Can Bacteria and Mold Grow on HEPA Filters?

HEPA filters are one option for removing particulate pollutants, including microorganisms like bacteria or mold spores, from the air in your home. However, one of the drawbacks often cited about HEPA and other mechanical filters is that microorganisms can survive on the filter medium for some time—and under certain conditions might even grow or be released back into the air.

This poses a serious question for the health of you and your loved ones: Rather than keeping the air clean, is it possible for a HEPA filter itself to become a source of microorganisms polluting your indoor air? We will take a look at the scientific research and see how much of a hazard microorganism build-up on HEPA filters can become, how much of a threat it could pose to you and your family, as well as what can be done to avoid it.

What conditions do mold and bacteria need to grow on HEPA?

Bacteria and mold are very different microorganisms, but they both require water and food to grow (and, to a lesser extent, proper temperature and pH). Bacteria are single-celled microscopic organisms that can be aerosolized (emitted into the air) through coughing, sneezing or other processes (Cox, 1989). Mold is a fungus that can emit spores in your home. These spores can multiply if they land on a surface with the right conditions.

So, the question remains: Does a HEPA filter provide the proper conditions for mold or bacteria to grow? The answer is yes. In a humid environment, it is possible for a HEPA filter to retain enough moisture to enable bacteria or mold to grow. A HEPA filter that is otherwise exposed to water, such as the condensate from an air conditioner, can provide the necessary moisture for microorganisms to grow.

The conditions for food for these microorganisms are a bit more difficult to determine than those of moisture—it depends on what the filter is made from as well as the specific type of microorganism. Bacteria and fungus can draw nutrients from sugars, starches and other organic materials; so dust, pollen, pet dander or other materials trapped in the filter along with the microorganisms could provide them with enough nutrients to become a food source. Moreover, if the filter is made from organic fibers such as cellulose, then it could become a food source as well (the HEPA standard specifies filter performance, not what the filter is made from, so it could be constructed of any number of materials).

Evidence that mold and bacteria have grown on HEPA filters and affected health

Given the evidence, it seems that microorganisms can theoretically grow on HEPA filters—researchers have proven that this does in fact happen under the right conditions. One study examined both HEPA and lower efficiency filters used in hospitals and commercial buildings for mold. They found that all filters made of cellulose-based materials, and those that had some form of moisture problem (i.e. filters installed in HVAC systems) had some mold growing on them—typically on the “load side” of the filter, or the side that polluted air is coming from. The study also found that filters “treated with an antimicrobial preservative tended to reduce or delay colonization,” however, two HEPA filters that were untreated were completely colonized by mold on both the load side and the downstream side [Price et. al, 2005]. Having mold growing on the clean air side of a filter is much worse than having it only on the load side.

Another study found that bacteria contaminating a HEPA filter could survive up to 210 days. And even if the bacteria did not have the proper conditions to grow, the bacteria would grow again if it was later introduced to a moist area with sufficient nutrients (Mittal et. al, 2011). However some of the bacteria tested did not even survive two days, so the extent of the problem depends on the specific type of bacteria.

A study examining HEPA filters from the homes of children with asthma or allergy symptoms also found several species of fungus growing on the filters, including molds known to cause allergic reactions (Seong et. al, 2014).

Mold spores are known to trigger or exacerbate asthma symptoms and allergic reactions, and numerous infections are caused by airborne bacteria which can stay viable for up to 45 minutes in the air. So having these microorganisms present on a HEPA filter creates a potential health risk.

When is the risk greatest?

The most significant health risk occurs when the bacteria or mold is able to grow through to the downstream side of the filter media. In this scenario, when air flows through the filter, it picks up bacteria and mold spores as it flows out of the unit and into the room—the filter has become a point of origin for contamination, rather than a way to remove pollutants.

The results of a study on mold spores and filters found that sudden increases in air velocity caused a greater number of mold spores to be released from the filter back into the air stream, a process known as “reentrainment” (Jankowska et. al, 1999). This study was focused on spores collected by the filter being “shaken loose” by air flow, but the researchers warned that a filter with mold actually growing on it would likely be even more of a hazard.

Another point of risk is when you change the old filter out to install a new one. The process of jostling and moving the old filter can release mold spores or bacteria into the air. Even if the only microorganisms present in the filter are ones that it trapped there, this could release a significant percentage of the bacteria or mold spores into your house. And, again, if fresh mold or viable bacteria are growing on the filter, the problem is even worse. Because many people forget to replace their filters on time, this can increase the likelihood of the problem occurring and especially the health of sensitive groups who are particularly affected by breathing in mold spores.

Ways to prevent mold from growing on HEPA filters

If you are concerned about mold or bacteria growing on your HEPA filter, there are a few steps you can take to minimize the problem:

  • Change filters frequently: HEPA filters gradually become clogged with the particles they remove from the air, reducing the amount of air that can flow through them. This is reason enough to change them on a regular schedule. But when you add the possibility of mold or bacterial growth, it becomes even more important to install a fresh filter on a regular basis. Most HEPA filters have recommended duty cycles, so set a reminder on your calendar and get rid of old, potentially contaminated filters.
  • Use filters with an antimicrobial treatment: Research has found that filters that have had an antimicrobial treatment applied to them have lower rates of microbial growth, delayed microbial growth and release fewer microorganisms back into the air [Verdenelli et. al, 2002; Price et. al, 2005].
  • Use PECO filter technology: If you would rather avoid the problem entirely, you could use an air purification technology such as PECO, which uses a photocatalytic reaction to destroy polluants, including mold spores and bacteria, rather than trapping them in a filter. To find out more about how Molekule works on mold, check out our article “How the Molekule Air Purifier Works on Indoor Mold” here.

The possibility of mold or bacteria growing on a HEPA filter is more than just hypothetical—it can happen both in laboratory tests and in real-world conditions. Bear in mind that this does not mean that a HEPA filter will always turn into a mold or bacteria source. It will only happen under certain conditions, with certain species of fungus or bacteria. But if your HEPA filter is in a very humid environment or is otherwise exposed to moisture, then it could be a more significant problem for you. And if you or someone in your family is immuno-compromised, then it is even more of a serious issue.

If you choose HEPA filters, make sure to use them correctly, with on-time filter changes, and look for antimicrobial filters when possible. If the Molekule air purifier is a part of your plan, the PECO technology destroys mold, so that no spores will escape the device and be released back into the air. For every air purifier, proper filter replacement is key, in order to receive the full benefits of the device.