Also, First Blood is based on a book published in 1972 right at the end of US involvement in Vietnam. I could not find a similar reference to Agent Orange in the book, but what would have been the understanding of its effects immediately following the Vietnam War?
The first hearing on the potential health implications of Agent Orange was held by the Senate in 1970, so concerns about the potential effects of Agent Orange existed even at the time it was in use.
But it took time for the effects to be proven. According to the Congressional Research Service's 2014 report(2), veterans first started making claims to the U.S. Department of Veteran Affairs (VA) in 1977. In 1979, Congress enacted several laws to examine the potential links between Agent Orange and long-term health effects and disabilities. Unfortunately, the Federal government did not appropriately implement these laws, with the CDC abandoning attempts to release a study report in 1987.
Two statutes have been enacted to compensate veterans exposed to Agent Orange: the Veterans’ Dioxin and Radiation Exposure Compensation Standards Act of 1984 (P.L. 98-542); and the Agent Orange Act (P.L. 102-4) enacted in 1991. And a class action filed in 1986, Nehmer v. U.S. Veterans’ Administration(3), alleged that veterans' claims for service-related disabilities connected to Agent Orange had been improperly denied. In 1989, the District Court held that the VA had improperly been using too stringent a standard in looking at the statistical correlation between Agent Orange and long-term health implications, and invalidated the VA regulations. The 1991 Agent Orange Act was a result of this case.
In the late 1970s, veterans and their families were speaking to the press about their exposure to Agent Orange and their health issues. "Agent Orange: the Deadly Fog", a documentary, aired in Chicago in 1978. (4) This documentary led to more frequent local and national press coverage of veterans' complaints about alleged Agent Orange-related issues. (5) Later that year, a former helicopter crew chief, Paul Reutersham, appeared on NBC's Today show, stating "I died in Vietnam, but I didn't even know it." (6) Reutersham described his experience of near-daily flights through clouds of herbicides, and his cancer diagnosis upon his return to the US. He died, at age 28, from cancer in his colon, liver, and abdomen.
To sum up an answer to your question, I would say that the effects of Agent Orange were well suspected by the early 80s. The proof was not fully accepted by then (certainly not by the U.S. Government), but between the timing of concerns raised by scientists and the media attention given to claims of veterans at the time, it makes sense that the claim appeared in the 1982 movie but not the 1972 novel.
(1) U.S. Congress, Senate, Committee on Commerce, Subcommittee on Energy, Natural Resources, and the Environment, Effects of 2,4,5-T and Related Herbicides on Man and the Environment, 91st Cong., 2nd sess., April 7, 1970 (Washington: GPO, 1970).
(2) Panangala, Sidath Viranga and Daniel T. Shedd, "Veterans Exposed to Agent Orange: Legislative History, Litigation, and Current Issues" (November 18, 2014), available at: https://sgp.fas.org/crs/misc/R43790.pdf
(3) Nehmer v. U.S. Veterans’ Admin., 712 F. Supp. 1404 (N.D. Cal. 1989).
(4) WBBM Channel 2 - Agent Orange: Vietnam's Deadly Fog (Partial Excerpt, 1978): https://www.youtube.com/watch?v=9AvroVBgjlo
(5) Institute of Medicine (US) Committee to Review the Health Effects in Vietnam Veterans of Exposure to Herbicides. Veterans and Agent Orange: Health Effects of Herbicides Used in Vietnam. Washington (DC): National Academies Press (US); 1994. 2, History of the Controversy Over the Use of Herbicides. Available from: https://www.ncbi.nlm.nih.gov/books/NBK236351/
(6) Ibid; see also Wilcox FA. 1989. Waiting for an Army to Die: The Tragedy of Agent Orange. Cabin John, MD: Seven Locks Press.
Weren’t there more dangerous chemicals used that might’ve been the cause?