Enrique Orozco, Camfil Farr’s laboratory technician, inserts a MERV 13 filter into one of the company’s filter testing ducts. (Photos courtesy of Camfil Farr.)
What is the role of air filters in controlling mold? What air filtration efficiency is necessary to remove airborne spores? These are some of the questions HVAC contractors are asking filter suppliers to address regarding their concerns about mold.

To answer these questions, Camfil Farr of Riverdale, NJ, says it has become one of the first filter manufacturers to develop specific guidelines on air filter selection for mold control in heating and cooling systems.

“Air filters serve the purpose of limiting the amount of spores in a given volume of air, thus limiting the reproduction capabilities of the mold,” stated Don Thornburg, Camfil Farr’s R&D manager.

“Our filter selection recommendations, which we have developed in response to current concerns about mold, are based upon the particle sizes of specific mold spores,” noted Charlie Seyffer, marketing manager for the company.

Shown here magnified 1000 times, Stachybotrys chartarum has been associated with Sick Building Syndrome in various studies.
“Mold spores are well over 1.0 micron in size and are readily removed by applying air filtration according to American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommendations,” Thornburg said. “Even a pleated panel filter with a minimum efficiency reporting value (MERV) 6 per ASHRAE Standard 52.2 will have an efficiency of higher than 80 percent against most molds.”

One key consideration, he added, is that air must be moved through the filter in order for it to remove contaminants. The more air that is moved through the filter, the better it is at removing mold spores and other particles. The company recommends at least six air changes per hour to achieve removal efficiencies above 80%.

“It is important to note that most systems operate at three to six air changes per hour during the heating and cooling seasons,” said Seyffer. “Ironically, temperate periods (spring and fall) have a reduced heating and cooling load requirement, thus less of a demand on the system fan, and fewer air changes. These seasons are also periods noted for high pollen and spore counts. Placing the system fan in constant-on position can significantly reduce exposure, as filters will only remove contaminants if you move air through them.”

Thornburg also pointed out, “Some molds require a higher level of filtration efficiency because of their smaller particle size.”

Cryptococcus neoformans is one of 37 species of Cryptococcus.
Cryptococcosis is considered opportunistic as it affects mainly immunosupressed individuals.


Table 1 lists various common mold species and the filter efficiency that should be applied to capture each using the company’s guidelines.Aspergillus, for example, has a particle diameter of 3.5 microns. If it becomes airborne, using a MERV 6 filter can remove it when matched with a proper number of air changes.

Based on this table, “When the entire spectrum of indoor air quality for commercial buildings is considered, a filter with an efficiency of MERV 13 or higher should be the product of choice,” advised Thornburg. “A MERV 13 filter will move the HVAC filtration system into the realm of controlling respirable-size particles, including most bacteria. Its higher efficiency will also address airborne mold spores.”

A MERV 13 filter with six air changes per hour would have “virtual 100% efficiency” in capturing all the molds indicated in the chart, agreed Seyffer. It would also be highly effective on bacteria, since “98% of all bacteria known to man are over 1 micron in size” and MERV 13 “has an efficiency on 1-micron size particles of over 85%.”

However, he said, “The problem with contaminants such as mold is that they are constantly regenerating unless they are eliminated at the source. In formulas applied to filtration or ventilation, this is defined as the ‘generation rate.’ Unless you eliminate the source, it is virtually impossible to obtain 100% removal efficiency. You can, however, reduce the risk by reducing the exposure rate.”

Filter manufacturers have upgraded their testing systems to perform filter testing to the new ASHRAE 52.2 test standard. Filters are now evaluated based upon particle size removal efficiency as opposed to the dust spot value of Standard 52.1.


Thornburg and Seyffer emphasize that air filtration is not a cure for mold proliferation, but one tool in a total control strategy. Mold requires three things to survive: appropriate temperatures, nutrients (a food source), and water. One of the most important components of control is the limitation of moisture in the environment and in the HVAC system.

ASHRAE recommends that spaces have a controlled humidity level of less than 60%. “Consistently exceeding this level is a prescription for problems,” declared Thornburg.

Excess humidity can cause mold and mildew to grow in ductwork and on coils. An increase in humidity beyond the recommended level can also increase the generation rate of the mold, Seyffer said. “If a filter has 90% efficiency on a particle size and it is challenged by 100 particles, 10 particles will return to the space,” he related. “If the same filter is challenged by 200 particles at the same 90% efficiency, 20 particles will return to the space. As defined by ASHRAE, spaces of human habitation should be maintained at 30 to 60 percent humidity, both for human comfort and for health.”

Dirty air filters themselves can provide a breeding ground for mold and mildew, underlining the importance of changing filters at proper intervals. “Given normal operation, filters should be changed when they reach the manufacturer’s recommended change point, based upon system resistance to airflow, or as part of a properly consulted preventive maintenance program (time period change),” stated Seyffer.

“In cases where mold contamination is found within a space, fix the problem, then change the air filters as an additional measure of protection,” he said. “Given the amount of air treated by a single filter, changing the filter is the cheapest form of insurance.” However, he again stressed that air filtration by itself is not a cure, but just one tool in an overall control strategy.

“Unfortunately, an HVAC system can present optimum temperatures for growth of most molds,” Seyffer explained. “For some, 80 to 95 degrees is excellent. Others can proliferate at the same temperature ranges that we find comfortable. When we have dark areas and crevices, system contaminants that supply nutrients, and moisture, we have an excellent breeding ground for mold. Therefore, besides keeping humidity under control, a good rule of HVAC operation is: Keep it clean.”

For a technical services bulletin on “Mold & Air Filtration,” which also includes background information on mold and recommendations for proper HVAC system design and maintenance, contact Camfil Farr at 973-616-7300, 973-616-7771 (fax), or literature@camfilfarr.com (e-mail). The bulletin is also posted on the company’s information website at www.camfilfarr.info.

Publication date: 11/04/2002