March 1, 2021 | Susan Napier-Sewell

Respirable Crystalline Silica Monitoring During Grounds Crew Operations Yields the Unexpected

crystalline silica incident

All forms of crystalline silica can be inhaled when broken into dust-size particles through chipping, cutting, drilling or grinding objects.

Crystalline silica is a basic component of soil, sand, granite, and many other minerals. All forms of crystalline silica can be inhaled when broken into dust-size particles through chipping, cutting, drilling or grinding objects. Inhaling respirable crystalline silica dust particles can cause fibrosis of the lungs and therefore has been classified as a human lung carcinogen.

In this Lessons Learned article you’ll learn why two PNNL ((Pacific Northwest National Laboratory) grounds crew exceed the occupational exposure limit of respirable crystalline silica after cleaning paved parking lots and removing leaves and why it’s so important to carefully evaluate the methods and means used to implement any scope of work.

Subtle differences in work scope cause higher than expected monitoring results of respirable crystalline silica

On November 2, 2019, nine PNNL grounds crew staff divided into teams of three worked overtime to clean paved parking lots and remove leaves from around the PNNL campus. Each team consisted of two staff using landscape backpack blowers and one driving a tractor pulling a landscape vacuum. Three of the nine staff members were monitored for respirable crystalline silica during the work evolution.

Based on PNNL’s 2015 baseline personal monitoring of parking lot clean-up
work, it was anticipated that respirable crystalline silica concentrations would be below the occupational exposure limit (OEL) set in 2016 by the American Conference of Governmental Industrial Hygienists (ACGIH) for respirable silica, which is 25 ug/m3 measured as a 8-Hr Time Weighted Average (8-Hr TWA). The objective of the 2019 sample collection effort
was to validate the 2015 baseline breathing zone exposures. However, the 2019 personal monitoring results showed two of the three samples collected exceed the ACGIH respirable crystalline silica OEL.

PNNL has adopted the ACGIH threshold limits for crystalline silica, which was set at a level that provides additional protections from excess cancer risks.

Details leading to the incident

Work planning included a review of previous baseline personal monitoring results for parking lot clean-up work. Total dust concentration measured during past performance of parking lot clean-up activities indicated the exposure to dust, including respirable crystalline silica, was considered to be well controlled and below the ACGIH OEL; therefore respiratory protection was not required for the November 2019 work. Full-shift personal monitoring was conducted on three of the nine staff performing parking lot clean-up work, which entailed use of backpack landscape blowers and tractor-pulled landscape vacuums to clean and remove
leaves from paved parking lots.

Analytical results reported for the personal sample collected in 2019 indicated that two of the three staff were exposed above the
ACGIH silica OEL. One sample was worn by a staff member operating a backpack blower and the second by a staff member driving a tractor while towing a landscape vacuum. Both grounds crew staff were sent to the on-site occupational medical provider where they were evaluated and released without restrictions.

Differences in work scope suspected to have caused the higher levels of crystalline silica in the air include:

-Season – The 2015 work was conducted in the Spring while the 2019 work was conducted in the fall.
-Equipment – The 2015 work included a variety of equipment to include backpack and buffalo blowers, shovels, tractors, tractor buckets and landscape vacuums, while the 2019 work was limited to backpack blowers and tractors pulling landscape vacuums.
-Time – The 2015 work was less than eight hours and consisted of multiple tasks, while the 2019 work was a full eight-hour shift performing the exact same task.

Lessons Learned


Since OSHA (Occupational Safety and Health Administration) enacted the reduction in the allowable 8-Hr TWA exposure limits to respirable crystalline silica, any work that could involve dust generating activities should consider the time of year, the equipment to be used and the time it takes to complete the task, as well as any other possible factors that could impact potential exposure.

Attention to methods and means used to implement the scope of work need to be carefully reviewed. Changes in the length of assignments and the sharing of multiple tasks can affect the overall exposure potential.

Alternative methods that could help reduce dust generation during those activities should be considered. There is currently very little literature available that examines crystalline silica exposure from grounds and landscaping activities.

However, the change in the ACGIH exposure limit is significant and tasks previously seen as low risk should be re-evaluated to verify controls and work practices are adequate. Outdoor environmental conditions are ever-changing and should be carefully considered for every task that potentially involves crystalline silica exposure. Relying on historical respirable crystalline silica monitoring data may no longer yield the results expected.

Article source: Origin: Department of Energy, OPEXShare, Office of Environment, Health, Safety, & Security. Publisher, Pacific Northwest National Laboratory, Richland, Washington,

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