Synthetic turf hardness testing
G-max is a measurement of impact attenuation - the ability of a surface to absorb energy. The harder a surface, the less impact it can absorb. A higher G-max measurement means a harder, and more dangerous surface. As an artificial field ages, hardness will increase.
The U.S. Consumer Products Safety Commission ASTM F1292 requires G-max levels (any single measured point on a field) to be less than or equal to 200 (F355). Most communities use this upper limit, but the synthetic turf industry, professional athletic leagues, and universities use lower, more protective standards based on updated science.
The Synthetic Turf Council recommends a limit of below 165 G-max using the F355 scale, and testing at least once annually. The NFL uses a different testing tool (Clegg Impact Soil Tester) with a different scale and sets their limit at about 100 G-max. They test prior to each game.
Natural grass fields typically measure 85 F355 (42 Clegg) units.
Diana Zuckerman, PhD, President of the National Center for Health Research says, “A Gmax score over 200 is considered extremely dangerous, and it is considered by industry to pose a death risk. However, the synthetic turf industry ... suggest scores should be even lower — below 165 to ensure safety comparable to a grass field.“
It's best for communities to not assume safety or protection from liability with an upper Gmax limit of 200 in light of the industry standard recommendation and advances in research on injury prevention.
The U.S. Consumer Products Safety Commission ASTM F1292 requires G-max levels (any single measured point on a field) to be less than or equal to 200 (F355). Most communities use this upper limit, but the synthetic turf industry, professional athletic leagues, and universities use lower, more protective standards based on updated science.
The Synthetic Turf Council recommends a limit of below 165 G-max using the F355 scale, and testing at least once annually. The NFL uses a different testing tool (Clegg Impact Soil Tester) with a different scale and sets their limit at about 100 G-max. They test prior to each game.
Natural grass fields typically measure 85 F355 (42 Clegg) units.
Diana Zuckerman, PhD, President of the National Center for Health Research says, “A Gmax score over 200 is considered extremely dangerous, and it is considered by industry to pose a death risk. However, the synthetic turf industry ... suggest scores should be even lower — below 165 to ensure safety comparable to a grass field.“
It's best for communities to not assume safety or protection from liability with an upper Gmax limit of 200 in light of the industry standard recommendation and advances in research on injury prevention.
How To Test
G-max testing needs to be done by a professional. Here is an example of a comprehensive G-max testing report prepared for a resident of one community in Massachusetts.
Fields should be tested upon installation and annually. The Sports Field Management Association says;
How often should field hardness be tested?
Field hardness levels should be tested upon installation of the field, then continue once per year or more. ASTM Standards recommend measuring field hardness at least once per year. While these are the written recommendations, testing once per year is not likely to hold up in a liability court case. To ensure field safety, sports turf managers at all facility levels are advised to test throughout the season.
https://www.stma.org/wp-content/uploads/2019/09/Field-Hardness-Testing-STMAInstitute-logo.pdf
It's important to note when reading the test results that any point on the field above the designated threshold* should result in a safety failure. An average of total field point measurements should not be used to determine the need for remediation, closure, or replacement. Athletes do not hit their heads on averages!
*Ideally below 165 G-max as recommended by the STC and SFMA
All data gathered from the public record.