Q: I’ve heard conflicting comments about misting to reduce dust in the air during mold remediation. My question is, is misting appropriate?
The use of misting during mold remediation is controversial. On one hand, misting can reduce suspended dust in the air, and on the other hand, there is evidence that misting may cause the dispersal of mold spores.
Dust is a general term that refers to minute solid particles that are finely divided or separated. They are generally less than 500 microns in size and can be comprised of a wide variety of substances. Ohio State University describes house dust as being generated from several sources including, but not limited to:
- The breakdown and release of plant and animal materials used in the home. These contaminants include such items as feathers, cotton, wool, jute, hemp, and animal hairs. They come from clothing, carpets, rugs and furniture.
- The disintegrated stuffing material from mattresses, pillows, quilts and upholstered furniture. Prolonged use seems to cause these resilient fibers to weaken and eventually break down into particles small enough to be inhaled.
- Human skin scales, animal dander, insect parts from cockroaches and dust mites, saliva, molds and mildew, bacteria, viruses, and pollen. As people go through their daily activities, particles that have settled onto the floor and other surfaces are stirred into the air.
- Other contaminants deliberately introduced into the indoor environment. These can include tobacco smoke from pipes, cigars and cigarettes, cosmetic powders, baby powder and some powdered laundry detergents, aerosols such as air fresheners, and cleaning products with strong odors.
During mold remediation it can be expected that mold spores, fungal fragments and abraded building materials will be added to whatever mixture might already exist. Some of the components of dust are hydrophilic (water loving) and other may be hydrophobic (water fearing or hating). The practice of using a mist of water to remove hydrophilic dust from the air has been around for centuries. Most recently, misting has been used to assist in removing asbestos particles from the air during abatement. Asbestos is hydrophilic and when water, in the form of mist, comes into contact with minute asbestos particles, they tend to combine. The combined weight of the asbestos and the water causes the particles to fall out of the air space. A similar process can be used in lead abatement when dust is created from abrading surfaces that potentially contain fragments of lead. Misting has also been used to control dust that is generated by stirring up dirt and the particles of gypsum board dust created during demolition.
However, it has also been reported that hydrophobic particles in the atmosphere are not necessarily removed by the formation of water droplets. Fan, S. et al, stated in their article entitled “Impact of air pollution on deposition of mineral dust: Implications for ocean productivity” that “African dust over the tropical Atlantic is mostly hydrophobic and removed by ice, but not droplet, nucleation.” The authors of a report entitled “Thunderstorm asthma - mold spores causing huge increases” suggested that asthma may have been caused by an increased release of fungal spores due to an initial rainfall. The study indicated that there was a reduction of certain airborne particulates and an increase of mold spores. The fact that many spores are hydrophobic can account for the increase. Professor of Botany George Wong from the University of Hawaii makes the following reference to aerosolized dry spores:
These spores do not readily soak up water and when clusters of these spores are splattered by water, as may often occur in those fungi that produce their spores directly on their mycelium, rather than absorbing the water, the impact dislodges the spores and scatters them into the wind. Because these spores do not readily absorb water, they are said to be hydrophobic. Although this may not be very intuitive, the initial resistance of these spores to water makes a great deal of sense.
If controlled demolition is employed, as described in the “IICRC S520 Standard and Reference Guide for Professional Mold Remediation,” dust generation can be minimized. Using HEPA-filtered negative air machines to capture any released particulates also minimizes the need to control dust by using water mist as a control measure. If misting is used, it adds a moisture source that needs to be controlled. If the use of misting results in workers being less diligent about controlled demolition, then more dust will be generated and more misting will be employed. The more moisture used, the greater the likelihood of increased labor and costs. There is also the potential for too much moisture being added to the environment, requiring dehumidification or causing secondary damage to building materials. We have seen cases where uncontrolled moisture from misting resulted in additional mold that wasn’t present before the remediation began. I’m not suggesting that there are not occasions where misting might be appropriate. Certainly there might be situations where the contractor has no choice. For example, a mold remediation performed on asbestos-containing materials. The asbestos construction standard requires that misting be used to control the asbestos particles. Of course, asbestos abatement requires special training and licensing and should never be performed by any firm that is not in compliance.
Even if containment and negative air are used, there is a risk to workers. In one situation, monitored by our company, an asbestos abatement company performing mold remediation on asbestos-containing materials utilized misting with water during the demolition. Air sampling during the demolition demonstrated 27,000,000 cfu/m3 (colony forming unit per cubic meter) of culturable water damage related mold spores in the air within the work area. Crew members were wearing full-face air-purifying respirators with HEPA cartridges, but even with that level of respiratory protection, the effective exposure that might be expected due to leakage around the face piece could be 540,000 cfu/m3. This represents a much higher concentration than we have ever measured during controlled demolition without the use of misting. Fortunately, containment and negative air were being used, which prevented the spread of contaminants to other parts of the building, but worker exposure was still an issue.
Remediators choosing to use misting should be prepared to monitor the contained environment. Thermo-hygrometers and moisture meters can be used to monitor humidity and moisture levels respectively. If moisture levels begin to increase, dehumidifiers might be needed to maintain acceptable conditions. Particle counters can be used to check for higher-than-normal levels of particulates in the air. In some circumstances the involvement of an indoor environmental professional may be appropriate to assess the work environment during remediation and misting operation. If extremely high levels of mold spores develop, the use of powered air-purifying respirators (PAPRs) might be necessary.
Author's Note: The S520 citations that appeared in "IEQ Q&A" in January were taken from proposed language for the update, not the 2003 version. The final language may ultimately be slightly different than what was quoted in the article. If you liked this article, circle 146 on the Reader Inquiry Card.