In general, the smaller the LD50 value, the more toxic the chemical is. The opposite is also true: the larger the LD50 value, the lower the toxicity.
The LD50 gives a measure of the immediate or acute toxicity of a chemical in the strain, sex, and age group of a particular animal species being tested. Changing any of these variables (e.g., type animal or age) could result in finding a different LD50 value. The LD50 test was neither designed nor intended to give information on long-term exposure effects of a chemical.
Once you have an LD50 value, it can be compared to other values by using a toxicity scale. Confusion sometimes occurs because several different toxicity scales are in use. The two most common scales used are the "Hodge and Sterner Scale" and the "Gosselin, Smith and Hodge Scale". These tables differ in both the numerical rating given to each class and the terms used to describe each class. For example, a chemical with an oral LD50 value of 2 mg/kg, would be rated as "1" and "highly toxic" according to the Hodge and Sterner Scale (see Table 1) but rated as "6" and "super toxic" according to the Gosselin, Smith and Hodge Scale (see Table 2). It is important to reference the scale you used when classifying a compound.
It is also important to know that the actual LD50 value may be different for a given chemical depending on the route of exposure (e.g., oral, dermal, inhalation). For example, some LD50s for dichlorvos, an insecticide commonly used in household pesticide strips, are listed below:
- Oral LD50 (rat): 56 mg/kg
- Dermal LD50 (rat): 75 mg/kg
- Intraperitoneal LD50: (rat) 15 mg/kg
- Inhalation LC50 (rat): 1.7 ppm (15 mg/m3); 4-hour exposure
- Oral LD50 (rabbit) 10 mg/kg
- Oral LD50 (pigeon:): 23.7 mg/kg
- Oral LD50 (rat): 56 mg/kg
- Oral (mouse): 61 mg/kg
- Oral (dog): 100 mg/kg
- Oral (pig): 157 mg/kg
Differences in the LD50 toxicity ratings reflect the different routes of exposure. The toxicity rating can be different for different animals. The data above show that dichlorvos is much less toxic by ingestion in pigs or dogs than in rats. Using Table 1, dichlorvos is moderately toxic when swallowed (oral LD50) and extremely toxic when breathed (inhalation LC50) in the rat. Using Table 2, dichlorvos is considered very toxic when swallowed (oral LD50) by a rat.
Routes of Administration | |||||
Oral LD50 | Inhalation LC50 | Dermal LD50 | |||
Toxicity Rating | Commonly Used Term | (single dose to rats) mg/kg | (exposure of rats for 4 hours) ppm | (single application to skin of rabbits) mg/kg | Probable Lethal Dose for Man |
1 | Extremely Toxic | 1 or less | 10 or less | 5 or less | 1 grain (a taste, a drop) |
2 | Highly Toxic | 1-50 | 10-100 | 5-43 | 4 ml (1 tsp) |
3 | Moderately Toxic | 50-500 | 100-1000 | 44-340 | 30 ml (1 fl. oz.) |
4 | Slightly Toxic | 500-5000 | 1000-10,000 | 350-2810 | 600 ml (1 pint) |
5 | Practically Non-toxic | 5000-15,000 | 10,000-100,000 | 2820-22,590 | 1 litre (or 1 quart) |
6 | Relatively Harmless | 15,000 or more | 100,000 | 22,600 or more | 1 litre (or 1 quart) |
Toxicity Rating or Class | Dose | For 70-kg Person (150 lbs) |
6 Super Toxic | Less than 5 mg/kg | 1 grain (a taste – less than 7 drops) |
5 Extremely Toxic | 5-50 mg/kg | 4 ml (between 7 drops and 1 tsp) |
4 Very Toxic | 50-500 mg/kg | 30 ml (between 1 tsp and 1 fl ounce) |
3 Moderately Toxic | 0.5-5 g/kg | 30-600 ml (between 1 fl oz and 1 pint) |
2 Slightly Toxic | 5-15 g/kg | 600-1200 ml (between 1 pint to 1 quart) |
1 Practically Non-Toxic | Above 15 g/kg | More than 1200 ml (more than 1 quart) |