Operation Ranch Hand was the US Air Force aerial herbicide spraying programme in South Vietnam from 1962 to 1971. Approximately 19 million gallons of herbicides — including Agent Orange, which contained dioxin (TCDD) as a manufacturing byproduct — were sprayed over approximately 4.5 million acres. Internal documents from Dow Chemical and Monsanto, the primary suppliers, show that both companies were aware of the toxicity of dioxin contamination in their products before and during production. The US government did not publicly acknowledge health risks to veterans until the late 1970s and did not establish presumptive service connection for Agent Orange-related diseases until 1991. The concealment of known dioxin risks is confirmed by the documentary record.