(CNN) When two hijacked passenger planes crashed into the World Trade Center towers on September 11, 2001, a massive cloud of dust swept across the New York skyline.
The looming cloud, caused by the twin towers' collapse and the digging in ground zero, carried chemicals and carcinogens such as perfluoroalkyl substances or PFASs, a class of chemicals used to make products stain-resistant, nonstick or waterproof.
Many dust particles brought such chemicals into New Yorkers' homes and intimate spaces.
Now, a study suggests that exposure to PFASs after the 9/11 attacks could be tied to abnormal cholesterol levels in teens and young adults who were children when the towers fell. Such high cholesterol can be a risk factor of heart disease.
The study was published in the journal Environment International on Thursday.
"We know in other studies, not of World Trade Center exposure, that levels of these PFASs have been associated with increases in cholesterol. So we're not the first to show an association of PFASs with cholesterol," said the study's lead author, Dr. Leonardo Trasande, a health epidemiologist and associate professor at the New York University School of Medicine.
"It's just that we found the same association in a population that we had already proven to have higher levels of PFASs in relationship to reports of dust cloud and home dust exposure," he said. "The increases in cholesterol raise concerns about potential longer-term consequences, and that simply reinforces the need for ongoing monitoring of heart health in kids who were exposed to the disaster, and the need to maintain a healthy diet and physical activity."
On 9/11, about 25,000 children younger than 18 were living or attending school in lower Manhattan, near the World Trade Center.
Separate studies have linked dust from the 9/11 attacks with high rates of asthma in children living near the site, as well as psychological impacts such as post-traumatic stress disorder.
The US Centers for Disease Control and Prevention's World Trade Center Health Program provides medical monitoring and treatment for responders and survivors who were in New York during the attacks.
The CDC said in an emailed statement that as of June, the program has 1,091 members who are 35 or younger, which means they would have been about 19 or younger on September 11, 2001.
Additionally, the World Trade Center Health Registry, based in the New York City Department of Health and Mental Hygiene, was developed to track and evaluate the short- and long-term physical and mental health effects of 9/11. Nearly 3,200 of its enrollees were younger than 18 on 9/11.
Often, if adverse 9/11-related health outcomes or symptoms are detected among the city's registry enrollees, they are referred to seek treatment through the CDC's program, said Mark Farfel, director of the World Trade Center Health Registry.
"The most common 9/11-related health outcomes are post-traumatic stress disorder, depression, respiratory-type conditions including asthma, and gastroesophageal reflux disease," Farfel said, adding that other potential conditions include heart disease in adults, the inflammatory disease sarcoidosis and increased smoking and drinking.
A hallmark of these 9/11 health conditions is that more than one can occur at the same time in a person, Farfel said.
"The registry is going to continue to follow all of our enrollees across time. So as the children become young adults and adults and later, we'll be able to follow up," he said. "Of course, there's always the potential for new emerging conditions."
Previously, Trasande and his colleagues measured levels of PFASs in children exposed to the World Trade Center collapse and compared them with New York children who weren't exposed.
"Generally, we found about a 20% or so increase in levels of PFASs among kids who had dust cloud exposure or home dust exposure," Trasande said of the previous study, published in the journal Environmental Research in April.
For the new study, the researchers used those data to determine whether there were differences in the overall health of the children exposed to PFASs compared with those not exposed.
The new study involved 123 New Yorkers born between September 11, 1993, and September 10, 2001, who are enrolled in the World Trade Center Health Registry and 185 New Yorkers in that age group who were not eligible to enroll in the registry.
The registry is the largest post-disaster registry in US history, according to its website.
The researchers collected and analyzed blood samples from all of the study participants, measuring their cholesterol levels, insulin resistance and triglycerides, or a type of fat in the blood, along with other health factors. The researchers also measured each participant's PFAS exposure.
For those participants with high PFAS exposure, "what we found was substantial increase in their cholesterol levels and in particular their LDL component, which is known to be a risk factor for later cardiovascular disease," Trasande said. LDL, or low-density lipoprotein, makes up the majority of the body's cholesterol and is known as "bad" cholesterol.
"For later cardiovascular disease, it begs the question whether we'll see downstream consequences in the years to come," he said.
It turned out that for each threefold increase in a person's level of PFAS exposure, they had a 9% to 15% rise in cholesterol, Trasande said.
"Fortunately, we didn't see increases in blood pressure or any signs of increases in stiffness of the arteries in the adolescents and young adults that we studied in relationship to these levels of chemicals," Trasande said.
As for what might explain the high LDL cholesterol levels, Trasande said, "there are two likely mechanisms. Inflammation may be one such mechanism, and then in particular there are concerns about specific hormonal disruptions."
Still, he added that the new study findings show a correlational relationship between 9/11 and cholesterol levels, not a causal one.
"It's very difficult to go from association to causation," said Dr. Pauline Thomas, professor and director of Preventive Medicine Residency at Rutgers New Jersey Medical School, who was not involved in the new study.
The researchers "are aware that they haven't taken into account the mental health trauma that the children may have had, and diet is a big source of the PFASs, and they did a dietary history, but dietary histories have a lot of limitations. So I think there is more work to be done," she said. Yet "this is a very interesting and well put-together paper. It's an important paper and intriguing."
Another limitation of the study was that even though some of the participants were not eligible for the World Trade Center Health Registry, that doesn't mean they were not exposed to the 9/11 attacks at all, Trasande said. After all, the attacks affected most New Yorkers.
"There were many children exposed to the World Trade Center disaster," said Thomas, who has conducted separate research on the link between 9/11 exposure and a high prevalence of asthma among children.
"Most of them have become young adults, and I think they have a very unique role to play in defining how a disaster can affect long-term health," she said. "In any environmental study, it's very difficult to sort out which exposure's which, but I think this group has taken a shot at looking at one particular exposure, and I hope they will continue to pursue their endeavors."
Being acutely exposed to the dust cloud or suffering injury on 9/11 was found to be associated with heart and respiratory diseases later in life in a study published in the journal Injury Epidemiology in July. However, that study did not involve children.
"We found that, in this particular group of people who had an acute exposure, injury was associated with heart attack and also angina (a type of chest pain) and that the dust cloud had an association with respiratory problems, like asthma and other respiratory diseases," said Dr. Robert Brackbill, director of research for the World Trade Center Health Registry.
"We actually published two previous papers on the heart as well, and ... we did find that, for instance, PTSD was associated with self-reported heart disease," he said. "Injury is a trauma and people who are injured in a disaster situation may have post-traumatic stress disorder, and PTSD is an established risk factor for heart disease, so that could be one way it's happening."
Yet Brackbill added that "we have a very large proportion of people on the registry who don't have PTSD and don't have health problems, so we want to make that point that ... there are many people who have continued on with their lives and have been able to not have this effect," he said.
Health conditions that have been associated with 9/11, such as those covered by the World Trade Center Health Program, include acute traumatic injury, airway and digestive disorders, cancer, mental health conditions and musculoskeletal disorders.
For instance, as of June 30, the CDC said in an emailed statement that 7,139 people enrolled in the program were certified with at least one type of cancer covered by the program.
As for children, Farfel said that the city's registry will plan to continue to track their health and refer them to the CDC's program if they have 9/11-related health needs.
"For those who do need care, it's known through the literature that health care utilization tends to be low following a disaster, and there are lots of reasons for that," he said. "People may not connect their symptoms to the disaster or may feel like others need it more, or maybe they're concerned about mental health stigma. They just don't know about the resources that are available."