How To Perform Clearance Sampling
I have had several questions lately about clearance or exit sampling. As we have discussed in the past. This task involves sampling for the negative case, which is always difficult, if not impossible.
As usual, how you do clearance sampling depends on exactly what you want to document. Do you want to prove that the remediation was designed and performed correctly? That the environment is better than it was before remediation? That the indoor environment is better than that outdoors? That exposure to occupants is below some predetermined limit? Each of these needs requires a different approach to sampling.
1) Was the remediation designed and performed correctly?
In this case you can inspect the work that was done, and use some sampling method (dust sampling?) to document that in the process of remediation you have not contaminated the rest of the space.
2) Is the indoor environment better (with respect to mold) than it was before remediation?
This question might be answered by inspection as above. However, this assumes that the principal problem with the environment was recognized during the initial inspections and design of remediation protocols. A series of air samples collected before and after remediation would provide some indication of the potential for hidden, unremediated mold. Combined with bulk or surface samples of discovered mold, it might also provide a clue as to whether or not all of the mold growth has been discovered.
3) Is the indoor environment better (after remediation) than that outdoors?
Here can collect a series of air samples outdoors and in for direct comparison, or for comparison using a statistical method such as the MoldScore.
4) Is exposure to occupants less than some predetermined limit?
In this case, you have to decide for yourself what the predetermined limit will be, since there are no reasonable guidelines to assist you. You might make this decision after collecting air samples to determine what the pre-remediation exposure are, and consider how much lower you want the exposure to be.
So obviously a single air sample or a few tape samples are not going to tell you much, and certainly won't be defensible if the occupants continue to have problems. It should also be clear that clearance protocols must be designed individually for each specific case, although a structural frame work can be developed and details added.
This article originally appeared in the April 2011 issue of Indoor Environment Connections. Reprinted by permission.
Dr. Harriet Burge is EMLab P&K's Director of Aerobiology and Chair of EMLab P&K's Scientific Advisory Board. Widely considered the leading expert in indoor air quality (IAQ), Dr. Burge pioneered the field more than 30 years ago. She has served as a member of three National Academy of Sciences committees for IAQ, including as Vice-Chair of the Committee on the Health Effects of Indoor Allergens.
View Dr. Burge's Curriculum Vitae.