Methamidophos is an extremely toxic organophosphate which causes serious adverse effects to human health.
Furthermore acute and long-term risks were identified for aquatic invertebrates and for beneficial arthropods. A risk to birds and mammals from consumption of dead insects and possibly other routes of exposure were also identified.
It is classified as a WHO Toxicity Class 1b.
The World Health Organization names four toxicity classes:
- Class 1 – a: extremely hazardous
- Class 1 – b: highly hazardous
- Class 2: moderately hazardous
- Class 3: 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.
The draft decision guidance document for methamidophos that formed the basis for the decision during the 7th meeting of the Parties to the Rotterdam Convention can be downloaded here.