Ethylene thiourea (ETU)

ETU's story begins with EBDC fungicides (maneb, mancozeb, zineb, nabam) developed in the 1940s and 1950s for controlling fungal diseases in grapes, potatoes, tomatoes, apples, and other crops [1]. These fungicides are applied in enormous quantities worldwide — mancozeb alone is one of the most widely used fungicides globally — and when the body metabolizes them or when treated produce is cooked, EBDC degrades to ETU [2]. The problem was discovered in the 1970s when NCI researchers found ETU caused thyroid tumors in rats at doses achievable by dietary exposure, and subsequent studies confirmed it was a thyroid carcinogen and potent thyroid-disrupting agent [1]. EPA classified ETU as a probable human carcinogen (Group B2) and restricted many EBDC uses, but they remain registered and widely used internationally, meaning imported produce continues to be a significant source [2].

Dietary exposure from consuming EBDC-treated produce is the primary pathway — and cooking increases ETU levels because heat converts EBDC residues on the surface to ETU [1]. Wine from EBDC-treated grapes may contain ETU. Leafy vegetables, root vegetables, and fruit from countries where EBDC fungicides remain unrestricted are higher-risk [2]. Occupational exposure occurs among farm workers who mix and apply EBDC fungicides — dermal absorption and inhalation of spray mist are significant routes [1]. Spouses and children of farm workers are secondarily exposed through take-home contamination on clothing and skin [2].

ETU specifically targets thyroid peroxidase — the enzyme essential for synthesizing thyroid hormones — inhibiting it at very low concentrations [1]. This inhibition reduces thyroid hormone production, triggering the pituitary to release more thyroid-stimulating hormone (TSH), which chronically stimulates thyroid cell division [2]. This cycle of hormonal disruption leads to thyroid hyperplasia and ultimately thyroid adenoma and carcinoma in animal studies. In pregnant animals, ETU causes developmental defects including brain abnormalities, cleft palate, and limb defects — reflecting the critical importance of adequate thyroid hormone for fetal development [1]. Human epidemiological data on ETU-specific cancer risk are limited, but thyroid disruption in exposed farm workers has been documented [2].

Pregnant women are most critically at risk: thyroid hormone is essential for fetal brain development throughout pregnancy, and ETU's thyroid-suppressing action during pregnancy could impair neurodevelopment even at subclinical levels [1]. Agricultural workers who apply EBDC fungicides, and their family members, face the highest exposures [2]. People with pre-existing subclinical hypothyroidism or thyroid nodules may be more vulnerable to additional thyroid burden from ETU. Children who eat large amounts of produce from EBDC-treated crops — particularly fresh fruit, grapes, wine residues in processed juices — have proportionally higher exposure relative to body weight [1].

1. Choose organic produce for fruits and vegetables where EBDC fungicides are commonly used: grapes, potatoes, tomatoes, apples, and leafy greens are the highest-risk categories [1]. 2. Wash all produce thoroughly under running water; for items with edible skins (apples, grapes), scrubbing reduces surface EBDC residues [2]. 3. Peel fruits and vegetables where practical — most EBDC residues concentrate on the surface. 4. Note that cooking does NOT eliminate ETU risk — it increases ETU by converting EBDC, so raw is not worse than cooked for ETU specifically [1]. 5. If you work with EBDC fungicides, use nitrile gloves, eye protection, and a respirator rated for pesticide mists during mixing and application; shower and change clothes before entering living areas [2]. 6. Check USDA's Pesticide Data Program annual report for current ETU detection rates in specific food categories [1].