Category 4 toxicity refers to the most dangerous category of toxic substances according to the Globally Harmonized System of Classification and Labelling of Chemicals (GHS). Substances are classified into this category if they are fatal, toxic or cause serious damage to human health or the environment after single or short-term exposure.
Category 4 toxicity is the highest level of acute toxicity, indicating that the substance can be lethal even in small amounts. Exposure occurs through ingestion, inhalation or skin contact and can result in severe health effects that may lead to death. This category is reserved for chemicals like hydrogen cyanide, strychnine and methyl isocyanate – substances that are highly toxic and can kill from a single exposure.
Origins of the Category 4 Toxicity Classification
The category 4 toxicity rating has its origins in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS). This system was developed by the United Nations to standardize the classification and labelling of hazardous chemicals around the world.
The first version of the GHS was published in 2003 after years of negotiations between experts from national regulatory agencies, chemical industries, transportation sectors and other relevant organizations. The aim was to develop an internationally-agreed approach for identifying hazardous chemicals and effectively communicating associated risks through standard symbols and terms on labels and safety data sheets.
Within the GHS, substances are classified into categories – Category 1 being the most hazardous through to Category 5 being the least hazardous. The Category 4 toxicity band specifically applies to chemicals that are fatal, toxic or seriously damaging to human health from a single exposure. It indicates the highest level of acute toxicity.
The GHS is not legally binding but governments are encouraged to implement it through their own national laws and regulations. As a result, Category 4 toxicity ratings are now applied in many countries as part of chemical control frameworks that align with the globally harmonized system.
How Category 4 Toxicity is Determined
Chemicals are assigned to Category 4 based on acute toxicity data from animal studies. Various international accepted test methods are used to determine the dose or concentration that causes severe adverse effects after a single exposure.
The key parameters are:
- LD50 – The dose that kills 50% of exposed animals from a single exposure
- LC50 – The concentration in air that kills 50% of exposed animals from a single exposure
The criteria for Category 4 toxicity are:
| Exposure Route | Toxicity Criteria |
|---|---|
| Oral (ingestion) | LD50 ≤ 50 mg/kg bodyweight |
| Dermal (skin contact) | LD50 ≤ 200 mg/kg bodyweight |
| Inhalation (gases) | LC50 ≤ 100 ppmV/4h |
| Inhalation (vapors) | LC50 ≤ 0.5 mg/L/4h |
| Inhalation (dusts/mists) | LC50 ≤ 0.05 mg/L/4h |
If a substance meets or exceeds the toxicity criteria through any route of exposure, it will be classified as Category 4. Data from human cases may also be considered where available. Expert judgment by regulatory toxicologists is used to interpret the data and assign an appropriate category.
Examples of Category 4 Toxic Substances
Some well-known examples of substances classified as Category 4 toxicity include:
Sodium Cyanide
Sodium cyanide is an inorganic salt used primarily in gold mining industries. It is a highly toxic chemical that interferes with cellular respiration. Exposure occurs through inhalation, ingestion or skin contact and can rapidly lead to death.
Toxicity data:
- Oral LD50 – 6.4 mg/kg (rats)
- Dermal LD50 – 10.4 mg/kg (rabbits)
- Inhalation LC50 – 323 ppm (mice)
Sodium cyanide is classified as Category 4 for all routes of exposure. The lethal dose for humans is only 1-3 mg per kg body weight.
Strychnine
Strychnine is a poisonous alkaloid used as a pesticide. It is toxic to humans and can cause violent convulsions leading to death through respiratory failure.
Toxicity data:
- Oral LD50 – 2 mg/kg (rats)
- Dermal LD50 – 200 mg/kg (rabbits)
- Inhalation LC50 – 0.167 mg/L/4h (rats)
Strychnine meets the Category 4 criteria for oral and inhalation toxicity. The lethal oral dose for an adult is between 50-100 mg.
Acrolein
Acrolein is a reactive aldehyde used as an aquatic herbicide and a chemical intermediate. It is extremely toxic if inhaled and can damage the lungs.
Toxicity data:
- Oral LD50 – 46 mg/kg (rats)
- Inhalation LC50 – 0.46 ppm (rats)
Acrolein is classified as Category 4 based on inhalation toxicity at very low concentrations in air.
Carbon Monoxide
Carbon monoxide (CO) is a toxic gas produced from incomplete combustion. It binds to hemoglobin and reduces oxygen transport in the body.
Toxicity data:
- Inhalation LC50 – 180 ppm (rats)
Carbon monoxide meets the Category 4 inhalation toxicity criteria. It is lethal to humans at concentrations above 100 ppm.
Hydrogen Cyanide
Hydrogen cyanide is an extremely toxic chemical asphyxiant. Exposure can occur through inhalation or ingestion and may result in death within minutes.
Toxicity data:
- Inhalation LC50 – 323 ppm (mice)
Hydrogen cyanide is classified as Category 4 based on inhalation toxicity data. 50 ppm for 30 minutes can be rapidly fatal to humans.
Implications of Category 4 Toxicity Classification
The Category 4 toxicity rating has a number of important implications:
Hazard Communication
Category 4 substances must be labelled with the GHS hazard pictogram “Skull and Crossbones” to indicate acute toxicity. This instantly recognizes them as highly toxic chemicals that can be fatal with short exposures.
The “Danger” signal word is also required on the label as well as precautionary statements indicating harmful routes of exposure and the need for prompt medical attention.
Safe Handling
Strict safety precautions are necessary when handling Category 4 substances to protect workers from harmful exposures. Closed systems, ventilation controls, specialized PPE and other engineering solutions may be required. Access is restricted to trained personnel only.
Emergency procedures like eyewash stations and spill containment must be in place where these chemicals are used or stored.
Transportation
For transportation purposes, Category 4 chemicals are classified as “Class 6.1 Toxic Substances” under the UN Recommendations for Dangerous Goods.
Special packaging, labelling, segregation, quantity limits and emergency response procedures apply for shipments of these highly toxic materials.
Waste Disposal
Safe disposal procedures are critical for Category 4 wastes which are intrinsically hazardous. Incineration or chemical decomposition may be required under regulated conditions. Disposal directly to landfill is generally not permitted.
Regulatory Oversight
Governments impose greater levels of regulatory control on Category 4 substances compared to less dangerous chemicals. Strict rules may apply for sale/supply, handling, storage, transportation, usage, waste disposal and emission controls.
Additional oversight, compliance audits and emergency planning are implemented proportional to the high risks these toxic substances present. Facilities handling Category 4 chemicals are subject to detailed risk assessments.
Conclusion
Category 4 is the highest toxicity rating in the Globally Harmonized System for chemicals. Substances in this category are fatal or toxic with single exposures and require special precautions to protect human health and the environment.
The rating is based on dose/concentration data from animal studies and is applied to chemicals like cyanides, pesticides and toxic gases. Category 4 hazards must be effectively communicated through labels and safety data sheets. Strict controls are enforced by governments due to the severe acute risks posed by these deadly poisons. Handling Category 4 substances requires utmost care to prevent catastrophic consequences from accidental releases or exposures.