What is the best HVAC filter for mold spores?

by | Apr 11, 2022 | Mold Advice & Tips

The toxic facts are that indoor air can be 2 to 5 times more polluted than outdoor air. (EPA) Cheap, defective, and dirty heating and air conditioning system (HVAC) filters are a poor line of defense against harmful airborne pathogens and debris.

They are also the perfect breeding grounds for mold.

For people who suffer from mold illness, the goal is to limit the spores in the air, thus limiting your exposure and the ability of the mold to reproduce and contaminate your home.

One of your best lines of defense against airborne mold in your home is having the right HVAC filter and changing it often.

However, in my experience as a certified mold inspector, most people do not even look at their filter or even know where it is located.

I would say that 9 out of 10 homes I inspect have an extremely dirty HVAC filter and they are rarely the proper kind to filter air contaminants.

For example, a cheap or dirty filter acts as a sponge for water, condensation, and mold, which poses a serious health threat to people who may be immune-compromised or who suffer from mold illness.

It takes only 24-48 hours for mold to start growing on it, and when you turn on your HVAC system, the spores can then flow all throughout your property, contaminating the air, every room, and all your furniture.

This can create what is called “sick building syndrome,” which can lead to serious mold illness and or fungal infections for the occupants.

A dirty and clogged filter can also lead to reduced or no airflow at all. The result will be higher energy bills and can lead to your entire HVAC system needing expensive repairs or to be replaced.

Hence, the wrong or dirty filter can cost you thousands of dollars, and even you and your family’s health. Therefore, it is imperative that you learn how they work, the best ones to buy for your home, and how to maintain them properly.

If you are looking to improve the overall indoor air quality of your home, most of the standard cheap HVAC filters that you can find at the local Walmart simply won’t do the job.

That is why having a highly efficient disposable filter is paramount if you want to reduce your exposure to harmful debris, dust, pollen, and microorganisms such as toxic molds.

How filters work is simply by dirty air passing through the filter to catch the various debris and microorganisms which then releases the filtered clean air.

The key to getting the right filter is to make sure that it will clean most of the air contaminants in your home so you can breathe fresh air.

This is done by making sure that it can capture the tiniest of microbes such as mold spores which are measured at the very smallest, 1-3 microns, and can easily be removed by using a good filter.

Minimum efficiency reporting value, commonly known as MERV rating, is a measurement scale designed in 1987 by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to rate the effectiveness of air filters.

The higher the MERV rating equates to less airborne contaminants that can pass through the filter and cleaner indoor air.

MERV filters with high capture efficiency can be a great help for people who are sensitive to mold because they help purify the air by removing most mold spores, along with other allergens. By capturing mold spores, MERV air filters help prevent the spread of mold and results in cleaner indoor air for the occupants.

The rule of thumb is the higher you go up the MERV scale, the better.

A really good filter with a MERV rating from 8-13 will capture 30 times or more pet dander, mold, and pollen than a standard filter, which makes it much easier for people to breathe, especially those with allergies.

For example, one of the most common indoor molds that we find is aspergillus, which has a particle diameter of 3.5 microns.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) says a MERV 6 filter can remove it when matched with a proper number of air changes.

Here is a table from the EPA showing the various ratings and filtering abilities.

Bacteria, droplet nuclei (sneeze), cooking oil, most smoke, and insecticide dust, most face powder, most paint pigments will have a micron size of 1-.3, so you would need a filter with a rating of a MERV 12 – 16 to properly filter the air against these contaminants.

Personally, I recommend the MERV 13 filter because it filters these other smaller contaminants from the air in addition to mold spores.

Here is a list of several different types of molds along with the MERV rating from Camfil, a manufacturer of high efficient filters for commercial HAVC systems.

What type of filter should you buy for your property?

There are many types of filters on the market.

Some are good, some great, and some are just terrible.

When you search for a good filter, you need to find one that has as a rating per the Occupational Safety and Health Administration (OSHA) minimum requirements with an efficiency of 50 to 60% or a rating of MERV 8, as determined by Test Standard 52.2 of the American Society of Heating, Refrigerating and Air-Conditioning Engineers may be appropriate.

(OSHA) recommends:

“High-quality filters must be used in an HVAC system during remediation because conventional HVAC filters are typically not effective in filtering particles the size of mold spores. Consult an engineer for the appropriate filter efficiency for your specific HVAC system, and consider upgrading your filters if necessary.

A filter with a minimum efficiency of 50 to 60% or a rating of MERV 8, as determined by Test Standard 52.2 of the American Society of Heating, Refrigerating and Air-Conditioning Engineers may be appropriate.

Remember to change filters as appropriate, especially following any remediation activities. Remove filters in a manner that minimizes the reentry of mold and other toxic substances into the workplace. Under certain circumstances, it may be necessary to wear appropriate PPE while performing this task.”

This is what the Environmental Protection Agency (EPA) says:

“Air filters should have a dust-spot rating between 35% and 80% or a Minimum Efficiency Rating Value (MERV) of between 8 and 13. The higher the rating, the better the protection for the equipment and the occupants.

It has been estimated that a 30% increase in static pressure across a coil results in a $200 per 10,000 cfm of air movement (at 7 cents per KWH). This does not include the added cost of cleaning dirty heating or cooling coils, drain pans, or air ducts.

Designers should consider specifying a low efficiency (~10%) pre-filter upstream of the main filters. The pre-filters are generally easy and inexpensive to change and will capture a significant amount of the particulate mass in the air, thereby extending the useful life of the more expensive main filters.”

Mold Safe Inspections Conclusion

Please keep in mind that this is not a complete cure for keeping your indoor air clean, but one of the most effective tools for the defense against harmful pathogens and to create clean air in your home.

One last note, even though these are great filters, you will still need to change them at least 3-4 times a year, and if you have severe allergies or a mold problem, I recommend once every other month or every 90 days maximum.

To do so, I recommend that you wear safety equipment such as a HEPA mask and you install a vapor barrier around the area or at the very least, use a HEPA vacuum and air filter right next to you so it will clean any dust that gets airborne.

You will also need to measure the size of your filter and the easiest way to do that is by finding the size of the one that is currently installed and in use.

If it is not listed on the current filter, take it out and measure the length, width, and depth, and then round the numbers up to the nearest inch. It is better for it to be a little too large than too small, so there are no gaps where debris and pathogens such as molds can bypass the system.

When buying a new filter, I recommend that you go as high as possible to a MERV rating of 12-13 to filter mold and other airborne contaminants such as smoke, insecticides, and auto-exhaust emissions. That way your indoor air is as clean as possible.

If you need a mold inspection or mold remediation, please call or text us at 760-818-6830.


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