Robert F. Kennedy Jr. has often trained his criticisms of vaccines on a common ingredient: aluminum, which he has suggested is responsible for a litany of childhood ailments, including food allergies, autism and depression.

“You wonder why a whole generation of children is allergic to stuff,” Mr. Kennedy said during a 2021 interview. “It’s because we’re inducing allergies, pumping them full of aluminum.”

To many vaccine scientists, aluminum is a strange target. It is among the most studied ingredients in vaccines, and perhaps in modern medicine.

“There’s a huge amount of information that’s gathered,” said Dr. Andrew Racine, a pediatrician and chief medical officer at the Montefiore Health System. “If there was something jumping out about a lack of safety, we would most likely have seen it someplace, and it just doesn’t appear.”

Aluminum salts, a more soluble form of the metal, are added to vaccines like the Tdap shot to bolster the body’s immune response. The ingredient has repeatedly been evaluated as a part of vaccines in clinical trials and administered in billions of doses over several decades.

Even so, as Mr. Kennedy prepares for confirmation hearings as secretary of health and human services, many experts fear the standby will face fresh scrutiny, and may even serve as a justification for limiting access to several childhood vaccines, like the shots for hepatitis B and pneumococcal disease.

Aaron Siri, a lawyer and close ally of Mr. Kennedy, has represented clients in petitioning federal regulators to pause the distribution of more than a dozen vaccines until the makers provided more information about the amount of aluminum in the formulations.

Mr. Kennedy and Mr. Siri did not respond to requests for comment.

The origins of added aluminum in vaccines can be traced back nearly a century. In a stable on the outskirts of Paris, a young veterinarian had made a peculiar discovery: mixing tapioca into his horses’ diphtheria vaccines made them more effective.

The doctor, Gaston Ramon, had noticed that the horses who developed a minor infection at the injection site had much more robust immunity against diphtheria. He theorized that adding something to his shots that caused inflammation — ingredients he later named adjuvants, derived from the Latin root “to help” — helped induce a stronger immune response.

After testing several candidates — including bread crumbs, petroleum jelly and rubber latex — he found success with a tapioca-laced injection, which produced slight swelling and far more antibodies.

Tapioca never caught on as an adjuvant. But in 1932, a few years after Dr. Ramon’s studies were published, the United States began including aluminum salts in diphtheria immunizations, as they were found to invoke a similar but more reliable effect.

Today, aluminum adjuvants are found in 27 routine vaccines, and nearly half of those recommended for children under 5.

This extra boost of immunity is not needed in all types of vaccines. Shots that contain a weakened form of a virus, like the measles mumps and rubella shot, or created with mRNA technology, like the Pfizer and Moderna Covid-19 vaccines, generate strong enough immune responses on their own.

But in vaccines that contain only small fragments of the pathogen, which would garner little attention from the immune system, adjuvants help stimulate a stronger response, allowing vaccines to be given in fewer doses.

Scientists believe that aluminum salts work in two ways. First, aluminum binds to the core component of the vaccine and causes it to diffuse into the bloodstream more slowly, giving immune cells more time to build a response.

It’s also thought that aluminum operates more directly, enhancing the activity of certain immune cells, though this mechanism is not fully understood.

Though aluminum salts are not the only adjuvants on the market, vaccine makers often prefer them because of a lengthy track record of safety data.

Each time a new vaccine that uses an aluminum adjuvant is developed, it undergoes lengthy clinical trials to evaluate its safety, and side effects are continuously monitored after approval. One of the earliest trials including an aluminum adjuvant was published in 1934.

Later trials revealed some minor side effects, like redness, body aches and, in rare cases, painful nodules at the injection site. But there is little credible evidence that aluminum in vaccines causes serious, long-term side effects, as Mr. Kennedy has suggested.

People who are routinely exposed to large quantities of aluminum — such as workers who breathe in aluminum dust, or dialysis patients who routinely receive aluminum-rich medications — may experience respiratory, bone and neurological complications.

The amount of aluminum in childhood vaccines, however, is trivial compared with what people are routinely exposed to via the environment and food, said Dr. Tony Moody, director of the Duke CIVICs Vaccine Center.

“We’re exposed to aluminum constantly,” he said. “If you inhale dust from the outside, you’re coming into contact with aluminum.”

In the first six months of babies’ lives, vaccines expose them to roughly 4.4 milligrams of aluminum. One slice of American cheese, by contrast, can contain as much as 50 milligrams of aluminum.

When scientists compared aluminum levels in the blood and hair of babies who had been given aluminum-containing vaccines versus those who hadn’t, they found no significant difference.

Some scientists believe that one potential side effect needs additional research.

In 2022, after a federally funded study found a minor association between aluminum exposure from vaccines and asthma, the C.D.C. noted that the link warranted “further investigation.” (The agency said it would not change vaccine recommendations “based on a single study.”)

Mr. Kennedy, who was then the chairman of Children’s Health Defense, a nonprofit frequently critical of vaccines, held out the study as evidence that immunizations were causing an “asthma epidemic.”

But the authors of the paper were careful to point out the limitations of their findings, most notably that the effect was small and that the study was observational, which means it cannot prove cause-and-effect.

In the paper, they cautioned that their results “do not constitute strong evidence for questioning the safety of aluminum in vaccines.”

Independent experts also noted that the authors did not collect data on several key risk factors, like whether the participants were exposed to cigarette smoke or had a family history of asthma — either of which could explain the difference they observed.

In response to the study, a group of researchers from the Statens Serum Institut, Denmark’s public health agency, ran a similar analysis on a national data set of more than 450,000 children.

Their preliminary results did not show an association between aluminum adjuvants and asthma, they told the C.D.C.’s Advisory Committee on Immunization Practices in 2023.

Still, a C.D.C. spokeswoman said the agency was “discussing additional studies” to investigate the potential risk.

But until rigorous research can confirm the finding, the link between aluminum and asthma is tenuous at best, said Dr. Stanley Plotkin, who played a pivotal role in helping create the rubella vaccine.

“You cannot change conclusions from any single paper,” he said. “You have to look at the overall literature.”

Anders Hviid, who led the Danish study, said he welcomed more research into the safety of adjuvants, but he added that these studies might never be enough to convince skeptics that aluminum does not cause long-term side effects.

“You cannot prove a negative,” he said. “There will always be this goal-post moving, saying, ‘Well, you didn’t look at this and you didn’t look at that in that way.’”