The number of artificial turf fields at public schools across San Diego County has exploded in the last decade – but a key measure of whether those fields are safe to play on hasn’t kept pace.

Hardly any of the region’s public schools have performed a G-Max test, which measures field hardness to ensure turf fields have not become too hard and too great a risk for concussion and injury for students who take a tumble.

One turf industry veteran said colliding with hardened turf can be like hitting “frozen Earth or concrete.” A turf field designer said not testing for field hardness is like “Russian roulette.”

In recent years, the NFL began performing a G-Max test before every game to make sure it is safe for players. Most San Diego County public school students get no such protection.

A Voice of San Diego survey found nearly none of San Diego County’s public schools are testing their fields for hardness regularly and only a few districts have tested them at all. Several districts reported G-Max testing their fields once after installation or not at all, even as fields remained in place for several years. Several local school districts are skipping the tests even though field contracts require them.

Southwestern College was the only public agency surveyed that produced reports showing regular G-Max testing spanning multiple years.

Most local turf fields come with crumb rubber infill, or chopped up tires that act as a cushion between blades of synthetic grass. Exposure to the material has raised its own health concerns locally and abroad, spurring new safety studies by state and federal agencies, under way now.

But while experts study crumb rubber exposure, turf field hardness is recognized as a legitimate safety issue industrywide. Though not legally required, field owners are urged to closely monitor their fields using G-Max tests, and some field construction contracts require regular testing.

Turf fields harden over time as the rubber infill pieces get washed out by rain and inadvertently carried off the field by players’ cleats and clothes. Sand used in the infill mixture can also get compacted.

To get a G-Max score, a special weight is dropped onto the field surface and equipment measures how fast the object slows down. A harder surface will cause the weight to stop more rapidly. Multiple drops are made on a single spot at various points across the field. The scores – measured in gs, a unit of gravitational force – are then averaged.

Typically, a new artificial turf installation may score under 100 gs or in the low 100s. After a few years, scores typically rise to the mid-100s and up.

Scoring below 165 is still considered safe by the Synthetic Turf Council, a nonprofit trade association. Above that level becomes precarious.

Another organization called ASTM, which issues standards for how G-Max tests are done, says turf fields should never score higher than 200 gs. Fields that score above 200 are unsafe for play and often need to be replaced. Below that level, fields might need more infill cushioning to make the field softer.

The 200 maximum “can cause death … 200 is the equivalent of frozen Earth or concrete,” said John Schedler, a 32-year turf industry veteran and ex-director at FieldTurf, one of the nation’s leading turf companies. “Very recently the NFL recognized the importance of the concussion data they were getting back and that’s when they enacted this (175 G-Max) gameday standard.”

Schedler, who now runs his own consulting firm, Baraka Sport, said, “G-Max will increase every year. … From a liability standpoint, it just makes no sense for schools to not make sure that that athletic field system is safe. It’s a very expensive piece of athletic equipment that requires maintenance and tuning and their kids play on it daily.”

Skipping the test “is a big deal, in my opinion,” Larry Foster, a Bay Area landscape architect who designs synthetic turf fields for schools, wrote in an email. Artificial turf fields “need to be tested every year. … It is Russian roulette otherwise.”

Concussion concerns hit close to home when a La Jolla High football player suffered a serious brain injury on the field in fall 2014.

The 17-year-old student was sent back into an Oct. 16, 2014, game after saying he was hurt. San Diego Unified changed how it handles student concussions last year.

But measuring field hardness is not part of the protocol.

San Diego Unified, the county’s largest district and turf field buyer, has G-Max tested only nine of its 44-plus artificial turf fields installed over the years. Those tests took place in 2008.

“In 2008, the district performed GMAX tests on several fields that were nearing the end of their useful life, and those fields were subsequently replaced. The district has not performed GMAX testing on any fields since 2008,” wrote Cynthia Reed-Porter, district spokeswoman, in an email. “Testing would typically occur when a field begins to show signs of wear. The district’s older, worn fields have been replaced.”

It turns out contract documents drafted by design firm Mele Amantea Architects II in 2011 for San Diego Unified’s fields require a G-max score between 125 and 175 for eight years following installation. Since tests aren’t being done, it’s unclear if the fields are meeting those standards.

Southwestern College spokeswoman Lillian Leopold said there’s a singular reason it tests its turf fields regularly: “Safety.”

Leopold said in an email that the college tests the fields annually.

“Fields rarely become unsafe overnight, so testing once a year provides adequate warning of emerging safety issues,” she said. “Significant changes in GMAX readings can be an early sign that problems are developing within the turf system.”

Most local districts haven’t been as diligent, but some – like Vista Unified and Carlsbad Unified – responded to Voice of San Diego’s inquiry saying they would do better in the future.

Others defended the decision to skip the tests.

“We don’t routinely conduct GMAX testing given the FieldTurf product that is installed at our sites is widely installed throughout Southern California and has undergone extensive industry-wide testing deeming it the safest synthetic turf field,” said Grossmont Union High School District executive secretary Catherine Melick in an email.

El Capitan High School was tested this year, and a few other Grossmont fields have been tested sporadically over the years, in 2003 and 2008, records show.

Poway Unified in North County and the Sweetwater Union High School District in the South Bay could only produce one G-Max report each, even though each district has had multiple turf fields for years. Sweetwater did not produce any tests for Montgomery High’s 2010 field, even though the G-Max is supposed to remain below 160.

San Diego State also said no testing had been done on its turf fields, and San Diego Community College District did not produce any reports for the Mesa College field nearly seven weeks after the request was made. Palomar College produced one test for Minkoff Soccer Field from 2013.

Coronado Unified recently tested the field at Coronado High while exploring litigation against the turf manufacturer FieldTurf, which has seen more than 20 fields fall apart prematurely across the region due to a defect. Coronado officials declined to release the results, citing attorney-client privilege.

Both turf consultant Schedler and architect Foster said they make sure projects they’re involved with include G-Max testing and maximum scores permitted throughout the warranty period, not just after installation.

Schedler recommends fields get tested after installation and “at least every other year.”

Records show project specifications for fields installed at La Costa Canyon High School last year and San Dieguito Academy in 2013 came with detailed testing requirements.

Upon installation, the fields needed to score no higher than 130 at any location, and during the entire warranty period, the field cannot score higher than 175. In any given year, the field score cannot increase more than 15 gs, Seattle-based landscape architecture firm D.A. Hogan & Associates wrote. Testing needed to occur after installation, and after year one, two, four, six and again 60 days prior to the warranty expiration. Costs would be paid for by the turf contractor.

Despite those requirements, San Dieguito Union High School District officials could only produce one test report from 2013 for Torrey Pines High.

The project specifications for Fallbrook High School’s 2008 field required a G-Max “between 110 and 125 upon installation and shall never exceed 135 at any test location for the entire warranty period … without the use of any supplemental padding.”

Fallbrook Union High School District Superintendent Hugo Pedroza said in an email, “When the turf was initially installed, it required GMAX testing and it was done. The second time around, when the ‘carpet’ was replaced, GMAX was not required because the area below the carpet was not disturbed.”

While it’s not uncommon for districts to skip the test, that may be changing.

“I think it’s becoming more and more common that it is written in the specifications and it is getting enforced because of the heightened awareness of concussions,” Schedler said. “There has been an awful lot of media publicity about concussions and parents and owners are becoming a lot more concerned and the specifiers and designers of these fields listen.”