LD₅₀ (Median Lethal Dose 50%) is a quantitative measure used in toxicology to indicate the dose of a substance (usually expressed in milligrams per kilogram of body weight) that is required to cause death in 50% of a test population, typically laboratory animals (usually rats or mice), under controlled conditions (e.g., oral, dermal, or intravenous) within a specified time. It provides a standardised way to compare the acute toxicity of chemicals.
Explanation of the Term “LD₅₀ (Lethal Dose 50%)”
The LD₅₀ is a quantitative measure that helps characterize the immediate (acute) toxicity of a chemical. A lower LD₅₀ value indicates a more toxic substance, meaning a smaller amount is needed to cause death in half of the test subjects. Conversely, a higher LD₅₀ value suggests lower acute toxicity. It helps in hazard classification and labelling of substances under systems such as the Globally Harmonised System (GHS).
While LD₅₀ testing on animals is still referenced in safety data, modern regulations often encourage alternative non-animal testing methods where possible.
It’s essential to understand a few key aspects of LD₅₀ in the context of occupational health and safety:
- Animal Testing: LD₅₀ values are primarily derived from animal studies. While these values provide a basis for understanding potential human toxicity, they are not directly transferable to humans due to species differences in metabolism, physiology, and sensitivity. They serve as an estimate and a tool for comparison of relative toxicity.
- Route of Exposure: The LD₅₀ value is highly dependent on the route of administration. For example, the oral LD₅₀ of a substance might be very different from its dermal (skin contact) LD₅₀ or inhalation (LC₅₀ – Lethal Concentration 50%) value. This highlights the importance of understanding how workers might be exposed to a chemical.
- Acute Toxicity: LD₅₀ specifically measures acute toxicity, which refers to adverse effects occurring rapidly after a single, short-term exposure. It does not provide information on chronic toxicity (effects from long-term or repeated low-level exposures), carcinogenicity, mutagenicity, or reproductive toxicity.
- Hazard Classification: LD₅₀ values are used in hazard communication systems, such as the Globally Harmonized System (GHS), to classify chemicals into acute toxicity categories, which then determine appropriate labeling and safety data sheet (SDS) information.
Example:
Sodium Azide (NaN₃):
Sodium Hypochlorite (NaOCl):
Oral LD₅₀ in rats: ~27 mg/kg
Oral LD₅₀ in rats: ~5,230 mg/kg (for a 12.5% bleach solution)
Sodium azide is highly toxic, particularly via ingestion or inhalation. It is used in laboratories and airbag propellants but poses serious health risks, including respiratory failure and central nervous system damage.
Sodium hypochlorite is the active ingredient in bleach. It is much less toxic than sodium azide in terms of acute toxicity but can still be hazardous, particularly to the skin, eyes, and respiratory tract.
Due to its low LD₅₀, strict control measures are required:
- Use in fume hoods,
- Air monitoring,
- Chemical-resistant gloves,
- Immediate containment and disposal procedures.
Common in cleaning and disinfection, its risks are primarily due to:
- Corrosive properties
- Potential to release toxic chlorine gas when mixed with acids or ammonia.
Despite its higher LD₅₀ (lower toxicity), sodium hypochlorite still requires safe handling practices such as proper dilution, adequate ventilation, and PPE use.
These examples show how LD₅₀ values help differentiate between highly toxic and moderately hazardous substances, guiding how each should be managed in the workplace.
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