GHS Classification and Labeling

Why GHS Classification and Labeling Matters

Chemicals cross borders, supply chains, and workplaces every day. Before the Globally Harmonized System (GHS), a single chemical could be labeled as toxic in one country, harmful in another, and carry no warning at all in a third. Different test methods, different cut-off values, and different symbols created confusion that led to real exposures, spills, fires, and environmental releases. GHS was developed by the United Nations to create one consistent, science-based system for classifying chemical hazards and communicating them through labels and Safety Data Sheets (SDS).

For the Certified Safety Professional (CSP), GHS knowledge sits at the intersection of several exam domains: Environmental Management (ecological hazards, waste characterization, spill response), Hazard Communication, Fire Protection, Industrial Hygiene, and Emergency Response. In the United States, OSHA adopted GHS through the revised Hazard Communication Standard (HCS 2012, 29 CFR 1910.1200), and later aligned the standard more closely with GHS Revision 7 in a 2024 final rule. The CSP exam expects you to read a label, identify the hazard class and severity, interpret pictograms and signal words, locate information in an SDS, and understand how GHS interacts with other systems such as NFPA 704, HMIS, DOT transport labels, and EPA regulations.

What GHS Is

GHS is the Globally Harmonized System of Classification and Labelling of Chemicals, published by the UN in a document commonly called the Purple Book. It is not a law by itself; it is a framework that individual countries adopt into their own regulations. GHS has two core functions:

1. Classification: defining the hazards of a substance or mixture using standardized criteria and test data.
2. Communication: conveying those hazards through standardized label elements and a 16-section SDS format.

GHS uses a building block approach, meaning a country may adopt some hazard classes or categories and not others. This is why OSHA's version differs slightly from the UN model (for example, OSHA does not enforce environmental hazard classes and did not adopt Acute Toxicity Category 5).

The Three Hazard Groups

Physical hazards (OSHA recognizes 16 classes): explosives, flammable gases, aerosols, oxidizing gases, gases under pressure, flammable liquids, flammable solids, self-reactive substances, pyrophoric liquids, pyrophoric solids, self-heating substances, substances which in contact with water emit flammable gases, oxidizing liquids, oxidizing solids, organic peroxides, and corrosive to metals. OSHA also added combustible dust, pyrophoric gas, simple asphyxiant, and Hazards Not Otherwise Classified (HNOC).

Health hazards (10 classes): acute toxicity (oral, dermal, inhalation), skin corrosion/irritation, serious eye damage/eye irritation, respiratory or skin sensitization, germ cell mutagenicity, carcinogenicity, reproductive toxicity, specific target organ toxicity single exposure (STOT-SE), specific target organ toxicity repeated exposure (STOT-RE), and aspiration hazard.

Environmental hazards: hazardous to the aquatic environment (acute and chronic) and hazardous to the ozone layer. These are important for the environmental management domain. OSHA does not enforce these because they fall under EPA jurisdiction, but manufacturers frequently include them on labels and SDSs, and the exam may test your understanding of them.

How Classification Works

Within each hazard class, substances are divided into hazard categories based on severity. The critical rule: Category 1 is the MOST severe, and higher numbers are less severe. This is the opposite of NFPA 704, where 4 is the most severe. Exam writers love this contrast.

Examples of classification criteria you should know:

Acute toxicity (oral LD50): Category 1 = 5 mg/kg or less; Category 2 = greater than 5 to 50 mg/kg; Category 3 = greater than 50 to 300 mg/kg; Category 4 = greater than 300 to 2,000 mg/kg; Category 5 (not adopted by OSHA) = greater than 2,000 to 5,000 mg/kg. Categories 1 through 3 use the skull and crossbones; Category 4 uses the exclamation mark.

Flammable liquids: Category 1 = flash point below 23°C (73.4°F) AND initial boiling point at or below 35°C (95°F); Category 2 = flash point below 23°C AND boiling point above 35°C; Category 3 = flash point at or above 23°C and at or below 60°C (140°F); Category 4 = flash point above 60°C and at or below 93°C (199.4°F). Note that GHS Category 4 overlaps what the old OSHA standard called combustible liquid.

Skin corrosion/irritation: Category 1 (subcategories 1A, 1B, 1C) is corrosive and uses the corrosion pictogram and the signal word Danger; Category 2 is irritant and uses the exclamation mark with Warning.

Serious eye damage/irritation: Category 1 = irreversible damage (corrosion pictogram, Danger); Category 2A = reversible irritation (exclamation mark, Warning); Category 2B = mild irritation (no pictogram, Warning).

Aquatic toxicity (environmental): Acute Category 1 applies when the 96-hour LC50 (fish), 48-hour EC50 (crustacea), or 72/96-hour ErC50 (algae) is 1 mg/L or less. Chronic categories 1 through 4 consider toxicity, bioaccumulation potential (log Kow or BCF), and rapid degradability. The environment pictogram (dead fish and tree) applies to Acute 1 and Chronic 1 and 2.

