organ toxicity, single exposure	3	Warning
H340	May cause genetic defects	Germ cell mutagenicity	1, 1A, 1B	Danger
H350	May cause cancer	Carcinogenicity	1, 1A, 1B	Danger
H400	Very toxic to aquatic life	Aquatic environment, acute	1	Warning
H410	Very toxic to aquatic life with long lasting effects	Aquatic environment, long-term	1	Warning

Notice the last two rows. The two statements that decide almost every disposal argument in this course carry the signal word Warning, not Danger, because the signal word ranks harm to the person holding the bottle and not harm to the river. A label is not a summary of everything that matters about a substance.

!Two statements that look alike and are not

H317 and H334 are both sensitisation, and they are different routes with different consequences. H317 is skin: allergic contact dermatitis, acquired by contact, triggered thereafter by contact. H334 is respiratory: occupational asthma, acquired by inhalation, triggered thereafter by inhalation at doses that affect nobody else. H334 is Danger where H317 is Warning, and H334 is the statement that put platinum on the Level D page.

H318 and H319 are eye damage and eye irritation, and the difference is whether the injury is permanent. It is also the difference between safety glasses and splash goggles on a page’s Required PPE line.

Precautionary statements

A P-code says what to do, and the codes are grouped by when: P1xx general, P2xx prevention, P3xx response, P4xx storage, P5xx disposal. The Rev. 11 summary assigns each hazard statement its own set in four columns — prevention, response, storage, disposal — so a supplier’s selection is traceable rather than arbitrary.

Three are worth recognising on sight.

P280 — Wear protective gloves / protective clothing / eye protection / face protection… The slashes matter: the supplier deletes the options that do not apply. A sheet that says only “wear protective gloves” has made a decision, not truncated a list.

P305+P354+P338 — IF IN EYES: Immediately rinse with water for several minutes. Remove contact lenses if present and easy to do. Continue rinsing. This is the pairing Rev. 11 assigns to H314 and H318, the two serious eye classifications. The older P305+P351+P338, which says rinse cautiously, is still current — for H319. The wording of the advice changed while the chemistry did not, which is what a revision date is for.

P301+P330+P331 — IF SWALLOWED: Rinse mouth. Do NOT induce vomiting. Assigned to H314. It is on the first aid reference as the one ingestion instruction the course quotes, because it is class-specific rather than general.

Rev. 11 also marked several familiar codes obsolete, including P310, immediately call a POISON CENTER or doctor, replaced by P316, get emergency medical help immediately. Obsolete codes are still printed on labels and sheets in circulation, so PubChem keeps them listed and annotated. A code you cannot find in the current list is probably not a misprint.

Reading an aggregated classification

This is the part that goes wrong, and it goes wrong the same way every time.

A classification on a record has one of two origins.

A harmonised classification is legally binding across a jurisdiction. In Europe these are set out in Annex VI of Regulation (EC) No 1272/2008. Silver nitrate, metol, hydroquinone, sodium hydroxide and acetic acid all have one, and nobody gets to disagree with it.

A notified classification is what individual companies have told ECHA they use. ECHA publishes them in the C&L Inventory and PubChem aggregates them, showing what share of the reports carries each statement. It is a survey, and it should be read as one.

✕The percentage is a share of notifiers, not a concentration

A line reading H315: Causes skin irritation — 84.9 % does not mean the substance is 84.9 per cent anything, and it does not mean the hazard applies 84.9 per cent of the time. It means that 84.9 per cent of the reports lodged with ECHA for that substance carry that statement. The denominator is companies, not molecules.

The corollary is the one that matters: a low percentage means thin evidence or a divided market, not a low hazard. A statement carried by 12 per cent of notifiers is a statement that most suppliers did not lodge — which may be because they classify a different grade, or a solution, or because they never looked. It is not a statement that has been measured and found to be one-eighth true.

Two numbers should always be read before the percentages: how many reports, and across how many notifications. They are printed on the record and they are the sample size.

