Monitoring for the presence of mold in the environment can be very complicated due to the selectivity of sample media and methods. Different methods and types of media can be used for the different species of mold. Conducting mold sampling may not be necessary in all cases and is not recommended by the Minnesota Department of Health. However, it is useful in determining the type of molds present in the environment and provides a rough estimate of the concentration of airborne spores within an environment. Sampling can also be used as a determination of clearance in an area cleaned of mold infestation.
Locating mold is usually done using human senses. Problematic mold is often present in large enough quantities, which allows us to use the senses of sight and smell to detect the presence of molds. However, some molds are present in smaller quantities or hidden in wall cavities and may not be visible or detected by human senses. There are several techniques to sample for molds, including air sampling and surface sampling.
(Photo:Mold growing on a humid room wall)
Airborne mold spores are of concern because they can be directly inhaled. Area samples are used to measure the concentration of mold in an environment. Air samples are collected using a high volume pump, which draws air through one of several different media types. The volume of air drawn through the pump is quantified by taking the product of the airflow rate of the pump and the amount of time the pump is running. The total volume is necessary in order to quantify the amount of spores in the air of the sampled environment.
Airborne spores can be collected by pumping air directly onto a growth media, pumping the air into a liquid, or pumping air through a filter. Once collected, the sample is either cultured to allow the spores to grow or directly analyzed by microscopy, depending on the sampling method.
(Photo: Mold growing on gypsum board)
A variety of samplers are available for collection of airborne spores. Selecting the type of sampler can be difficult because each sampling method has its strengths and weaknesses for analyzing mold. Cultured sample analysis is greatly influenced by the type of media selected. Some media is better for certain situations. The most common media choice is malt extract agar (MEA). Additionally, some sampling methods are more efficient at collecting spores and some methods are more sensitive to overloading. Cultured samples take approximately seven days to grow and analyze while filter and impaction cassettes can be analyzed immediately.
Common samplers include the New Brunswick slit to agar, Andersen sampler, SAS sampler, RCS sampler, and cassettes. Andersen samplers have been found through use to be the most accurate in sampling airborne spores due to their ability to collect a wide range of species of spores, but they are also believed to be susceptible to overloading in high mold concentrations. Overloading a sample deems it as not being able to be analyzed because of the extremely high concentration.
High mold concentrations in air can be analyzed using an impinger. Impingers use a liquid to trap airborne spores. A pump draws the airborne contaminants into the liquid medium. This liquid is then cultured for analysis. This method works great for longer sampling periods, which better represents the average airborne concentrations of molds.
(Photo: Mold growing on a suitcase stored in a humid basement)
Most of the air sampling techniques pose a bias in sampling. They tend to underestimate the amounts of mold spores that are present in the environment. There are other problems that exist with taking airborne concentration samples.
Collecting accurate airborne concentration samples of mold is difficult. Problems with airborne sampling include: sampling too far from the source of mold, abnormal conditions during sampling period, medium and spore variability, and ability of certain mediums to overload. Being too far from the source of the mold can lead to a gross underestimate of the mold concentrations. It is difficult to determine the normal conditions of an area, since the measured concentrations could be higher or lower than what is actually present.(Photo: Mold growing on a wooden headboard in a room with high humidity)
Another method of monitoring mold spores in the air is through open media collection where mold spores and dust settle onto an open medium. These are typically placed on a tabletop near the mold source for 1-hour worth of exposure to the environment. However, there are several downsides to this method. These include the fact that spores have different settle rates with some having the ability to stay airborne for hours, so that there must be a significant quantity of spores present to get a reliable result, plus you also need an outdoor comparison. This method is seldomly used.
Collecting samples to identify molds may or may not provide the answers to the questions of the individuals with mold problems. One sampling method cannot identify all types of mold present within a given environment. Other issues that may not be addressed through sampling is whether the molds that are present are toxic, whether presence of the mold makes an association with experienced health complaints, determining a safe level of exposure, or proving that cleanup is needed.
If a person decides to go beyond the physical senses of identifying the presence of mold, it is recommended that a professional be consulted to minimize bias sampling and to ensure accurate interpretations of results. As of June of 2003, the state of Minnesota did not have any requirements for mold investigators to hold any certification. Be sure to conduct a brief background check of qualified firms prior to selecting a firm to complete the sampling.
Several other methods also exist for sampling for the presence of mold spores in the environment. These include collecting samples of settled dust with a wipe, surface swabbing, contact plate and scrapings. It is important that sterile techniques be used to prevent cross-contamination of samples. Some of these methods include:
1) Contact Plates Take a media plate, like MEA, and press it on a surface. The sample is cultured and analyzed by microscopy.
2) Tape Lift Clear tape is pressed on a surface. The tape is analyzed by direct examination by a microscope.
3) Surface Swab A cotton swab is used to collect surface mold. The swab is placed in sterile water. The water is cultured and then analyzed by microscopy.
4) Dust Wipe A surface is wiped with a watered wipe and cultured. The area of wipe is known, typically 1 square foot.
5) Bulk Sample Pieces of bulk building material is collected for direct examination or culturing.
6) Micro Vac A paper filter cassette to collect particulate material from surfaces such as carpets and upholstery. The collected dust is then cultured for analysis.
Minnesota Department of Health Environmental Health in Minnesota: Testing for Mold. 9/29/03
Sampling methodology for fungal bioaerosols and amplifiers in cases of suspected indoor mold proliferation. 9/18/2003
Levy, Barry S.; Wegman, David H. Occupational Health: Recognizing & Preventing Work-Related Disease & Injury, 4th Edition; Lippincott Williams & Wilkins 2000.
Minnesota Department of Health Environmental Health in Minnesota: Mold Questions and Answers. 9/29/03