(CNN) Jean Bryant, a Seattle mother of three boys, remembers the moment when her friend Amy Griffin first mentioned those "little black dots."

It was about seven years ago, and their children were in preschool together. Griffin, assistant head coach of women's soccer at the University of Washington, mentioned to Bryant how "it was weird" that some of her current and former players, especially goalkeepers, had been diagnosed with cancer.

Griffin turned to Bryant and said she wondered whether cancer was somehow associated with those "little black dots" on the artificial turf fields where they play, which were then replacing natural grass fields. Those dots are recycled tire crumbs.

"I didn't think much of it until my son," Bryant said. Years after that conversation, just before he turned 14, Jack Bryant was diagnosed with cancer.

He's a soccer goalie.

Coach and her cancer list

Artificial turf fields made of synthetic fibers to mimic grass and tire crumbs to mimic soil, are often used for most outdoor sports due to their weather resistance, low cost, no need to be watered or fertilized, and year-round accessibility, which some turf industry experts argue could promote more exercise and then even reduce cancer risk

Artificial turf is used in thousands of parks, sports fields, and stadiums across the country.

chemicals Yet Griffin couldn't get the little black dots out of her mind. She searched online for more information about artificial turf fields and discovered that the tire rubber contains that can be harmful if you are exposed at high levels, she said.

Then, in 2014, she compiled a list of the players she was connected to who were diagnosed with cancer. When the list began drawing attention, the Washington State Department of Health and researchers at the University of Washington School of Public Health reached out to conduct an investigation into whether the cancer rate seen in Griffin's list was unusual. The study, published last week , concluded it was not, and recommended that "people who enjoy soccer continue to play irrespective of the type of field surface."

Among the 53 players on Griffin's list, many of the diagnoses were blood cancers, including leukemias, non-Hodgkin's lymphomas and Hodgkin's lymphomas. More than 60% of the players were goalkeepers, Griffin said.

Jack Bryant playing in his last middle school soccer game in May 2015

Bryant's oldest son, Jack, was added to the list in 2015. He was diagnosed with Hodgkin's lymphoma after he visited his doctor with a tenacious cough and was found to have swollen lymph notes in his chest.

"On Mother's Day, I told my son that he had been diagnosed with Hodgkin's lymphoma," Bryant said, adding that she received the news from a nurse's phone call. "My commitment was not to break down in tears and to fall apart. That wasn't my job as his parent. My job was to let him know that it was going to be OK."

Jack Bryant during his first cancer treatment in the ICU in June 2015.

Jack continued to play soccer after his first cancer treatment in an effort to continue normal activities, Bryant said. But he relapsed, and he hasn't played on a soccer field since.

"No one at this point can convince me that playing on ground-up tires is a healthy option for kids," she said.

However, the study results from the Washington investigation did not suggest that soccer players are at an increased risk for cancer due to the tire crumbs.

'It's the one thing that I still don't feel great about'

The results showed that the cancer rate among soccer players in the investigation, based on Griffin's list, was actually less than expected compared with the cancer rate among all Washington residents of similar ages.

For the players' cancer cases to be a cluster, there would have to be a greater-than-expected number of cases of the same type of cancer, according to criteria listed by the Centers for Disease Control and Prevention

"It's important to note that the investigation was not designed to discover the causes of cancer among the people, nor was it designed to add to our understanding of the risks or benefits of crumb-rubber fields," said Cathy Wasserman, state epidemiologist for non-infectious conditions, in her opening statements about the investigation last week.

In other words, the study wasn't designed to identify whether exposure to tire crumbs caused cancer among some of the players. Rather, the investigation was to determine whether the cancer diagnoses were higher than would be expected and then qualify as a cluster.

Griffin, the soccer coach, still has concerns, and she wants more studies.

Other than the turf, Griffin said, she couldn't point to any other potential factors that might be linked to cancer among the goalkeepers on her list.

"When I keep adding up these things, in my head, it's the one thing that I still don't feel great about," she said of crumb rubber being a possible factor.

"Goalkeepers get it in their sides, hips, elbows, abrasions from sliding on the stuff. So if they have an open sore, not only the black dots but the dust particles that you can't even see when the tire crumb breaks down so small get in there. I'm sure you eat it and inhale it," she said. "Just in a 10-minute warmup, our keepers will hit the ground anywhere from 50 to 100 times."

Since the investigation, Griffin said many more people have reached out to add names to her list.

Now, Griffin's list of soccer players with cancer has grown to 237, she said, and her team still uses two fields, one grass and one artificial.

For Jack Bryant, whose hair is growing in curly after chemo, "life is back to normal," he said. "I'm doing basketball right now. It's fun."

A growing debate over turf

More than 12,000 synthetic turf sports fields are in use throughout the United States, according to the Synthetic Turf Council, a trade association representing the synthetic turf industry, and they're used by many types of athletes, including football players.

Daniel Bond, president and CEO of the council, issued a statement in response to the Washington investigation that said "We applaud the Department of Health's recommendation that children and athletes should continue to play on these surfaces and enjoy the many benefits of exercise and physical activity.

"In fact, the Department of Health's finding of less cancer in this group of youth athletes than would be expected -- given background rates in Washington -- goes to show yet again how physical activity is among the strongest strategies for preventing cancer, and that, if anything, synthetic turf fields aid this fight against cancer," the statement said.

The crumb rubber used as infill for the fields typically comes from recycled tires and contains manmade chemicals such as polycyclic aromatic hydrocarbons and volatile organic compounds , said Michael Peterson, a board-certified toxicologist at the environmental and risk sciences consulting firm Gradient . He is also a scientific adviser to the Recycled Rubber Council.

