Testing for Mercury in Poured Rubber Flooring

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As part of its ongoing commitment to Indoor Environmental Quality (IEQ), HCPSS has initiated a project to identify, test, and remediate poured rubber flooring material in school gymnasiums. This type of flooring may be produced with a mercury catalyst that can emit mercury vapors under certain conditions. Four school locations were determined to have this type of flooring. To determine if there is any health risk, air sampling is being conducted at each location periodically during the year, because mercury vapor concentrations vary seasonally due to changes in temperature. The results of the preliminary air quality tests taken thus far show that none of the floors tested pose a health risk during normal school operations.

HCPSS plans to remove and replace the flooring during summer 2021, while school is not in session. This measure will be taken out of an abundance of caution, and not because of any suspected health risk. As an additional precaution until the flooring is replaced, the HVAC systems at each of the four schools will be run for additional hours to enhance air circulation and further dilute the levels of mercury in the air.

It is important to note that the flooring test and mitigation program was initiated as a proactive measure to improve IEQ, as part of HCPSS’ overall commitment to practices that support a healthy environment. No health concerns have been reported, and the sampling/removal of the flooring is not required by federal, state, or local regulations.


HCPSS has identified four facilities that contain a poured rubberized gym flooring. This type of flooring was widely installed in school gymnasiums across the United States from the 1960’s until approximately 2005. Poured rubberized flooring can contain a mercury catalyst to aid in its installation. This catalyst can emit mercury vapor under certain conditions. These emissions can reach levels of concern in some cases, especially where flooring condition has deteriorated and air temperature is elevated and/or lacks dilution through the Heating Ventilation and Air Conditioning (HVAC) system.

After extensively reviewing the construction documents for each school building, HCPSS identified only four locations with poured rubber gymnasium floors:

  • Bollman Bridge Elementary School*
  • Northfield Elementary School*
  • Old Cedar Lane School
  • Waterloo Elementary School*

*rubber floor covered with standard wood flooring at these locations

Analysis and results

At these four school gymnasiums, HCPSS collected and analyzed preliminary air samples at various locations and heights to determine if the flooring emits mercury vapor during normal hours of occupation. While there are no federal or Maryland state regulations or guidelines regarding rubberized gym flooring containing mercury, the sample results were compared to widely-accepted guidelines from the Minnesota Department of Health and New Jersey Department of Health, which indicate that indoor air concentrations below 750 nanograms per cubic meter (ng/m3) are protective of preschool-aged children, and thus also deemed safe for older children and adults.

Preliminary air quality testing has indicated that none of the floors at these school locations pose a health risk during normal school operations, because all samples taken were well below the 750 ng/m3 threshold. Mercury vapor concentrations vary seasonally due to changes in temperature. The results for each season, and the yearly average, are provided in the table below. These results will be updated as additional seasonal data becomes available.

Sample Results
School Summer 2020
Fall 2020
Winter 2021
Spring 2021
Waterloo ES 149 - 206 0.5 - 307 118 - 150 45 - 65 78 - 182
Old Cedar Lane 85 - 116 64 - 77 53 - 73 44 - 154 62 - 105
Northfield ES - - 84 - 491 26 - 153 55 - 322
Bollman Bridge ES - - 3 - 54 0.5 - 33 2 - 44

Management and Remediation

HCPSS will remove and replace the flooring with a standard wood gymnasium floor, out of an abundance of caution and not because of any suspected health risk. This process will take place during summer 2021 so that it will not interfere with the instructional program.

Until the flooring can be removed and replaced, risk will be avoided through operation of the HVAC system, which dilutes the levels of mercury in the air. As an additional precaution, HVAC systems will start two hours before student arrival, to help further dilute mercury vapor prior to building occupation. This is being done simply an extra precaution, and not because of any perceived risk to people in the buildings.


Christopher Madden Certified Industrial Hygienist/Indoor Environmental Quality Manager, HCPSS Office of the Environment

Frequently Asked Questions

How has HCPSS determined which buildings contain this type of flooring?

Staff in the Office of the Environment carefully reviewed construction documents and/or conducted on-site inspections to identify schools and other locations having poured rubber flooring. Three schools were identified, as well as one former school that is currently used as an office building.

What are the health risks of mercury?

Mercury is a neurotoxin. Health risks can vary significantly depending on the level and length of exposure and other factors such as the age and health of the individual exposed. The effects of mercury exposure can range from severe to none at all. While exposure can occur in various ways, the most common route is through consumption of seafood containing mercury.

What should I do if I/my child has been in the gymnasium?

Based on the air sampling data obtained and compared to guidelines from the Minnesota Department of Health and New Jersey Department of Health, no further action is warranted. If you have further concerns, you are advised to contact your primary care provider. Note that the Howard County Health Department has also reviewed the data collected by HCPSS. If you have further concerns, you are advised to contact your primary care provider.

How did HCPSS test the samples and determine the level of risk?

Indoor air samples were collected in gymnasiums of concern using a real-time mercury vapor analyzer. Air samples were collected on Monday mornings while the HVAC was operating normally to simulate the “worst case scenario” a teacher or student would be expected to encounter. Results were then compared to the Minnesota Department of Health’s action level of 750 ng/m3. The New Jersey Health Department also has a similar action level of 800 ng/m3.

My activity room/weight room has rubber tiles. Is this a concern?

No. Rubber tiles are not poured into place; therefore, a mercury-based catalyst is not needed for installation. Only flooring constructed of a soft rubber material with no seams may potentially be of concern. At HCPSS, this type of flooring is present only in a small number of gymnasiums.