But the condition is difficult to diagnose, with relatively few doctors understanding the scope of the disease. And of the PACT Act's listed presumptive illnesses, it is the only one without a proper medical code in the VA's claims system, presenting a roadblock that keeps sick veterans from receiving timely benefits.

During the panel Thursday, hosted by the American Legion, veterans and physicians said this all must change.

"We need a protocol and a system within the VA to help evaluate these people, and we need disability benefit structure for them as well," said Dr. Robert Miller, a professor at Vanderbilt University School of Medicine who was among the first physicians to diagnose constrictive bronchiolitis in combat veterans in the mid-2000s.

Constrictive bronchiolitis, also known as obliterative bronchiolitis or bronchiolitis obliterans, is the narrowing or obstruction of the lungs' smallest airways -- the bronchioles -- by scarring or fibrous tissue. The scarring prevents oxygen exchange between the lungs and the bloodstream, causing extreme fatigue.

Other symptoms include shortness of breath and unexplained wheezing or dry cough in the absence of a cold or allergies.

The only current way of definitively diagnosing constrictive bronchiolitis is via a lung biopsy -- a painful procedure that many veterans fear and doctors are reluctant to perform because it is invasive.

And even if veterans receive a diagnosis, Miller said, they face challenges getting disability compensation because the VA's disability claims system rejects the disease without a designated medical code.

Since they display no other symptoms, the likelihood of getting approval for another malady is slim, according to Miller. "Unfortunately, the current patient who has a biopsy-proven diagnosis for constrictive bronchiolitis is not going to be eligible for disability benefits, because their X-rays and pulmonary function tests are normal," he said.

The VA is currently working to create a constrictive bronchiolitis diagnostic code for its claims system through the federal rulemaking process, retired Army Col. Steve Miska, the VA's executive director for the PACT Act, said during the panel discussion.

Even as the department works toward the change, affected veterans may still file a claim, he added.

"If there's not a diagnostic code, we're not going to stop it. Sometimes, it requires your own advocacy ... or other[s] advocating on your behalf. And we recognize that should not be the case, and we're working to make sure that all our claims processors understand toxic exposure and how to adequately classify conditions," Miska said.

Affected veterans also would like to see advancements in diagnostic tools, such as imaging, for constrictive bronchiolitis that are less invasive than a lung biopsy. Dr. Trishul Siddharthan, an expert in functional lung imaging at the University of Miami Miller School of Medicine, told the assembled crowd that state-of-the-art scans, which can detect lung function and capability in real time, can help.

"Functional lung imaging can detect disease at a much higher rate than you can with traditional CTs. ... These modalities can be deployed in clinical settings and have the potential to screen larger [numbers of] patients," Siddharthan said.

Still, affected veterans see functional lung imaging as a tool that the VA should invest in. Daniels, an advocate for the group Burn Pits 360, has made expansion of its use at VA a priority.

"If you can use this type of technology and figure out what the veteran has without them having to go through the biopsy, why would we not do that?" Daniels asked.

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Danielle Robinson, whose husband, Ohio Army National Guard Sgt. 1st Class Heath Robinson, was the namesake for the PACT Act, said she is fully on board with the expansion of imaging and improved services for sick veterans.

According to Danielle, Heath Robinson's complaints of fatigue and difficulties working out were dismissed by his doctors as low testosterone and a "changing body" as he aged. By the time he was diagnosed nine months later with lung cancer, his prognosis was terminal. A doctor later described his lungs following an autopsy as "obliterated."

"If this imaging would have been there for Heath to do, who knows, he could have been here today," Danielle Robinson said.

Miller, the Vanderbilt professor, said he also is pressing for a "network" of providers nationwide who are familiar with burn pit-related lung conditions.

While Miska, the PACT Act executive director, said the VA has trained 90% of its providers about toxic exposures, has more advanced specialized training and operates six centers of excellence for toxic exposures, Miller said the centers don't have the "capacity to evaluate the population that we've taken care of."

[RELATED: Lawmakers Urge VA to Reduce Backlog, Wait Times on Veterans’ Claims for Benefits]

Miller did not provide any data on the number of patients he has seen with constrictive bronchiolitis or the numbers nationwide.

"What I talked to senior [VA] leadership about today was to enhance the system so that when the local providers see somebody that they can't diagnose, they have a place to refer them to," Miller said, referring to a meeting before the event with veterans organizations and the under secretaries of health and benefits at the VA. "And then the second thing is that we need a structure to give them a disability [rating]."

Miska said the VA is teaming with other federal agencies to conduct research on toxic exposures, assessing the technology, and standardizing protocols to treat veterans with environmental illnesses.

"We're not going to stop until we get it right," he said.