How Evaporative Coolers Cause Mold – And How To Prevent It


More than half of all American homes have mold issues. Since evaporative coolers can increase the moisture content in a room and mold thrive in humid environments, it makes sense to wonder if they grow or cause mold. 

Evaporative coolers can cause mold if the relative humidity is increased above 50%. This is more likely to occur in already humid environments. Evaporative coolers can also breed mold and disperse mold spores around the home due to inadequate maintenance or irregular cleaning.

This article closely examines how evaporative coolers work and how they may facilitate mold growth. We also discuss some helpful maintenance tips that can prevent mold breeding inside the cooler and restrict mold growth in the room.  

What Is Mold?

Molds are complex organisms that are found across all ecosystems, (i.e., air, water, and soil), and are actually classified as fungi.  

Occurrence

Mold spores measure about 3 to 40 microns and occur in large concentrations in our environment. In fact, the number of mold spores per cubic meter of air can be as high as 50,000.  

While the spores are usually airborne, mold colonies can form on all surfaces, from paper and wood to wall paint and dust.  

Cladosporium, penicillium, aspergillus, and acremonium are some of the most common molds in the environment.  

Toxicity

Molds are germs with various side effects. While inhaling spores is the most common way of acquiring mold infections, food contamination is also one of the primary sources of mold exposure in our homes.  

Mold infection usually appears as a combination of allergic symptoms that include: 

  • Watery eyes 
  • Runny nose 
  • Itching 
  • Coughing 
  • Shortness of breath 
  • Headache 
  • Fatigue  

Molds are toxigenic and release many harmful chemicals. One of the most potent of all these substances is a mycotoxin. It can have severe acute and chronic health implications, such as liver cancer, immune system impairment, gangrene, convulsions, respiratory distress, etc. 

Evaporative Coolers, Humidity And Mold

Evaporative coolers are environmentally friendly alternatives to air conditioners. Unlike AC units, coolers do not use ozone-depleting greenhouse chemicals (refrigerants) to extract heat from the air.  

Instead, they work on the principle of evaporative cooling. Heat is transferred from the air into the water that an evaporative cooler contains, and as a result, the air temperature drops. This is why evaporation is a cooling process.  

There are two types of evaporative coolers that use this principle to cool air down by as much as 35°F. These coolers use slightly different mechanisms and influence the growth of mold in different ways.

1. Direct / Swamp Coolers

This system consists of a rotating fan, evaporative pads or honeycomb filter, water reservoir, water pump, spray water box, thermostat, and blower.  

The pump propels water to the spray water box, from where it is sprayed onto the evaporative pads or honeycomb corrugated filter.  

  • First, the fan rotates to draw dry and hot air inside the system. 
  • Next, the air encounters the wet hydromel film on the pad or filter.  
  • Due to the temperature difference, heat exchange occurs, i.e., the water absorbs heat from the incoming air and converts it into vapors.  
  • Lastly, the blower fan disperses cool and humid air into the room.  

A thermostat regulates the entire process. Once the temperature drops to the preset level, the sensor temporarily stops the cooling process to maintain a controlled, comfortable environment.  

2. Indirect Coolers

Indirect evaporative coolers are the same as swamp coolers, except they don’t humidify the original steam.  

In indirect coolers, air does not directly come in contact with a water film. Instead, water is sealed in a set of closed tubes. Therefore, while the heat transfer does occur, no moisture is added to the air (which also flows in the duct system).  

How Humidity Affects Mold

The addition of moisture to the original stream of warm air in evaporative coolers is the main cause of mold growth.

Moisture plays a crucial role in mold breeding because mold spores cannot sprout without adequate humidity. Spores remain in a dormant phase until the relative humidity reaches over 55% (at 65 degrees).

Therefore, maintaining the indoor relative humidity within the ideal range, which is between 30 to 50%, is crucial to prevent mold from establishing.  

Do Evaporative Coolers Cause Mold?

When used in a damp climate, direct evaporative coolers can cause mold (since indirect evaporative coolers do not add moisture to air).  

While direct evaporative coolers are more efficient than indirect units (because swamp coolers are mobile and typically require less maintenance), they are not the best choice in humid areas (i.e., where relative humidity exceeds 50%).  

