Summary
Most people learn LD50 as “the dose that kills five out of ten animals.” It gets you through an exam, but it is not what the number is.
The 50 is a probability, not a headcount. Results from several dose levels are fitted statistically to estimate the point at which 50 percent of the test population would be expected to die. Nobody watched half a group die at that dose.
Once that lands, the rest follows: why modern tests use one animal at a time, and why the same substance carries several different numbers.
What LD50 and LC50 each measure
The names look alike. The axis they split on is different.
| Meaning | What it measures | Unit | Usual route | |
|---|---|---|---|---|
| LD50 | Lethal Dose 50 | Amount that entered the body | mg/kg | Oral, dermal |
| LC50 | Lethal Concentration 50 | Concentration in the air | mg/L, ppm | Inhalation |
Swallowed or absorbed through skin is measured as an amount per body weight. Breathed in is measured as a concentration in air.
An oral LD50 of 50 mg/kg means 50 mg per kilogram of body weight is the dose at which half the population would be expected to die.
So how is the test actually run?
The old approach placed many animals across many dose levels to draw a curve. Modern guidelines are built to use far fewer animals, so the method now varies by guideline.
| Route | OECD guideline | Method | Typical animal count | Main purpose |
|---|---|---|---|---|
| Oral | TG 420 Fixed Dose | Set doses (5, 50, 300, 2,000 mg/kg) | 5 per dose | Signs and class |
| Oral | TG 423 Acute Toxic Class | Step dose up or down on results | 3 per step | Toxicity class |
| Oral | TG 425 Up-and-Down | One at a time; survive means up, die means down | 1 at a time | Estimate LD50 |
| Dermal | TG 402 | 24-hour contact on clipped skin | 1 then 2 more | Dermal class |
| Inhalation | TG 403 | About 4-hour exposure | 5 male + 5 female per level | Derive LC50 |
| Inhalation | TG 436 Acute Toxic Class | Step concentration up or down | 3 male + 3 female per step | Inhalation class |
| Inhalation | TG 433 Fixed Concentration | Signs rather than lethality | 5 of one sex per level | Signs and class |
This table is why “how many animals does an LD50 test use” has no single answer. Depending on the guideline it is 1, 3, 5 or 6 at a time, and the total depends on how the results come out.
The purpose also splits. Only TG 425 is really aimed at estimating an LD50 value; the others mostly assign a class. Regulation needs the band more than the decimal.
Do not compare a one-hour value to a four-hour value
This is where inhalation numbers go wrong.
Transport classification may use a one-hour LC50, while OECD testing typically runs a four-hour exposure. Same substance, same unit, different exposure duration, different number.
Before putting two values side by side, check the conversion or applicability rule the regulation sets. Matching units do not make matching values.
Where these numbers get used
Acute toxicity values do not stop at the laboratory. They decide which box a substance goes into when it is shipped.
The common skeleton for dangerous goods worldwide is UN-RTDG, the United Nations Recommendations on the Transport of Dangerous Goods. It is not itself law; it is the model each transport mode and each country builds its rules from.
| Regulation | Mode |
|---|---|
| UN-RTDG | Common model |
| ADR | Road |
| RID | Rail |
| ADN | Inland waterway |
| IMDG Code | Sea (under IMO) |
| ICAO-TI | Air |
The same substance falls under different rules depending on whether it travels by truck or by ship. Same skeleton, different detail.
Class and packing group answer different questions
The second place people get tangled.
- Class — “what kind of hazard is it” (Class 3 flammable, Class 6.1 toxic, Class 8 corrosive)
- Packing Group — “how much hazard within that class” (I high, II medium, III relatively low)
Lower numeral means higher hazard. And Packing Group III does not mean “safe.” It is still a dangerous good; it is simply lower within its class.
Acute toxicity values are exactly what feeds this assignment. Lower LD50 or LC50 pushes a substance toward a higher group.
The X, Y and Z stamped on packaging
You can read this straight off the container marking.
| Mark | Packing groups it may be used for |
|---|---|
| X | I, II and III |
| Y | II and III |
| Z | III only |
A matching performance mark is still not the end of it. Container material, capacity, liquid density, vapor pressure and the substance’s own packing instruction all have to be checked. The mark is necessary, not sufficient.
What I took away
- The 50 in LD50 is a statistical 50 percent probability of death, not an animal count
- LD50 is an amount entering the body (mg/kg); LC50 is a concentration in air (mg/L, ppm)
- Modern tests step through 1, 3, 5 or 6 animals, and totals depend on results
- Most guidelines aim at a class, not a precise value
- A one-hour transport LC50 and a four-hour OECD value are not directly comparable
- Class is the kind of hazard; packing group is the degree
- X, Y and Z on a container tell you which packing groups it may carry
In one line: LD50 is an estimated point, not an observed fact. Which is why comparisons made without checking conditions are usually wrong.
On the strength of this evidence
Test methods and animal counts change with revisions. Check the current edition before relying on any of it.
- OECD Test Guidelines, Section 4 (Health Effects) : source text for TG 402, 403, 420, 423, 425, 433 and 436
- UN Recommendations on the Transport of Dangerous Goods, Model Regulations (published by UNECE) : class and packing group criteria
- ICAO Dangerous Goods : technical instructions for air transport
- Other modal rules : ADR (road), RID (rail), ADN (inland waterway), IMDG Code (sea)
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