Mixtures: When a mixture has not been tested as a whole, classifiers use bridging principles (dilution, batching, concentration of highly hazardous mixtures, interpolation, substantially similar mixtures, aerosols) or additivity formulas and cut-off values. Key cut-offs: generally 1% for most health hazards, but 0.1% for carcinogens, germ cell mutagens, reproductive toxicants, and respiratory sensitizers. Ingredients present at or above these cut-offs must be disclosed in SDS Section 3.

How Labeling Works: The Six Required Label Elements

Every shipped container must carry a GHS-compliant label with:

1. Product identifier: the name or number that matches the SDS.
2. Signal word: either Danger (more severe) or Warning (less severe). Only one signal word appears; if any hazard warrants Danger, Warning is omitted.
3. Hazard statements: standardized phrases describing the nature and degree of hazard, coded as H-statements. H2xx = physical hazards, H3xx = health hazards, H4xx = environmental hazards (for example, H225 Highly flammable liquid and vapour; H301 Toxic if swallowed; H400 Very toxic to aquatic life).
4. Pictograms: black symbol on a white background inside a red diamond-shaped (square set on point) border.
5. Precautionary statements: coded as P-statements. P1xx = general, P2xx = prevention, P3xx = response, P4xx = storage, P5xx = disposal.
6. Supplier identification: name, address, and telephone number of the manufacturer, importer, or responsible party.

The Nine GHS Pictograms and What They Mean

Flame: flammables, pyrophorics, self-heating, self-reactives, organic peroxides, emits flammable gas with water.
Flame over circle: oxidizers (gases, liquids, solids).
Exploding bomb: explosives, self-reactives (Types A and B), organic peroxides (Types A and B).
Gas cylinder: gases under pressure (compressed, liquefied, refrigerated liquefied, dissolved).
Corrosion: skin corrosion, serious eye damage, corrosive to metals.
Skull and crossbones: acute toxicity Categories 1, 2, and 3 (fatal or toxic).
Health hazard (silhouette with starburst on chest): carcinogen, mutagen, reproductive toxicity, respiratory sensitizer, STOT, aspiration hazard.
Exclamation mark: irritant (skin and eye), skin sensitizer, acute toxicity Category 4 (harmful), narcotic effects, respiratory tract irritation, hazardous to ozone layer.
Environment (dead fish and tree): acute and chronic aquatic toxicity. Not mandatory under OSHA, but required under EPA and in many other jurisdictions.

Pictogram precedence rules (frequently tested): If the skull and crossbones appears, the exclamation mark is not used for acute toxicity. If the corrosion pictogram appears, the exclamation mark is not used for skin or eye irritation. If the health hazard pictogram appears for respiratory sensitization, the exclamation mark is not used for skin sensitization or skin/eye irritation. For transport, if a DOT hazard label addresses the same hazard, the corresponding GHS pictogram may be omitted from the shipped package.

Workplace Labeling

Shipped containers must have the full GHS label. For workplace containers (secondary containers filled on site), OSHA allows either the full GHS label or an alternative system such as NFPA 704 or HMIS, provided the system conveys the same information, is consistent with the SDS, and employees are trained to understand it. Portable containers for immediate use by the person who filled them do not require labels. Labels must be legible, in English, and prominently displayed; other languages may be added.

The 16-Section Safety Data Sheet

GHS standardized the SDS into a fixed order. Memorize it:
1. Identification
2. Hazard(s) identification
3. Composition/information on ingredients
4. First-aid measures
5. Fire-fighting measures
6. Accidental release measures
7. Handling and storage
8. Exposure controls/personal protection
9. Physical and chemical properties
10. Stability and reactivity
11. Toxicological information
12. Ecological information
13. Disposal considerations
14. Transport information
15. Regulatory information
16. Other information (including date of preparation or last revision)

OSHA does not enforce Sections 12 through 15 because they fall under the jurisdiction of other agencies (EPA, DOT), but they must still be present in the SDS. For environmental management questions, Section 12 (ecotoxicity, persistence, bioaccumulation, mobility in soil) and Section 13 (waste handling, RCRA considerations) are especially relevant. Section 6 (spill response) and Section 14 (UN number, proper shipping name, DOT hazard class, marine pollutant status) also connect directly to environmental and emergency planning.

Relationship to Other Systems

NFPA 704: the diamond with blue (health), red (flammability), yellow (instability), and white (special) quadrants rated 0 to 4, where 4 is most severe. It is designed for emergency responders viewing a building or tank, not for container-level hazard communication. Rating direction is opposite to GHS categories.

HMIS: a color-bar system (blue, red, orange/yellow, white for PPE) rated 0 to 4, used for workplace labeling.

DOT/UN transport: uses nine hazard classes with distinct placards and labels on packages and vehicles. Transport classification is separate from GHS, although GHS Section 14 reports the transport information.

EPA: governs environmental hazard communication, pesticide labeling under FIFRA (which has its own signal words: Danger, Warning, Caution), and RCRA waste characterization. GHS environmental classes feed into these programs but are not OSHA-enforced.

