Diallate

Diallate (S-(2,3-dichloroallyl) diisopropylthiocarbamate) was developed by Monsanto and introduced commercially around 1960 under trade names including Avadex. [1] It belongs to the thiocarbamate class of herbicides, which disrupt lipid synthesis by inhibiting the enzyme that converts very-long-chain fatty acids — a process essential for waxy cuticle formation in grass weeds. Diallate was selective for grass weeds (particularly wild oats, Avena fatua) in beet, cereal, and oilseed crops, and was applied preplant incorporated — mixed into the upper soil horizon before crop planting. It was used extensively in the UK, Europe, and parts of North America through the 1970s and early 1980s. NTP carcinogenicity bioassays found evidence of multiple tumors in mice including hemangiosarcomas and hepatocellular carcinomas, supporting an EPA Group B2 (probable human carcinogen) classification. [2] These findings led to cancellation of US registrations, and most other countries followed with registration withdrawals through the 1980s–1990s. A related compound, triallate (also an Avadex product), continues to be registered in some countries and shares similar structural and toxicological properties. [3] Diallate is a volatile compound that can move from treated soil into the atmosphere, and its persistence in soil (half-life 1–3 weeks in warm conditions) made it detectable in environmental samples for years after application in historically treated areas.

Agricultural workers who mixed, loaded, and applied diallate through the period of active registration (roughly 1960–1990) were the primary occupationally exposed population. Inhalation of vapors during preplant incorporation operations and dermal exposure during mixing were the main routes. People living in agricultural areas where diallate was used could have been exposed through air drift and potentially groundwater. Current exposures are negligible given canceled registrations.

The hemangiosarcomas (vascular tumors, the same type associated with vinyl chloride exposure) and hepatocellular carcinomas found in NTP studies established the probable carcinogen classification. [2] Acute thiocarbamate toxicity involves inhibition of carbamate-dependent metabolic pathways and may cause skin and eye irritation, nausea, and neurological effects at higher doses. The VOC nature of diallate creates inhalation exposure during application. Environmental persistence, though moderate, allowed accumulation in soil with repeated annual applications.

Former agricultural applicators who used diallate extensively through the 1970s–80s carry the primary historical risk. The general public's current risk from diallate is negligible given canceled registrations.

1. Diallate is no longer registered in most countries — verify current registration status before any use. 2. For former applicators concerned about past exposure, discuss cancer surveillance with a physician, particularly for hemangiosarcoma and liver cancer. 3. Those with contaminated soil from historical use should test before using land for food production.