Is it true: Operation Ranch Hand / Agent Orange dioxin contamination concealment (1962-71)?

Partly — it's more complicated than the claim suggests.

The claim that the US government and the makers of Agent Orange knew its dioxin was seriously toxic and deliberately concealed it is partly true. Dow had a chloracne outbreak among Midland workers in 1964 and convened rival makers in March 1965, and a Monsanto medical director wrote that month that dioxin might conceivably be a potent carcinogen. But in 2008 the Second Circuit found that the makers' known dangers were chloracne and liver damage in workers, with no evidence they knew of cancer or long-term harm to people exposed in use, and nothing withheld that could have influenced the military (a ruling about the military, not about troops, veterans or the public). The government held Bionetics evidence of birth defects in mice from 1966 but restricted 2,4,5-T only after the study's public release in October 1969, and suspended Agent Orange use on April 15, 1970. Official recognition of veterans' harm was slow (1984 and 1991 Acts), and allegations that a 1980s veterans' study was obstructed remain contested. Deliberate concealment of known long-term risk is not established.