Exam Tips: Answering Questions on GHS Classification and Labeling

Tip 1: Lock in the severity direction. In GHS, Category 1 is the most hazardous. In NFPA 704 and HMIS, 4 is the most hazardous. If a question describes a substance with an oral LD50 of 3 mg/kg, it is Acute Toxicity Category 1, signal word Danger, skull and crossbones. Do not let NFPA habits pull you toward a higher number.

Tip 2: Match pictograms to hazard classes, not to effects you assume. The health hazard pictogram covers chronic and target organ effects (carcinogen, mutagen, reproductive, sensitizer, STOT, aspiration). The exclamation mark covers less severe, generally reversible effects (irritation, skin sensitization, Category 4 acute toxicity, narcotic effects). The skull and crossbones is reserved for Categories 1 through 3 acute toxicity. If a question asks which pictogram applies to a respiratory sensitizer, the answer is the health hazard pictogram, not the exclamation mark.

Tip 3: Remember the signal word hierarchy. Only one signal word appears on a label. Danger always overrides Warning. Some low-severity categories have no signal word or pictogram at all (for example, Eye Irritation 2B has Warning but no pictogram; Flammable Liquid Category 4 has Warning and no pictogram).

Tip 4: Know the six label elements cold. Questions often list five correct elements and one distractor such as hazard rating number, CAS number, or date of manufacture. The six are product identifier, signal word, hazard statements, pictograms, precautionary statements, and supplier identification.

Tip 5: Decode H and P statement numbers. H2xx = physical, H3xx = health, H4xx = environmental. P2xx = prevention, P3xx = response, P4xx = storage, P5xx = disposal. If a question shows H410, you know immediately it is an environmental hazard (very toxic to aquatic life with long lasting effects) and that the environment pictogram applies.

Tip 6: Memorize the SDS section order and the OSHA non-enforcement rule. Expect questions such as: Where would you find spill cleanup procedures? (Section 6). Where is the exposure limit and recommended PPE? (Section 8). Where is ecotoxicity data? (Section 12). Which sections does OSHA not enforce? (12 through 15). Which section contains transport information? (14).

Tip 7: Distinguish shipped containers from workplace containers. A shipped container always needs the full GHS label. A workplace secondary container may use GHS, NFPA 704, HMIS, or another system as long as the information is consistent and employees are trained. Portable containers for immediate use by the person who filled them need no label.

Tip 8: Know what OSHA did NOT adopt. OSHA did not adopt Acute Toxicity Category 5, Flammable Liquid Category 4 was adopted but has no pictogram, and environmental hazard classes are not enforced by OSHA. If a question asks which hazard group is outside OSHA's HCS enforcement, the answer is environmental hazards (EPA jurisdiction).

Tip 9: Apply the flash point and boiling point thresholds precisely. The dividing lines are 23°C (73.4°F) for flash point between Categories 2 and 3, 60°C (140°F) between Categories 3 and 4, and 35°C (95°F) boiling point between Categories 1 and 2. A liquid with a flash point of 30°C is Category 3 (Warning, flame pictogram). A liquid with a flash point of 10°C and boiling point of 25°C is Category 1 (Danger).

Tip 10: For mixture questions, think cut-off values. A carcinogen at 0.2% must be classified and disclosed (threshold 0.1%). A skin irritant at 0.5% generally does not trigger classification (threshold 1%). Bridging principles let you classify untested mixtures based on tested similar mixtures.

Tip 11: Watch for the pictogram precedence rules. If a product is a Category 1 skin corrosive and also a Category 2 eye irritant, only the corrosion pictogram appears; the exclamation mark is dropped. Similar logic applies to skull and crossbones over exclamation mark for acute toxicity.

Tip 12: Connect GHS to environmental management. Expect scenario questions that pull SDS data into spill response, waste characterization, or stormwater protection. A chemical with H400/H410 statements and the environment pictogram signals that releases to storm drains or surface water must be prevented, that it may be a DOT marine pollutant (check Section 14), and that disposal (Section 13) must follow RCRA or state hazardous waste rules.

Tip 13: Use process of elimination on scenario questions. Identify the hazard class first, then the category, then derive the signal word and pictogram. Most GHS questions can be solved in this three-step sequence even if you do not immediately recognize the answer.

Tip 14: Remember the regulatory anchors. GHS comes from the UN (Purple Book). OSHA implements it through 29 CFR 1910.1200 (HCS 2012, updated in 2024 toward GHS Revision 7). Employers must have a written hazard communication program, maintain SDSs accessible to employees, ensure containers are labeled, and train employees on label elements and SDS format.

Quick Reference Summary

Category 1 = most severe. Danger beats Warning. Nine pictograms, red diamond border, six label elements, sixteen SDS sections. H2 physical, H3 health, H4 environment. P2 prevention, P3 response, P4 storage, P5 disposal. OSHA enforces SDS Sections 1 through 11 and 16; EPA and DOT cover 12 through 15. Shipped containers need full GHS labels; workplace containers may use alternatives with training. Flash point cut-offs: 23°C, 60°C, 93°C; boiling point cut-off: 35°C. Carcinogen/mutagen/reproductive/respiratory sensitizer cut-off 0.1%; most other health hazards 1%. Master these and the GHS portion of the CSP exam becomes a reliable source of points.