The One-stop Guide to Working with Chemicals that Cause Acute Toxicity: from Hazard Classification to Mitigating Risks

What are acutely toxic chemicals?

Acutely toxic substances or mixtures are those that cause serious adverse health effects or death upon a single or short-term oral, dermal or inhalation exposure. Numeric acute toxicity values are generally determined in terms of the amount of toxic material necessary to cause death in 50% of the tested animals (usually rat or rabbit). They are expressed as (approximate) lethal dose (LD50) values for the oral and dermal route, lethal concentration (LC50) values for inhalation, or as acute toxicity estimates (ATE) derived from LD50 or LC50 data. The more toxic a material is, the less of it is necessary to cause harmful effects. GHS ATE cut-off criteria allocates toxic substances into categories with the first category being the most severe.

Moreover, many toxic chemicals pose additional hazards. Nitric acid for instance is a corrosive oxidizing agent that is toxic if inhaled. Symptoms of this exposure may include burning of the mucous membranes, cough, shortness of breath, bloody vomiting, and damage to the respiratory tract.  

The ‘skull and crossbones’ pictogram is used to represent 1 of 10 GHS health hazard classes

Category Signal Word Hazard Statement Hazard Code

Category 1

Oral (mg/kg bodyweight): ATE ≤ 5

Dermal (mg/kg bodyweight): ATE ≤ 50

Gases (ppmV): ATE ≤ 100

Vapours (mg/l): ATE ≤ 0.5

Dusts & mists (mg/l): ATE ≤ 0.05


Fatal if swallowed

Fatal in contact with skin

Fatal if inhaled




Category 2

Oral (mg/kg bodyweight): 5 < ATE ≤ 50

Dermal (mg/kg bodyweight): 50 < ATE ≤ 200

Gases (ppmV): 100 < ATE ≤ 500

Vapours (mg/l): 0.5 < ATE ≤ 2.0

Dusts & mists (mg/l): 0.05 < ATE ≤ 0.5


Fatal if swallowed

Fatal in contact with skin

Fatal if inhaled




Category 3

Oral (mg/kg bodyweight): 50 < ATE ≤ 300

Dermal (mg/kg bodyweight): 200 < ATE ≤ 1000

Gases (ppmV): 500 < ATE ≤ 2500

Vapours (mg/l): 2.0 < ATE ≤ 10.0

Dusts & mists (mg/l): 0.5 < ATE ≤ 1.0


Toxic if swallowed

Toxic in contact with skin

Toxic if inhaled





Category 4 (no pictogram)

Oral (mg/kg bodyweight): 2000 < ATE ≤ 5000

(mg/kg bodyweight): 2000 < ATE ≤ 5000

Gases (ppmV): equivalent doses, not specified

Vapours (mg/l): equivalent doses, not specified

Dusts & mists (mg/l): equivalent doses, not specified


May be harmful if swallowed

May be harmful in contact with skin

May be harmful if inhaled





How to mitigate the risks of working with acutely toxic substances

1. Receiving:
    • Maintain an inventory of toxic chemicals including how much is used and by whom.
    • Order the minimal amount and concentration required for your lab.
    • Before receiving chemicals, review SDS information and take appropriate precautions.
    2. Storage:
      • Maintain appropriate ventilation. Do not store or handle acutely toxic chemicals in facilities with recirculating exhaust systems.
      • Store in designated areas with restricted access. This may include having caution tape and signs.
      • Store breakable containers in a chemically resistant tray.
      3. Handling:
        • Do not work alone: have a colleague nearby in case of emergencies.
        • Perform all work involving toxic chemicals in a fume hood or glove box.
        • Use disposable bench protectors to protect your work surfaces.
        • When not in use, tightly seal all toxic chemical containers.
        • Wear appropriate PPE including gloves, lab coats, closed toed shoes and goggles. Consider additional PPE such as splash goggles, chemical resistant gloves, disposable lab coats, respirators or masks.
        • If transferring to a secondary container (e.g. for dilutions), make sure to label it as “toxic”.
        4. Health and environmental considerations:
          • Take extra precautions if you have any personal health This may include:
            • Allergies to certain chemicals
            • Medical conditions and potential interactions between toxic chemicals and prescription medications
            • General body size and the dosage required to induce a toxic effect
          5. Emergencies and spills:
            • Ensure emergency irrigation including showers and eyewash stations are within a 10-second travel from your workspace.
            • If you spill any toxic reagents on yourself, use the emergency shower as soon as possible. Remove your clothing, wash with soap and water, and call for assistance.
            • In case of a major spill outside the fume hood, evacuate the area and call for help.
            • If possible, perform chemical conversions or neutralizations of spilled chemicals.
            6. Alternatives:
              • Review your SOP to determine if it is possible to substitute your reagents with less toxic chemicals.
              • If chemical substitutions are not possible, determine if a less toxic form can be used instead (e.g. solid pellets are less dangerous than fine powders).
              7. Waste:
                • Review ‘Section 13: disposal considerations’ of the SDS before disposal. According to the United States Environmental Protection Agency (EPA), “U-list” hazardous waste is segregated from “P-list” hazardous waste. For material to be considered as P-or U-listed waste, it must contain one of the chemicals listed on the P or U lists, the chemical must be unused, and in the form of a commercial product.


                By Qinling Li


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