Signal Word Vol. 2, No. 18
Did you ever wonder just how toxic that pesticide you were about to mix and apply really is? The answer is right there on the label. Most pesticides are required to carry a “Signal Word” that indicates their acute toxicity level. The following table shows these signal words and the level of toxicity they represent. These four toxicity classes consider both oral and dermal (absorption through the skin) toxicity, as well as other hazards not discussed here such as inhalation toxicity and potential to cause eye damage, but signal words do not provide information about chronic toxicity.
In addition to the signal words “Danger-Poison,” all pesticides in Toxicity Class I are also required to bear the Skull and Crossbones. Regardless of toxicity class, all pesticide labels are also required to carry “Keep Out of Reach of Children.” Note that the signal word on a pesticide label indicates the relative toxicity of the mixture or formulation in that container and not the toxicity of the pure active ingredient. Two products that contain the same active ingredient, but in different concentrations, may have different signal words.
| Signal Word | Toxicity Class | Toxicity Description | Acute Oral LD50 Range mg/kg |
Acute Dermal LD50 Range mg/kg |
| Danger-Poison | I | Highly Toxic | 0 - 50 | 0 - 200 |
| Warning | II | Moderately Toxic | 50 - 500 | 200 - 2000 |
| Caution | III | Low Toxicity | 500 - 5000 | 2000 - 5000 |
| Non-required (may have “caution”) |
IV | Very Low Toxicity | > 5000 | > 5000 |
“LD50” stands for ‘lethal dose 50%” and represents the dose of product required to kill 50% of a test population. The lower the LD50 the higher the toxicity. Because LD50s are based on toxicity to laboratory animals such as rats or rabbits, they do not directly indicate toxicity to humans, but LD50 values are useful indicators of general mammalian toxicity. As points of reference, the LD50 of pure caffeine is 127 mg/kg, and the LD50 of table salt (NaCl) is about 3000 mg/kg. Most pesticides sold to homeowners are in Toxicity Class III or IV.
Of course, you need to know more than just the signal word to be able to handle and apply pesticides safely and effectively. How do I apply this stuff and how much do I use? What kind of protective equipment do I need? How long do I have to wait before picking the crop? The label has the answer to all these questions and more. Always read the product label carefully before applying any pesticide.
See Extension Publication 2291, Applying Pesticides Correctly, for more information on safe pesticide handling and application: http://extension.msstate.edu/sites/default/files/publications/publications/p2291.pdf.
