Study lays groundwork for preventing dangerous falls on dry spills

Signs reading “slippery when wet” frequently warn about the dangers of slipping and falling. But floors made slick by dry spills are also a significant hazard—one that’s overlooked and understudied, according to University of Arizona Health Sciences researchers who developed a new way to assess floor slipperiness caused by dry contaminants.

“Most people think materials like oil or soapy water are the main cause of slips,” said study lead Jonathan Lee-Confer, Ph.D., an assistant professor of physical therapy in the School of Health Professions at the U of A Mel and Enid Zuckerman College of Public Health. “This study shows that particles like salt, flour or dust can be just as dangerous.”

Lee-Confer and his collaborators found that a layer of salt so sparse as to be nearly invisible reduced floor friction by 28%, making tile far slicker than permitted by common safety thresholds. Bringing dry slip testing up to the standard used for wet conditions could significantly improve safety practices, he said.

The paper was published in the Journal of Forensic Sciences.

Slip resistance testing provides important data for those making safety decisions and investigating accidents, Lee-Confer said. Regulatory agencies use it to set standards, including building codes and workplace safety regulations enforced by the Occupational Safety and Health Administration.

Manufacturers, architects, engineers and designers choose flooring based on the tests. Facility managers set cleaning protocols based on accepted levels of floor grip, or slip resistance.

“Consumers want to make sure these professionals are using legitimate methodologies for understanding resistance,” he said.

Methods for testing wet floors are well researched, Lee-Confer said, but the few existing studies that have examined slip resistance with dry particles present didn’t account for how particle distribution could affect testing.

Slip resistance is measured with an instrument that repeatedly strikes a surface. When those strikes occur in the same spot, contaminants are pushed to the side, which can affect results.

Lee-Confer and his team fabricated 3D-printed combs that raked salt to precise, uniform levels before each instrument struck. This approach more closely relates to testing methods used for liquid contaminant effects on slipperiness.

The researchers applied iodized salt to a porcelain floor tile at three levels, the thinnest of which was about .005 inches, slightly thicker than a human hair and just beyond easy visual detection. The next salt layer assessed was about twice as thick, and the third was around double that of the second.

When the salt was evenly distributed, each thickness significantly reduced the coefficient of friction—less friction translates to a slicker surface compared with a clean, control tile. The thinnest layer reduced friction by more than 28%, while the thickest caused a drop of more than 20%.

However, when the salt redistributed itself around the tile due to the impacts of traditional instrument strikes, there was no significant friction difference between the thinnest and thickest layers. The middle layer showed about 8% less friction.

“The results point to the need to standardize dry particle slip resistance measurement and validate a replicable, low-cost method,” Lee-Confer said.

The researchers, including U of A College of Medicine—Tucson undergraduate student Lila Wayman, plan further study with additional surfaces and substances such as flour and cat litter. Wayman, who in April presented the team’s findings at the annual Mel and Enid Zuckerman College of Public Health Poster Forum, says the study also provides actionable guidance for people in their homes.

“If you spill salt in your kitchen, you might overlook it. It’s a little bit of salt, but it dramatically increases the risk of falling,” Wayman said.

Signs reading “slippery when wet” frequently warn about the dangers of slipping and falling. But floors made slick by dry spills are also a significant hazard—one that’s overlooked and understudied, according to University of Arizona Health Sciences researchers who developed a new way to assess floor slipperiness caused by dry contaminants.

“Most people think materials like oil or soapy water are the main cause of slips,” said study lead Jonathan Lee-Confer, Ph.D., an assistant professor of physical therapy in the School of Health Professions at the U of A Mel and Enid Zuckerman College of Public Health. “This study shows that particles like salt, flour or dust can be just as dangerous.”

Lee-Confer and his collaborators found that a layer of salt so sparse as to be nearly invisible reduced floor friction by 28%, making tile far slicker than permitted by common safety thresholds. Bringing dry slip testing up to the standard used for wet conditions could significantly improve safety practices, he said.

The paper was published in the Journal of Forensic Sciences.

Slip resistance testing provides important data for those making safety decisions and investigating accidents, Lee-Confer said. Regulatory agencies use it to set standards, including building codes and workplace safety regulations enforced by the Occupational Safety and Health Administration.

Manufacturers, architects, engineers and designers choose flooring based on the tests. Facility managers set cleaning protocols based on accepted levels of floor grip, or slip resistance.

“Consumers want to make sure these professionals are using legitimate methodologies for understanding resistance,” he said.

Methods for testing wet floors are well researched, Lee-Confer said, but the few existing studies that have examined slip resistance with dry particles present didn’t account for how particle distribution could affect testing.

Slip resistance is measured with an instrument that repeatedly strikes a surface. When those strikes occur in the same spot, contaminants are pushed to the side, which can affect results.

Lee-Confer and his team fabricated 3D-printed combs that raked salt to precise, uniform levels before each instrument struck. This approach more closely relates to testing methods used for liquid contaminant effects on slipperiness.

The researchers applied iodized salt to a porcelain floor tile at three levels, the thinnest of which was about .005 inches, slightly thicker than a human hair and just beyond easy visual detection. The next salt layer assessed was about twice as thick, and the third was around double that of the second.

When the salt was evenly distributed, each thickness significantly reduced the coefficient of friction—less friction translates to a slicker surface compared with a clean, control tile. The thinnest layer reduced friction by more than 28%, while the thickest caused a drop of more than 20%.

However, when the salt redistributed itself around the tile due to the impacts of traditional instrument strikes, there was no significant friction difference between the thinnest and thickest layers. The middle layer showed about 8% less friction.

“The results point to the need to standardize dry particle slip resistance measurement and validate a replicable, low-cost method,” Lee-Confer said.

The researchers, including U of A College of Medicine—Tucson undergraduate student Lila Wayman, plan further study with additional surfaces and substances such as flour and cat litter. Wayman, who in April presented the team’s findings at the annual Mel and Enid Zuckerman College of Public Health Poster Forum, says the study also provides actionable guidance for people in their homes.

“If you spill salt in your kitchen, you might overlook it. It’s a little bit of salt, but it dramatically increases the risk of falling,” Wayman said.