Human Health Hazards, Toxicity, and Signal Words
Toxicity vs hazard
Toxicity is the inherent ability of a material to injure; hazard is toxicity combined with exposure. Two products with the same active ingredient can carry different hazards because solvents and carriers change absorption. Toxicity is expressed as LD50 (lethal dose for 50% of test animals, mg/kg of body weight) for oral and dermal routes and LC50 (concentration in air) for inhalation. Lower numbers mean more toxic.
Exposure routes
Dermal (usually the largest in agriculture), inhalation, oral, and ocular. Effects may be local (skin burn, eye irritation) or systemic (absorbed and carried to organs). Acute effects occur soon after a single exposure; chronic effects (cancer, nerve damage, reproductive harm) follow repeated exposure over time.
The signal word system
| Category | Signal word | Oral LD50 (mg/kg) | Dermal LD50 (mg/kg) |
|---|---|---|---|
| I | DANGER (plus POISON and skull/crossbones when highly toxic) | ≤50 | ≤200 |
| II | WARNING | >50–500 | >200–2000 |
| III | CAUTION | >500–5000 | >2000–5000 |
| IV | CAUTION | >5000 | >5000 |
The signal word reflects the most severe hazard of the formulated product, including eye and skin irritation and corrosivity - not the percentage of active ingredient. "DANGER" alone (without "POISON") typically signals corrosive eye/skin damage.
Special concerns
Organophosphates and carbamates inhibit cholinesterase; repeated exposure can cause illness at doses that would otherwise be safe, which is why some labels require cholinesterase monitoring. Some products are skin sensitizers or suspected carcinogens.
Traps
Questions linking a higher active-ingredient percentage to a more severe signal word are wrong; the formulated product's hazard sets the word. "CAUTION" covers both Category III and IV.
Rule of thumb: lower LD50 = more toxic, and a smaller amount can harm a child, so never estimate dose by volume alone.