Secret biological and chemical warfare tests conducted on U.S. Navy ships in the 1960s do not appear to have affected the health of participating sailors and Marines, according to a report released earlier this month.

The Institute of Medicine study found that troops who took part in Project Shipboard Hazard and Defense, or SHAD, which was part of a larger chem-bio program known as Project 112, had no significant difference in mortality rates or diseases than veterans of comparable age or time in service.

However, the researchers noted, the study was limited by data availability. Much of the measurement readings and information on the concentrations of contaminants still remain classified, and many Project SHAD veterans are in their 70s and 80s or have passed away, according to the report, published by the non-profit Institute of Medicine, a division of the National Academies of Sciences, Engineering, and Medicine.

Project SHAD was designed to study ship vulnerability to chemical and biological warfare and develop ship-board procedures to respond to such an attack.

More than 5,900 sailors and Marines participated in the tests. Most of them were unaware of the nature of the experiments, which spanned nearly a decade and involved spraying biological agents, simulated agents, tracers and even mosquitoes on ships and sailors and then decontaminating them with a sporicide that is a known carcinogen.

The tests remained secret until the late 1990s, when veterans began seeking health care and disability compensation for illnesses they believed were related to the experiments. The Defense Department released fact sheets on the tests in the early 2000s but still has not declassified much of the related material.

Veterans Affairs Department studies and a 2007 Institute of Medicine report found no evidence of increased incidents of disease among Project SHAD veterans, with the exception of a higher risk of heart disease, although for the most part, the authors found no consistent, specific patterns of ill health among veterans.

But SHAD participants and lawmakers challenged the methodology of the study and in 2010, Congress ordered another Institute of Medicine review.

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The most recent report found there is no increased risk of any disease for the studied veterans, including the George Eastman crew.

David Tollerud, an environmental health sciences professor at the University of Louisville who chaired the most recent SHAD review committee, said the members recognize the limitations of studies that look at a population over time, adding that the report’s negative findings "cannot unequivocally rule out some potential effect from the SHAD testing."

"However within the limits of the data available, the results of the analysis provide no evidence that the health of SHAD veterans is significantly different from that of similar veterans," Tollerud wrote.

Retired Navy Lt. Cmdr. Jack Alderson served as commodore for five Army tug boats that participated in Project SHAD. He has had multiple melanomas, open heart surgery, prostate cancer and allergies and respiratory issues he said began after he received Project SHAD-related vaccines.

Now 83, he continues to advocate for participating veterans and said he takes issue with the IOM's most recent findings.

"They didn't take into account any of the sailors who died before 1979 and they didn't look at our medical records, because they can't find them," Alderson said.

"When we first went to the VA for medical care, many of us were turned away. No one believed that the government would do that to its service members," he added.

Project 112 was a series of experiments designed to test the military's readiness against biological and chemical attack. It involved all branches of the service, with the maritime portion dubbed Project SHAD.

The ship-board tests took place in the North Atlantic, open water of the Pacific Ocean and Marshall Islands, Hawaii, Puerto Rico and the California coast.

Alderson remembers military aircraft spraying a variety of substances over the tugs, later identified as E. coli, Coxiella burnetii, an organism that causes Q fever, and Bacillus globigii, a microbe used as an anthrax simulant.

Some tests involved barges of rhesus monkeys, which were sprayed with sarin nerve gas and VX, an extremely toxic nerve agent.

Jack Barry served in the Army chemical corps for five years and later worked as a biological test officer for the Department of Defense on Project 112/Project SHAD. He believes that participating troops were not exposed to actual chemical or biological weapons but added that they did come into contact with simulants and decontaminants that are known to cause illness.

Barry said the most recent IOM report does not take into account the psychological impact of having participated in the tests.

"It says nothing about post-traumatic stress disorder from having participated and having lifelong worries about your health," Barry said. "What's so baffling to me is there were so relatively few sailors involved. I don't know why VA just doesn't give them the benefit of the doubt and award health benefits and disability compensation."

An IOM spokewoman said the committee did not try to get the military medical records of sailors while they were on active duty but did use information from VA and Medicare since 1999.

She added that committee members included mental disorders, including depression in the range of disease they examined.

Project 112/Project SHAD veterans are eligible for VA health care as members of Priority Group Six, meaning they have access to care along with veterans of the Vietnam and Persian Gulf Wars as well as those who experienced environmental exposures, such as ionizing radiation and contaminants in the Camp Lejeune drinking supply system until 1987,

No diseases are considered to be presumptive for disability compensation related to Project SHAD; all veterans claims for Project 112/Project SHAD are decided on a case-by-case basis.

Rep. Mike Thompson, D-California, has pressed for transparency on Project 112/Project SHAD since he was voted into office in 1999. The congressman, who pressured the Defense Department to release information about the secret experiments, said he will continue to advocate for veterans of the tests despite the most recent report findings.

"The DoD has admitted that it tested harmful chemical and biological agents by spraying them on ships and sailors, exposing them without their knowledge to … some of the most deadly chemicals on Earth: VX nerve gas, Sarin nerve gas and E. Coli," Thompson wrote in an email to Military Times. "Our country owes it to those still living to make sure they get the treatments and benefits they have earned."

The number of Project SHAD veterans continues to dwindle, but Alderson said he will not stop fighting for shipmates who have chronic pain, respiratory and cerebrovascular diseases.

"Nowadays, even though almost all of us have health care, some still have to make co-payments for service-connected conditions, and others deserve disability payments," Alderson said. "If they aren't going to take care of us, at least they could acknowledge our service to the country."

This story was updated Wednesday to include clarifications from the Institute of Medicine.

Patricia Kime covers military and veterans' health care and medicine for Military Times. She can be reached at pkime@militarytimes.com