The dust in the air was so thick after the World Trade Center’s twin towers fell, it reminded Brian McGuire of a blizzard.

On Sept. 11, 2001, he was off duty from his job as an emergency medical technician with the New York City Fire Department (NYFD), when he saw United Airlines Flight 175 hit the south tower.

Together with a group of other off duty firefighters, McGuire, then 23, rushed to lower Manhattan.

By the time they arrived, both towers had collapsed.

“You couldn’t see what street you were on because the dust was just as thick as snow, and you really didn’t have much vision,” McGuire told Healthline.

That dust, which remained in the air for days, covered everything and everyone in the area.

It held a mixture of toxins and irritants that included asbestos, polychlorinated biphenyls (PCBs), benzene, dioxin, glass fibers, gypsum, cement particles, and heavy metals such as lead, among other substances.

The enormous mass of debris from the fallen towers, referred to as the Pile, continued to smolder until mid-December, heating and combining the toxins.

McGuire worked in the area as part of the search and rescue effort to find survivors, and later to recover bodies, until the end of October.

He recalled the Environmental Protection Agency (EPA) announcing in the days following the terrorist attack that the air was safe to breathe.

But the EPA was wrong.

Fifteen years later, McGuire, now 38, suffers from several illnesses certified by the World Trade Center (WTC) Health Program as related to 9/11. These include chronic bronchitis, reactive airway disease syndrome, gastroesophageal reflux disease, sleep apnea, and sinusitis so severe that he required surgery.

And he isn’t alone.

According to the WTC Health Program, more than 37,000 people have at least one medical condition related to the 9/11 attacks.

Read more: Why rheumatoid arthritis is plaguing 9/11 first responders »

Treatment for illnesses

In January 2011, nearly a decade after the attacks, President Obama signed into law the James Zadroga 9/11 Health and Compensation Act of 2010.

The Zadroga Act created the WTC Health Program, which provides treatment and covers medical expenses for a list of conditions directly linked to 9/11.

So far, that list has more than 90 health conditions.

They include numerous aerodigestive disorders, such as asthma and chronic obstructive pulmonary disease (COPD), which were likely caused by the toxic dust people inhaled.

It also includes mental health conditions such as depression and anxiety disorder, which have been linked to the trauma of exposure to the 9/11 attacks.

It also includes more than 60 types of cancer.

Before the Zadroga Act, many 9/11 responders and survivors had to rely on their own health insurance — which, the New York City Department of Health reports, did not always cover their conditions — and had to pay extra medical expenses on their own.

Today, McGuire told Healthline, “You don’t pay a penny out of your pocket.”

The WTC Health Program also offers yearly medical monitoring to 9/11 responders, which includes thousands of people who worked or volunteered as part of the emergency, recovery, and cleanup efforts in lower Manhattan, at the Pentagon, or at the crash site near Shanksville, Pennsylvania.

Even responders who aren’t sick have the option of enrolling for annual physicals that include blood work, a breathing test, and an in-depth health questionnaire.

McGuire values the yearly testing because it could help catch a disease early, allowing for quicker treatment.

Prevention is a big focus of the monitoring program, but it also has a secondary purpose.

Responders can consent to have their yearly health data made available to researchers.

Dozens of research projects are devoted to understanding the health effects of the terrorist attacks.

Over time, researchers may find links to more diseases, and the list of health conditions covered by the Zadroga Act could grow even longer.

That research is vital for 9/11 responders and survivors, since the WTC Health Program doesn’t generally provide treatment for health conditions that aren’t on its list.

Read more: 9/11 survivor shares her story of addiction and recovery »

The link between 9/11 and disease

It hasn’t been easy to prove a clear connection between 9/11 exposure and the various diseases that followed.

It’s taken years of research.

In general, studies that found a link between 9/11 toxins and specific diseases were based on tracking people who were exposed to see if they had higher rates of illness than the general population.

That means people were already sick, and had potentially been sick for years or died from their illnesses, before officials acknowledged the connection.

Last year, a study in the journal Arthritis and Rheumatology found that first responders who had prolonged exposure to the 9/11 site were at higher risk for immunological diseases, like rheumatoid arthritis.

That disease still isn’t on the WTC Health Program’s list of covered conditions.

As research continues, the process to identify new conditions linked to 9/11 will likely get quicker, according to Dr. Benjamin Luft, director of the Long Island Clinical Center of Excellence for the WTC Health Program.

