There is no single definition of “Category 3” or “Category 5.” The number only makes sense within a named scale or classification system: Category 3 can describe hurricane winds, uncertain public-health evidence, a Canadian occupational code component, a radioactive sealed source, or a chemical hazard. Identify the system before interpreting the number.
Why “Category 3” has no universal meaning
Category numbers are labels within specific frameworks, not a shared ranking used across subjects. A Category 3 in one system cannot be compared directly with a Category 3 in another, and a higher number does not always mean greater danger. The definition also depends on details such as the authority, jurisdiction, edition, hazard class, or scale.
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If you saw the phrase without its surrounding context, look for the organization or document that assigned it. The examples below show how widely the meaning can differ.
What Category 3 and Category 5 mean in specific systems
ATSDR public-health conclusions
The U.S. Agency for Toxic Substances and Disease Registry (ATSDR) uses five categories for public-health conclusions. Category 3 means the available data are insufficient to determine whether harmful health effects may occur. It is an uncertain conclusion, not a finding that harm is occurring or that there is no risk.
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Category 4 means exposure may be possible, but is insufficient to cause harmful effects. Category 5 means no exposure is occurring. Categories 1 and 2 are health-hazard conclusions for acute/intermediate and chronic exposure contexts, respectively. These are assessment conclusions, not a universal scale of danger. ATSDR’s guidance on developing conclusions explains the categories.
Hurricane wind categories
On the Saffir–Simpson Hurricane Wind Scale used by Environment and Climate Change Canada, categories are based on sustained wind speed. Category 3 covers 178–209 km/h; Category 5 means more than 249 km/h.
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| Category | Sustained wind speed |
|---|---|
| 1 | 119–153 km/h |
| 2 | 154–177 km/h |
| 3 | 178–209 km/h |
| 4 | 210–249 km/h |
| 5 | More than 249 km/h |
The category describes wind speed, not the full local impact. It does not by itself say how much damage a storm will cause or how much rain it will bring; effects also depend on local landscape, vulnerability, and prior weather. See the Canadian Hurricane Centre’s frequently asked questions.
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In Canada’s NOC 2021, a category number is part of a hierarchical occupational code, not a danger rating. The first digit identifies one of 10 broad occupational categories; the second identifies one of six TEER categories, which reflect training, education, experience, and responsibilities. Further digits identify increasingly specific groupings. These details apply to NOC 2021 Version 1.0. Statistics Canada’s NOC 2021 introduction describes the structure.
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Canadian radioactive sealed-source categories
In the Canadian Nuclear Safety Commission’s sealed-source classification, Category 3 describes a source that, if mishandled or inadequately protected, could cause permanent injury after some hours of contact. Fatal proximity to unshielded material for days to weeks is possible but unlikely, according to the regulator’s glossary. Category 5 sources could not permanently injure someone.
These definitions concern sealed sources and their potential effects under specified exposure conditions; they are not general consumer “radiation levels.” The CNSC also cautions that sealed-source categories are different from nuclear-material categories. See the CNSC glossary, REGDOC-3.6.
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Canadian WHMIS chemical-hazard categories
Under Canada’s Workplace Hazardous Materials Information System (WHMIS), category meaning depends on the hazard class and its classification table. Category order generally follows decreasing severity within a table, and a product that meets criteria for multiple categories in the same table is assigned the most severe applicable one. Exceptions include specific target organ toxicity after a single exposure and gases under pressure. Therefore, a category number cannot be interpreted reliably without the hazard class and applicable rules. Health Canada’s WHMIS supplier guidance explains the classification requirements.
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How to find the right definition
- Find the source. Check the surrounding text, label, report, code, or website for the organization that assigned the category.
- Name the framework. Look for a scale or classification name, such as the Saffir–Simpson Hurricane Wind Scale, ATSDR public-health conclusions, NOC 2021, CNSC sealed-source categories, or WHMIS.
- Check the relevant details. Confirm the jurisdiction, edition or date, and—in a hazard classification—the specific class or table.
- Interpret the number only within that framework. Do not assume its meaning or severity carries over to a different system.
If the source gives no framework or context, the number alone is not enough to define the category. Ask where the label came from or what subject it refers to.
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