Homogeneous Areas of ACM

Question: While sampling the plaster at a large facility I collected eighteen samples from what appeared to be a single homogeneous area. The initial results showed four low positives, but after point-counting only one sample exceeded 1%. Does my client have to treat all areas of the plaster as ACM?


While inspecting a building the caller found 20,000 SF of plaster. Thon collected and submitted eighteen samples for analysis. Four samples came back >1% asbestos by PLM, so thon had them point-counted. One of the four was above 1% (1.5%) but all others analyzed as non-ACM.

The Inspector was conducting a pre-demolition NESHAP inspection when the materials were found. Since plaster is costly and difficult to remove, thon was looking for a way to avoid any unnecessary costs for thon's client. There was no pattern to where the 4 in 18 were located in the building, so drawing a clear line around the areas of ACM- and non-ACM-plaster was not obvious.


Collect and analyze additional samples to better define the area that is ACM. (See the Work Plan below for additional detail.)


The Law.

In its AHERA guidance, the U.S. EPA made clear that two things are true:

  1. If even one sample in a homogeneous area has asbestos above the 1% level, the entire homogeneous area must be considered ACM.
  2. It is up to the inspector to determine the limits of the homogeneous area. (None of the states in our area have a method for determining ACM that is different from the EPA's, so presumably the answer is not state-dependent; if the EPA would call it non-ACM, presumably so would the state.)

The Issues:

Plaster, like drywall, is notoriously difficult to divide into homogeneous areas, since once it is applied and painted there are often no color and texture indicators to help in subdividing the building. Plaster on large projects was often done by several subcontractors, some of whom may have used asbestos while others did not. Furthermore, damaged areas of plaster may have been patched; prior to their ban by the CPSC in 1973, asbestos-containing spackle, drywall mud, etc., were quite common. Thus, there are two approaches (not mutually exclusive) that one can take to try to isolate the relevant homogeneous area:

  1. Definition by secondary characteristics. We encourage inspectors to note the color, texture, and thickness of materials during sample collection. In many cases there may be enough information contained in these notes to allow dividing the plaster. This is especially valuable, since once a distinction is defined, it can be rapidly applied throughout the structure without requiring any laboratory analysis.
  2. Finding the homogeneous area boundary. This is more open-ended and costly. If the plaster found is only a patch, then sampling other walls in the same room may be enough to establish that the sample collected was not representative of the homogeneous area. In that case the ACM material can be removed and the rest of the building demolished as non-regulated material; if the plaster is removed by cutting the wall into large sections, the amount of RACM (Category II nonfriable material made friable by the operation) can probably be kept under the 3 square feet that would necessitate licensed people. We would advise that you use misting or HEPA dust collection during sawing; waste should be wrapped (wetting is neither required nor recommended) prior to disposal at an asbestos landfill (so it does not become friable during disposal).

The Work Plan

  1. If the plaster was the work of one subcontractor, take and analyze samples moving away from the room containing the ACM in a defined pattern until you are confident that there is no longer ACM-plaster in the area.
  2. All plaster in the newly delineated homogeneous area thus defined would need to be removed prior to demolition. This would certainly require the use of licensed people, full OSHA Class II protections (since the material will probably not be removed substantially intact) and appropriate protective measures for the personnel and the environment.
  3. I would suggest you consider sawing into sections as a way to reduce exposures and dust generation. Since the building is slated for demolition, no clearance sampling should be needed, but a visual clearance (to ensure that materials were not spread and would thus endanger workers entering to complete the demolition) by an independent examiner is strongly recommended.


The inspector collected additional samples and then discussed the results with the state compliance inspector. They agreed that the reception area of the building (about 10% of the plaster) was RACM, but that the rest did not have to be removed before demolition.

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One responding asbestos inspector noted that, after conversations with plastering contractors, he always separates plaster ceilings from plaster walls as homogeneous areas. The contractors told him that they were more likely to use the asbestos plaster on the ceilings since greater strength was required. (Thanks to Richard Griffith at Workplace Safety & Health.)

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