In the meantime, he noted, “It’s very frustrating for patients who are debilitated by these diseases to get the help they need.”

Today, there’s generally no problem getting coverage for conditions that are obviously related to 9/11 exposure, Luft said, such as chronic breathing or sinus problems.

But it can be more difficult for diseases that have a delayed onset.

“The real concern is that there were thousands of toxins that people were exposed to. It would be one thing if you had just one toxin and people were exposed to it,” Luft told Healthline. “But the different types of chemicals in the air and in the environment were really quite vast. That’s one of the problems, that the different manifestations are so myriad and so broad.”

Read more: PTSD can last for years for people who witness attacks like 9/11 »

Cancers linked to 9/11

Many cancers have long latency periods, meaning the disease can show up years after the toxic exposure that caused it.

A study by Mount Sinai’s WTC Health Program found that 9/11 rescue and recovery workers had a 15 percent higher risk of all types of cancer compared to the general population, with especially high rates of prostate and thyroid cancer.

Luft, who was not involved in the research, expects that cancer rates will continue to rise among those exposed to the toxic dust.

Other doctors told Healthline that cancers related to 9/11 seem to occur at unusually young ages.

“We see a lot of patients with cancers that are presenting younger than expected or with multiple cancers,” said Dr. Denise Harrison, director of New York University School of Medicine’s WTC Health Program.

In 2012, the first types of cancer were added to the list of conditions eligible for treatment through the WTC Health Program.

That was a little late for Howie Scott, who was diagnosed with colorectal cancer in 2010, before the Zadroga Act was approved.

A born-and-raised New Yorker, Scott was 39 years old and an NYFD firefighter during the 9/11 attacks.

He was at the site when the towers collapsed and remained all day, helping to evacuate the area and search for survivors.

For the next eight months, he worked regularly at the 9/11 site.

Less than nine years later, when Scott was 47, he found out he had colorectal cancer.

He underwent seven weeks of radiation and chemotherapy, along with surgery to remove the cancer.

Scott recovered, but the lingering effects of the condition and treatment forced him to retire from firefighting.

He had health insurance through the NYFD, but his co-payments and extra medical expenses still added up to thousands of dollars.

Today, those expenses would be covered under the WTC Health Program.

Since 2012, more than 5,400 first responders and survivors with cancer have had their condition certified as related to 9/11.

Scott, now 54, told Healthline that he’s just glad that his cancer was detected early and his treatment was successful.

He’s learned of other 9/11 firefighters who passed away from the same cancer he had.

“As firefighters, you know, we’re macho, we think we can deal with anything, we can overcome anything, and we’ve come to learn that, ‘You know what? We’re just as vulnerable as the next guy,’” Scott said.

Read more: Link between PTSD and cognitive impairment found in 9/11 responders »

A national issue

Nearly 3,000 people died during the 9/11 terrorist attacks.

No one knows exactly how many people have died from illnesses related to 9/11 since then.

But there’s at least one place that honors the memories of responders who passed away from those illnesses: 9/11 Responders Remembered Park, in Nesconset, New York.

There, the names of 588 responders are etched into three, 6-foot-high walls.

All are believed to have died from health conditions linked to 9/11.

John Feal, who co-founded the park, told Healthline that 99 more names will be added at a ceremony on Sept. 17 this year.

Feal, a former construction worker, helped with the rescue and recovery effort in the days just after the attacks.

He later founded the FealGood Foundation, one of the leading organizations that campaigned for federal lawmakers to pass the Zadroga Act in 2010, and to extend the law before it expired in 2015.

“For years, they said we weren’t sick and we were making it up,” Feal told Healthline. “And then science finally caught up to us.”

Now, the legislation will provide health benefits to 9/11 responders and survivors for 75 years.

The next pressing issue is to make sure that everyone who is eligible for the WTC Health Program is actually enrolled.

In the days and months following 9/11, people from every state traveled to work or volunteer at areas affected by the attacks.

Under a grant from the National Institute for Occupational Safety and Health, Feal and several firefighters travel to different states to reach out to responders who don’t know about the WTC Health Program.

So far, they’ve visited 10 states and helped about 400 responders enroll.

“It was not just a New York, New Jersey thing,” Jim Preston, one of the firefighters involved with the FealGood Foundation, told Healthline. “It’s a national issue.”