Substance	Reports	Notifications	What the aggregate is worth
Formaldehyde	6,451	102	Near-settled. Only 3 of 6,451 reports say it meets no criteria
Sodium sulfide	1,100	24	Strong for a notified entry: H314 and H400 in 100 per cent
Potassium dichromate	491	19	Close to complete agreement on the severe statements
Chrome alum, as the anhydrous salt	53	5	Small, and a second ECHA record for the same material holds 4 reports, 3 of which say it meets no criteria
Uranyl nitrate hexahydrate	39	2	The smallest behind any classification this course cites; 100 per cent agreement on 39 reports is not the same evidence as 100 per cent on 6,451

The uranyl nitrate row is the point of the table. Unanimity on a tiny sample is a weak claim wearing a strong number.

When the aggregate says a substance is not classified

Some records carry a line saying a share of notifiers report that the substance does not meet GHS hazard criteria. That line is data, and it is not a clearance.

Erythrosin is the course’s worked example. Thirty-seven of thirty-nine notifiers report that it does not meet the criteria — but only about five per cent of companies supplied information at all, and two of the thirty-nine did lodge hazard codes. The honest form of words is therefore “unclassified on a thin evidence base”, and the word “harmless” is not one the record supports. There is no British workplace exposure limit for it either, which under COSHH means exposure is reduced as far as is reasonably practicable with no figure to work to, so dust discipline applies regardless of the absent classification. Part V’s fifth project states it exactly that way.

When two records disagree

Chrome alum is the course’s worked example of the other failure. The dodecahydrate actually sold under that name carries no GHS classification section at all; the notifications sit under the anhydrous salt, under two different EC entries which give different answers on a sample of 53 reports and a sample of 4. The course quotes the larger record, names it as a transfer from the anhydrous salt, and points out that the two salts differ in molar mass by nearly a factor of two — so the classification transfers and the dose does not. The chromium policy sets that out in full.

Exposure limits are a different document

A GHS classification says what kind of harm a substance can do. A workplace exposure limit says how much of it in the air is judged tolerable over a working period, and the two are set by different bodies for different purposes. In Great Britain the limits are in HSE’s EH40, they are legally binding, and they carry four annotations worth knowing:

• Carc — capable of causing cancer and/or heritable genetic damage.
• Sen — capable of causing occupational asthma.
• Sk — can be absorbed through the skin, so an air measurement alone does not describe the dose.
• BMGV — a biological monitoring guidance value exists; the recognised check is a test on the person, not on the air.

Two cautions from EH40 itself belong with every figure the course quotes from it. The Carc, Sen and Sk notations are not exhaustive, so an unannotated entry is not a clearance. And the absence of a substance from the list does not indicate that it is safe — which is the sentence that governs erythrosin, glyoxal and every other reagent for which the course reports no limit.

One divergence a British reader will meet

Since 1 January 2021, chemicals placed on the Great Britain market have been regulated under the assimilated CLP Regulation as amended for Great Britain — GB CLP — with HSE rather than ECHA acting as the agency. Northern Ireland continues under EU CLP through the Northern Ireland Protocol and the Windsor Framework. In practice the classifications are largely the same, but the two lists are maintained separately and can drift, so a British reader should treat an ECHA record as a cross-check rather than as the governing text for a product bought in Great Britain.

How this course uses each

• A harmonised classification is quoted as fact, with the regulation named.
• A notified aggregate is quoted as a survey, with the percentage and the sample size attached.
• An exposure limit is quoted with its source, its averaging period and its notations.
• Neither replaces the sheet for the product you bought. An aggregate over two thousand notifications tells you what the market thinks of a substance; your supplier’s sheet tells you what is in your jar, at what strength, in what grade.
• Where a classification does not exist, the course says that it does not exist and says what follows — never that the substance is unclassified and therefore fine.

=The version fields every claim on this site is checkable against

Every chemical page in this course carries its hazard block with a ref and a verified date, so a claim can be checked against the record as it stood. Do the same for your own materials: product name as printed, supplier, document version or reference from section 1, revision date from section 16, the date you read it, and where the file is. OSHA’s hazard communication standard requires a sheet to carry those sixteen sections in that order and to mark a sub-heading with no relevant information rather than leave it blank, so a sheet with silent gaps is a badly written sheet rather than a substance with nothing to declare.