Toxicity Class refers to a classification system for pesticides created by a national or international government-related or -sponsored organization. It addresses the acute toxicity of agents such as soil fumigants, fungicides, herbicides, insecticides, miticides, molluscicides, nematicides, or rodenticides.  

General considerations

Assignment to a Toxicity Class is based typically on results of acute toxicity studies such as the determination of LD50 values in animal experiments, notably rodents, via oral, or sometimes inhaled, or external application. The experimental design measures the acute death rate of an agent. The Toxicity Class generally does not address issues of other potential harm of the agent, such as bioaccumulation, issues of carcinogenicity, teratogenicity, or mutagenic effects, or the impact on reproduction.

Regulating agencies may require that packaging of the agent is labeled with a Signal Word, a specific warning label to indicate the level of toxicity to the public.

EPA (United States)

The Environmental Protection Agency knows four Toxicity Classes. Class I to III are required to carry a Signal Word on the label to warn users of the toxicity. Pesticides are regulated by the Federal Insecticide, Fungicide, and Rodenticide Act(FIFRA).

Toxicity Class I

• most toxic;
• requires Signal Word: "Danger-Poison", with skull and crossbones symbol

Possibly followed by:

"Fatal if swallowed", "Poisonous if inhaled", "Extremely hazardous by skin contact--rapidly absorbed through skin", or "Corrosive--causes eye damage and severe skin burns".

Toxicity Class II

• moderate toxic
• Signal Word: "Warning"

possibly followed by:

"Harmful or fatal if swallowed", "Harmful or fatal if absorbed through the skin", "Harmful or fatal if inhaled", or "Causes skin and eye irritation".

Toxicity Class III

• slightly toxic
• Signal Word: Caution

possibly followed by:

"Harmful if swallowed", "May be harmful if absorbed through the skin", "May be harmful if inhaled", or "May irritate eyes, nose, throat, and skin".

Toxicity Class IV

• practically nontoxic
• no Signal Word required since 2002.

Furthermore, the EPA classifies pesticides into those that can be applied by anybody: General Use Pesticides, and those that are restricted, meaning, their application needs to be done by or under the supervision of a certified individual. Application of Restricted Use Pesticides requires that a record of the application is kept.

Generally, agents of Class I will kill an adult person at a dose of less than 5 grams (less than a teaspoon), of Class II at 5-30 grams, and of Class III at more than 30 grams.

WHO

The World Health Organization names four toxicity classes as follows:

• Class I – a: extremely hazardous;
• Class I – b: highly hazardous;
• Class II: moderately hazardous;
• Class III: slightly hazardous.

The system is based on LD50 determination in rats, thus an oral solid agent with an LD50 at 5mg or less/kg bodyweight is Class I-a, at 5-50 mg/kg Class I-b, at 50-500 mg/kg Class II, and at more than 500 mg/kg Class III. Values may differ for liquid oral agents and dermal agents.

European Union

There are three toxicity classes in the classification system by the European Union, regulated by Directive 67/548/EEC, namely:

• Class I: very toxic
• Class II: toxic
• Class III: harmful.

Very toxic and toxic substances are marked by the European toxicity symbol.

See also

• Dangerous goods
• Hazard symbol

References

• WHO Classification document
• Toxicity Rating Scale
• Reading the label