This is because swamp coolers may increase moisture levels above the ideal range, i.e., 30 to 50%, making the environment favorable for mold growth.  

Therefore, people living in humid areas should either switch to indirect units or run a dehumidifier to keep the humidity in check.  

How Evaporative Coolers Grow Mold

Here are the primary conditions in evaporative coolers that can facilitate mold growth in the cooler itself and around your home:

Obstruction

The air flowing inside the cooler is polluted and laden with various contaminants, ranging from dust and debris to pet dander, hair, germs (including mold spores), pollens, etc.  

Evaporative coolers do not generally have air filters that can remove airborne contaminants. While most of the pollutants are blown along with cool air, some of them can accumulate inside the unit. 

Over time, these pollutants build up and form clogs that can collect and block any component of the cooler, such as the fan blades, the water sprayer, inlet, blower, honeycomb filter, evaporative pads, etc.  

As the interior environment of evaporative coolers is particularly moist, any mold spores contained in the dirt deposits can grow and multiply into numerous colonies in no time.  

Fix

Cleaning the unit adequately to remove build-ups can resolve the issue. Running an antimicrobial air purifier, i.e., the ones with a True HEPA filter or electrolytic sterilization techniques, can minimize the problem by ensuring clean air inside the room.

Mineral Residues

The water that the pump supplies to the spray box and eventually to the evaporative pads comes from the reservoir. It stores a specific quantity of water and periodically supplies it to carry out the cooling process. 

If you add hard, mineral-rich water or unfiltered tap water to the reservoir, the particulate matter is left behind as a residue, while the water is used up. The mineral build-up (which occurs as yellowish scales) provides nutrition to mold, facilitating their growth. 

Fix

Descaling and disinfecting the cooler, especially the reservoir, regularly and using only distilled water is essential to get rid of and prevent mold growth. 

A 1:1 vinegar solution is generally the best for descaling. Allow any components to soak in the solution and use a soft brush to remove any hardened deposits.

How To Prevent Mold Growth In Evaporative Coolers

While the cleaning instructions may vary slightly for different models, here are some general maintenance tips for evaporative coolers: 

1. Dust The Exterior

Turn the cooler off and disconnect it from the main power supply. Then use a clean cloth and wipe the external body of the cooler. 

2. Drain The Reservoir

It is crucial to empty the tank every two days. Standing water is a breeding ground for mold, and therefore, it is essential to drain the tank, no matter how much water is left.  

3. Disinfect And Descale

Rinse the tank with clean water, and then wipe it with a cloth. 

  • Prepare a white vinegar solution with vinegar and water in a 1:1 ratio, and fill half of the tank with it. 
  • Allow the solution to sit in the tank for 20 minutes. The acidic nature of vinegar will kill the microbes and soften the scales. 
  • Drain the tank, and rinse it until the vinegar smell is gone. 

4. Brush And Vacuum

Use a soft, bristle brush or handheld vacuum to remove the build-up of particulate matter (e.g., dirt, debris) inside the cooler. 

How Do I Get Rid Of Mold In My Evaporative Cooler?

A white vinegar solution (1:1 ratio of water and vinegar) is an effective disinfectant and can remove mold growths from evaporative coolers. Soak the reservoir and components in the solution and use a soft brush to remove any physical signs of the mold. Rinse the evaporative cooler thoroughly before using again.

Do Evaporative Coolers Make A Room Humid?

Swamp (or direct) evaporative coolers add moisture to the air and can increase a room’s humidity.  However, in excessively humid environments swamp/evaporative coolers have a reduced effect in both reducing the temperature and increasing the humidity. Indirect evaporative coolers do not affect the humidity of a room.

Can You Get Sick From Evaporative Cooling?

Evaporative cooling does not have negative impacts on health. However, if the cooler is not cleaned and maintained well, it can breed and disperse various germs, which can cause health complications.

Russell Singleton

Russell holds a Bachelor of Science (Environmental and Marine Geoscience) with Class I Honors. He is currently completing his doctorate in science and is passionate about all earth processes, especially isotope geochemistry and paleohydrology.

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