Active Mold vs. Inactive Mold

A very common misconception in the world of mold is that there is only one type of mold that is dangerous and needs to be removed. The type in question is referred to as active mold. However, nothing could be further from the truth.

As a mold inspector I get a lot of calls and emails asking me for advice on what to do about inactive mold. My advice is always the same. Mold is mold whether active or inactive. Remove the mold using proper containment methods and remediate.

Lets take a look at active mold versus inactive mold so you are well armed with the information you need to make a proper decision about your mold situation.

Active Mold

Active mold is alive and producing spores as it has the right set of conditions to grow. This means it has moisture, food, and an ideal temperature. Mold spores thrive in temperatures 32 and 120 degrees Fahrenheit. Temperatures from about 70 – 90 degrees are the most conducive for mold growth. Chances of mold growth are heightened greatly between those temperatures. Mold also prefers stagnant air but doesn’t shy away from air flow.

Active mold is typically breaking down it’s food source which may result in irreparable damage to building materials and other organic materials.

Active mold also releases mycotoxins and MVOCs. Both can be extremely harmful to health.

Inactive Mold

Inactive mold is also commonly referred to as dormant mold, dead mold, non-viable mold, and old mold. Basically, all of these terms refer to the same mold state.

Mold starts with spores that can lie dormant indefinitely. Dormant spores stay inactive if they don’t sense the right combination of moisture, temperature, air, and food. When something changes, the dormant spores start actively growing mold.

Conversely, when conditions for growth are not met, once active mold becomes dormant.

If a mold colony is in the inactive stage, think of it as hibernating. It’s not growing because it doesn’t have any food or moisture.

When mold becomes inactive, the now dry mold spores become light as they do not have moisture weighing them down. This weightlessness allows them to become airborne rather easily. Airborne spores are the easiest ways for mold to spread and to find areas with the proper conditions to then become active again.

Does Mold Go Dormant During The Winter?

Mold is often times thought of as a seasonal problem. While warm summer air mixed with high humidity offers the perfect conditions for mold to grow, mold doesn’t just disappear during the winter months, especially in an indoor environment.

Cold weather will not kill mold. Mold spores are opportunistic in the sense that they wait for the right conditions and then begin to spread. Different types of molds thrive under different conditions, and some molds are more likely to grow in the winter months than others. This is especially true around windows.

During the winter months, moisture collects on and around windows due to condensation. When the temperature drops during the winter, the warm air in your home comes into contact with the cool glass of your window panes, depositing water vapor and leading to moisture buildup around your windows. This offers mold the perfect conditions in which to grow.

There are many other ways in which mold can flourish in the winter. Be sure you are properly winterizing your home if you live in a colder climate.

What Temperature Does Mold Start To Go Dormant?

Most indoor molds cannot grow below 40 degrees Fahrenheit (F). This is why food is refrigerated at 39 degrees F. The official temperature at which almost all types of mold become inactive is 32 degrees F.

Dead Mold

It is important to note that mold does not die on its own. When left alone, mold is either in an active state or an inactive state. It is never dead.

Some people confuse mold staining with dead mold. These are not the same. Some molds can indeed leave behind stains on certain building materials after the mold itself has been removed. A word of caution with this however. A mold stain may still have inactive mold spores lurking deep inside the building material. Removing or cleaning mold from building materials (especially wood) always leaves room for error. It is best to replace moldy building materials.

How Can You Tell The Difference Between Active Mold and Inactive Mold?

Visually, active and inactive mold CAN be very different. I emphasize can because there are always exceptions.

Active mold in the early stages has hair-like filaments in webs, which develop a more fuzzy or textured appearance as the bloom matures. It does not come off from the surface it is growing on as easily.

Inactive mold is dry and often powdery. A gentle nudge may send spores to the ground. It can generally be readily brushed off the surface. 

Active Mold vs. Inactive Mold – Which Is More Dangerous?

This is a hotly debated question. While active mold is releasing things like mycotoxins and MVOCs, it doesn’t typically move around as much. So if it is contained to a small area, it may not present a huge risk if remediated properly. Widespread active mold is a different story entirely and is no doubt much more dangerous than inactive mold.

Both active and inactive mold present health risks as most allergenic type of reactions are caused by mold spores themselves, whether active or dormant. The more severe health issues associated with mold are typically caused by the mycotoxins they release which they can only do in an active state.

The biggest risk with inactive mold is that it can move around very easily since it is so lightweight. This allows it the opportunity to find the conditions it needs to become active again.

Mold Prevention

The key to mold comes down to prevention! I have an entire library of articles on mold prevention so please be sure to spend some time reading through these.

Mold maintenance is hands down the most important step you can take to preventing inactive mold from becoming an actively growing colony. Regular cleaning with the proper products as well as quarterly or even monthly fogging can make the difference between a healthy home and a mold contaminated home.