Mold spores are microscopic reproductive units produced by mold, a type of fungus. These tiny particles are the primary way mold spreads and are constantly present in both outdoor and indoor air. How long they remain suspended in the air is highly variable. The duration ranges from a few minutes to an almost indefinite period, depending on the spore’s physical characteristics and surrounding environmental forces.
The Physical Properties of Mold Spores
The ability of mold spores to remain suspended for extended periods results from their extremely small size and low mass. Most mold spores range from 2 to 10 microns, though some can reach 30 microns in diameter.
This minute scale results in a very low terminal settling velocity, which is the speed at which a particle falls through air due to gravity. Scientists use “aerodynamic diameter” to describe a particle’s behavior, accounting for both its physical size and density. Because mold spores have a small aerodynamic diameter, they behave more like gas molecules than dust, making them easily influenced by air currents. Their lightweight structure allows them to resist gravity and travel vast distances from their source.
Environmental Factors Affecting Suspension
The time mold spores remain airborne is largely determined by external forces, especially air movement. Minor air currents, such as those generated by an HVAC system, an open window, or a person walking, easily overcome the spore’s low settling velocity. These constant disturbances keep the microscopic particles in a continuous state of re-suspension.
In an environment with steady air turbulence, spores can remain suspended for hours or even days until they are physically removed or the air becomes completely still. Humidity also influences the physical state of the spores. High indoor humidity, ideally kept between 30% and 50%, promotes mold growth, constantly releasing new spores into the air.
Moisture can also cause spores to aggregate or clump together. When spores absorb moisture, they stick to one another, forming heavier clusters that settle out of the air more quickly. Temperature has a less direct influence, primarily affecting air density and the rate of air movement.
How Spores Are Removed From the Air
Airborne mold spores are removed through two main processes: natural deposition and active intervention. In a completely still environment, spores eventually succumb to gravity and settle onto surfaces like floors, furniture, and fabrics. This natural settling process, known as dry deposition, is slow due to the spores’ small size, but it is the ultimate fate of any particle not actively moved.
Once settled, spores can be removed through cleaning or re-suspended into the air by activity like vacuuming or dusting. Active removal often involves air filtration systems. High-efficiency particulate air (HEPA) filters are effective because they capture 99.97% of particles as small as 0.3 microns. Since most mold spores are larger, HEPA air purifiers and HVAC filters significantly shorten their airborne duration by physically trapping them.
Ventilation also removes spores by introducing fresh outdoor air and exhausting spore-laden indoor air. This air exchange actively flushes out airborne particles, reducing their concentration indoors. Controlling moisture with dehumidifiers is another effective strategy, as it prevents the underlying mold growth that sources new spore production.