PROVIDENCE — Exposure to Agent Orange is known to have caused birth defects and developmental disabilities in babies born to some women in the vicinity of military fighting during the Vietnam War, according to government reports. The United States sprayed nearly 20 million gallons of Agent Orange during the war, when hundreds of thousands of US soldiers were exposed to it — many of whom were drafted into military service.

In the decades since the war ended, studies have shown that exposure to Agent Orange, a synthetic defoliating herbicide, was associated with an increased risk of certain cancers, diabetes, and cardiovascular disease.

Until recently, the long-term impact of Agent Orange exposure on the brain has been a mystery. But a new study at Brown University reveals a link between Agent Orange and certain aging-associated diseases, including how this wartime chemical compound can lead to neurodegenerative diseases similar to Alzheimer’s.

Dr. Suzanne M. De La Monte, a Brown University physician-scientist and a lead author on the study, said her findings could have critical implications for aging veterans who were exposed to Agent Orange during the Vietnam War.

Q. What inspired you to begin looking into Agent Orange exposure?

A. Years ago, a veteran called me up and said he had been denied help by Veterans Affairs. He explained he had been exposed to Agent Orange and had all of these problems. At the time, I didn’t know anything about it, but he was only asking to have a hearing and get some help. I went over his record, did some research, and wrote a convincing letter to get him a hearing and at least some coverage to address his medical issues. This poor man was suffering from all of these diseases as a veteran. It inspired me to find a stronger connection between exposure and corresponding, lifelong complications in hopes that something could be done so our veterans could get the care and coverage they deserve.

[For years, Veterans Affairs denied most disability claims by Vietnam veterans for certain conditions associated with exposure to Agent Orange. In 2015, a group of federal scientists agreed that more than 2,000 Air Force veterans should qualify for cash benefits for ailments they claimed had stemmed from flying aircraft contaminated by Agent Orange. An investigation by ProPublica showed there was a fierce internal debate about whether the government should dole out those benefits, since it would also mean other veterans would need to be paid as well.]

Q. What do we know about the exposure to Agent Orange, and how did you prepare for this study?

A. The goal of this research was to dive into the long-term effects of Agent Orange exposure decades earlier. Much of the initial work that we did was to do an epidemiological survey of veterans who had been exposed, and it was then that it became clear that they had quite a breadth of diseases, including increased rates of diabetes, skin disease, lots of cancers (which are still being identified), and then neuropathies that were debilitating. There was a suggestion that perhaps they had neurodegeneration, which was becoming evident. The rates of both Parkinson’s disease and Alzheimer’s tend to be much higher among the people who have some kind of exposure to Agent Orange.

Q. Is it hard to prove that these diseases or conditions are due to Agent Orange exposure and not just due to natural aging?

A. It’s hard to prove because you’re looking at a population in age who would ordinarily be in the group to get an aging-associated disease. But if the rates are higher, you have to suspect that there’s an additive effect, especially if there are no genetic or risk factors associated with it. We tested our hypothesis through experiments: Is it possible that exposure to these agents causes a neurological disease? And do those neurological diseases that you get to look at resemble anything that we know of that is like Alzheimer’s and Parkinson’s?

If you make the connection, you won’t get much sympathy from the military. They don’t want a connection, they want data. And it’s very hard to prove without a clear-cut biomarker.

Q. How do you prove these findings to the military?

A. You prove it experimentally, and that’s by looking at what’s in Agent Orange. There are two main toxins in Agent Orange, which we tested in the laboratory to see if the brain develops damage, and if that damage is what resembles what one gets in the early stages of Alzheimer’s. We investigated the effects of the toxins on markers of Alzheimer’s neurodegeneration using the samples from the frontal lobes of laboratory rats.

Q. Is Agent Orange still used today?

A. The use of Agent Orange was prohibited by the United States in 1971. Yet, the chemicals have remained in the environment, and there’s uncontrolled use of the toxins in herbicide and pesticide products. The use has been so widespread that one in three Americans has biomarker evidence of prior exposure.

The two main constituents of Agent Orange are 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid. The toxic effects of 2,4-dichlorophenoxyacetic acid are known, but federal agencies have not banned its use.

Q. So, technically, we’re still being exposed to these toxins?

A. These toxins are used in some herbicides. It’s everywhere. If you get your lawn done by a service, or purchased a product at a hardware store, this toxin is likely in it. They are in the water; they are everywhere. We’ve all been exposed.

Q. What’s the next phase of your research?

A. The next phase is finding veterans who have had their brains examined and look for the cardinal signs. I want to see if they’ve had Alzheimer’s and if it’s any different than regular Alzheimer’s. What other things were wrong with these patients? I don’t think anyone has done that study in any depth, and that’s what I plan to do next when I go down to Washington, D.C., and look at the cases myself.