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Short answer
A dangerous chemical, in the regulatory sense used by OSHA, is a chemical classified as a physical hazard or a health hazard, a simple asphyxiant, combustible dust, or a hazard not otherwise classified. There is no single scale that ranks chemicals from least to most dangerous, so “the most dangerous chemical” has no single answer. What a lab can do is look up how a specific chemical is classified, using its label, its safety data sheet and the NIOSH Pocket Guide.
What counts as a hazardous chemical
The OSHA Hazard Communication Standard, 29 CFR 1910.1200, defines a hazardous chemical as any chemical classified as a physical hazard or a health hazard, a simple asphyxiant, combustible dust, or a hazard not otherwise classified (HNOC). The definition is about classification, not about a popular idea of how deadly a substance sounds.
The HNOC category exists for an adverse physical or health effect that scientific evidence identifies during classification but that does not meet the criteria of any of the specified physical or health hazard classes. OSHA’s HazCom page notes that the standard is aligned with the Globally Harmonized System of Classification and Labeling of Chemicals (GHS). For how GHS categories work in practice, see the CASRAI guide to GHS classification of chemicals.
Health hazards and physical hazards
“Dangerous” splits into two families of hazard class. A chemical can be hazardous without being toxic at all.
| Family | Hazard classes listed in 29 CFR 1910.1200 |
|---|---|
| Health hazards | Acute toxicity (any route of exposure); skin corrosion or irritation; serious eye damage or eye irritation; respiratory or skin sensitization; germ cell mutagenicity; carcinogenicity; reproductive toxicity; specific target organ toxicity (single or repeated exposure); aspiration hazard |
| Physical hazards | Explosive; flammable (gases, liquids, solids); aerosols; chemical under pressure; oxidizer (gases, liquids, solids); self-reactive; pyrophoric (liquid or solid); self-heating; organic peroxide; corrosive to metal; gas under pressure; in contact with water emits flammable gas; desensitized explosive |
| Other categories in the definition | Simple asphyxiant; combustible dust; hazard not otherwise classified |
The table is the practical gain over a ranked list: it shows that the regulatory idea of danger covers fire, explosion, pressure and corrosion of metal as well as poisoning and cancer. Acute toxicity, which covers any route of exposure, is only one of the nine health classes.
Why there is no single “most dangerous chemical”
Lists of the “deadliest” or “most dangerous” chemicals rank substances on one axis, usually lethality. The classification system does not work that way. A chemical is assigned to hazard classes, and within each class to a category, so two chemicals can be hazardous for entirely different reasons: one because it is a carcinogen, another because it is a pyrophoric solid, another because it is an oxidizer. Comparing them requires choosing a hazard class first, and the choice depends on the work being done.
The same logic explains why the questions below each need a class-based answer. Which chemicals are poisonous points to the acute toxicity and target organ toxicity classes. Which are corrosive points to skin corrosion and corrosive to metal. Which are likely to burn or detonate points to the flammable, explosive, pyrophoric and organic peroxide classes.
Where to find a chemical’s hazards
For any chemical in a lab, three documents answer the question “what makes this dangerous?” without relying on a ranking.
1. The label
Under OSHA’s HazCom standard, labels must include a harmonized signal word, a pictogram and a hazard statement for each hazard class and category. This is the fastest check at the bench. The CASRAI guide to safety symbols covers the pictograms.
2. The safety data sheet (SDS)
The standard specifies a 16-section SDS format (paragraph (g)(2) of 1910.1200):
- Identification
- Hazard(s) identification
- Composition/information on ingredients
- First-aid measures
- Fire-fighting measures
- Accidental release measures
- Handling and storage
- Exposure controls/personal protection
- Physical and chemical properties
- Stability and reactivity
- Toxicological information
- Ecological information
- Disposal considerations
- Transport information
- Regulatory information
- Other information, including date of preparation or last revision
Section 2 gives the classification. Sections 4, 7, 8, 10 and 11 hold the first-aid, handling and storage, exposure control and personal protection, stability and reactivity, and toxicological detail behind it. Older sheets in the legacy format are covered in SDS vs MSDS, and the laboratory-specific application of the standard is in the HazCom standard for laboratories.
3. The NIOSH Pocket Guide
The CDC NIOSH Pocket Guide to Chemical Hazards gives general industrial hygiene information for hundreds of chemicals and chemical classes. An entry can include names and synonyms, CAS and RTECS numbers, DOT identification numbers, NIOSH recommended exposure limits, OSHA permissible exposure limits, NIOSH immediately dangerous to life and health (IDLH) values, physical properties, respirator selection, incompatibilities, exposure routes, symptoms, target organs and first aid. The CASRAI NIOSH Pocket Guide page explains how to read an entry.
Common misunderstandings
“Dangerous means toxic.”
The definition in 1910.1200 lists physical hazard classes alongside health hazard classes. A flammable liquid, an oxidizer or a chemical that is corrosive to metal is a hazardous chemical on classification alone.
“Hazardous chemicals are a short, known list.”
Hazardous status follows from classification, so it is assessed chemical by chemical. The definition also reaches simple asphyxiants, combustible dust and hazards not otherwise classified, none of which fit a “poisons” mental model.
“If a chemical is not on a famous list, it is safe.”
The check is the chemical’s own classification on its label and SDS. For turning those hazard facts into a decision about a task, see the risk assessment matrix for laboratory hazards.
FAQ
What are dangerous chemicals?
Under OSHA’s definition, they are chemicals classified as physical hazards or health hazards, simple asphyxiants, combustible dust or hazards not otherwise classified.
What are the most dangerous chemicals?
There is no single ranking. Chemicals are classified by hazard class and category, so the answer depends on whether the concern is toxicity, fire, explosion, corrosion or another class. Look up the specific chemical’s SDS and NIOSH Pocket Guide entry.
What are poisonous or toxic chemicals?
These fall under the health hazard classes, notably acute toxicity (any route of exposure) and specific target organ toxicity from single or repeated exposure. The SDS covers them in Section 11, toxicological information.
Where can I see examples of toxic chemicals?
The NIOSH Pocket Guide lists hundreds of chemicals and classes, each with exposure limits, IDLH values, exposure routes, symptoms and target organs.
How do I know how dangerous a chemical is?
Read the label’s signal word, pictogram and hazard statement, then Section 2 of the SDS, and check the NIOSH Pocket Guide for exposure limits and IDLH values where an entry exists.
Sources
- OSHA, 29 CFR 1910.1200, Hazard Communication, checked 2026-10-29.
- OSHA, Hazard Communication, checked 2026-10-29.
- CDC NIOSH, NIOSH Pocket Guide to Chemical Hazards (page last reviewed February 18, 2020), checked 2026-10-29.