"For many of those chemicals, the levels found in crumb rubber are very similar to what you find in natural soil," Peterson said.

"Out of the more than 100 studies that have been conducted in the past, there is no consistent evidence that inhalation, contact or ingestion of crumb rubber infill poses a cancer risk," he said.

Documents prepared by the US Environmental Protection Agency and the CDC have also noted that chemicals in synthetic, crumb-rubber fields are often found in surface soil and likely to be present in natural grass fields (PDF).

Last month, Peterson said, the Dutch National Institute for Public Health and the Environment published one of the most recent studies that "shows it is safe for people to play sports on synthetic turf fields with an infill of rubber granulate," as written on the institute's website.

On the other hand, a 2013 study conducted by researchers in Spain, published in the journal Chemosphere , found that concentrations of polycyclic aromatic hydrocarbons in recycled rubber tires are high enough to be "a matter of regulatory concern."

Vasilis Vasiliou, a molecular toxicologist and professor of epidemiology at the Yale School of Public Health, wants more data.

He said he is partnering with Griffin to conduct additional research to dig deeper into exactly how many chemicals and carcinogens from crumb rubber might end up in athletes' bloodstreams.

'We need to know how safe it is'

"The question for me is, do these chemicals get into the blood of our kids?" Vasiliou said. "We need these comprehensive studies to really give us some answers. We need to know how safe it is for our kids to be playing on these fields."

Vasiliou, along with his colleagues at Yale and researchers at the University of Washington, recently applied for a research grant from the National Institute of Environmental Health Sciences . Their application is scheduled for review in early March, Vasiliou said.

The researchers plan to assess human exposure to chemicals in crumb rubber through analysis of biological samples taken from college soccer players.

Once research sheds light on how chemicals may enter the blood through exposure to artificial turf, that is a starting point to further investigate possible cancer risks or other health risks, such as asthma, among athletes, Vasiliou said.

"I don't let my kids play on artificial turf," he said, adding that his 9-year-old twin boys and 11-year-old daughter play basketball in the winter instead of playing on artificial turf fields for indoor soccer.

Some experts aren't sure whether additional research will result in any new findings.

Andrew McNitt, a professor of soil science and director of the Center for Sports Surface Research at Pennsylvania State University, said the recent Washington study findings are not that surprising. He was not involved in the investigation.

"As a scientist who is not a toxicologist, I don't see any trends in existing data indicating any real significant issues with tire crumb or synthetic surfaces and cancer-causing agents," said McNitt, who gave the opening remarks Wednesday at the national Sports Turf Managers Association conference in Orlando.

"But with that being said, no research is ever completely definitive," he said. "If there is a significant issue, we have a much bigger issue in this country than synthetic turf. Tires are constantly degrading in the environment near you. Whenever you are driving down the road, you are exposed to tire dust."

Further investigations

Washington is not the only state to have conducted an investigation into synthetic turf fields.

"We found that health risks were not increased by the vapors and particulates that could be inhaled from playing on the fields. This included cancer and non-cancer health effects and agrees with findings from a number of other studies and analyses," Maura Downes, a spokeswoman for the department, wrote in an email.

"Synthetic turf is comprised of both the plastic blades of grass and crumb rubber. The blades of grass are made of plastic and are colored with paint that in some cases has been found to contain lead. We tested for lead in the blades of grass on the fields we studied in Connecticut and did not find elevated lead concentrations," she said.

Downes said the investigation also analyzed a variety of chemicals in crumb-rubber turf and determined that the exposure to these chemicals was not enough to pose a health concern.

Connecticut did "an amazing amount of work in their studies," Yale's Vasiliou said, but he added that for indoor synthetic soccer fields, the state still recommended that indoor building operators provide adequate ventilation to prevent a buildup of rubber-related volatile organic chemicals, found in indoor and outdoor fields.

Poor ventilation can lead to a concentration of such chemicals in the air, Vasiliou said. And "I can assure you that many of these indoor facilities are not properly ventilated," he said. "It is a serious public health concern that needs to be addressed."

On the federal level, the EPA, the CDC and other agencies launched plans last year to study key environmental and human health questions related to artificial turf safety . The research is still ongoing

Tips for playing on artificial turf

What other choices are there for sports fields, aside from artificial turf? Fields could be installed with natural grass or organic material to replace crumb rubber infills, Vasiliou said.

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"The Baltimore Ravens replaced the artificial turf in their stadium, and they put in grass," he said. "Also, there is organic material like coconut shells."

Some organic alternative infill is composed of coconut husks and fibers. However, Griffin, the soccer coach, noted that organic options tend to be expensive.

Vasiliou said Ajax, a professional soccer club in Amsterdam, reportedly made plans to remove all artificial turf from its training fields as a precaution over possible health risks, and the club is still examining which material to use to replace the turfs.

For athletes -- and soccer moms and dads who are concerned about their children's safety -- the Washington State Department of Health offered tips on how to minimize any potential exposures to chemicals from synthetic turf fields.

Always wash your hands after playing on the field and before eating.

Take off your shoes to prevent tracking crumb rubber into the house.

Shower after playing on the field and quickly treat any cuts or scrapes.

Don't swallow crumb rubber.

Bryant, the Seattle-based mother, said her youngest son still plays soccer, and her family insists that the 12-year-old goalkeeper follow the preventive measures to minimize tire crumb rubber exposure, such as removing his clothes and showering.

"He is aware of the issues," she said. "In Seattle, in order to play soccer or lacrosse or football, and even in a lot of cases baseball, there are turf fields.

"I'm not asking people or kids not to exercise or not to play soccer," she said. "I'm asking the city and the school districts to be responsible with health and safety."