How can I determine how well the air conditioning system in my home is filtering the air, and how could I improve it to make it suitable for an asthmatic? -Reojo

Hi Reojo,

How good a job your central heating and cooling (HVAC) system does in cleaning the air in your home largely depends on the quality of the air filter used with it. Filters are rated from 1 to 16 on the Minimum Efficiency Reporting Value (MERV) scale, based on the percentage of particles from 0.3 to 10 microns in size that are filtered out. The higher the MERV number, the better it filters the air.

Air filters made by Filtrete use a different rating system, the Microparticle Performance Rating (MPR), which measures only very small particles (between 0.3 and 1 microns). Ratings on the MPR scale range from 0-2200. The higher the number, the smaller the particles that are filtered.

To give you an idea of the size particles we are talking about, mold spores and pollen are usually in the 10-30 micron range, bacteria 2-3 micron, and tobacco smoke 0.5 micron. Here are some general guidelines to consider when choosing an air filter for your HVAC system:

• MERV 1-4 Air Filters: These inexpensive fiberglass mesh filters only remove particles over 10 microns in size and do very little to improve the indoor air quality in your home.
• MERV 5-8 Air Filters: Medium quality pleated air filters that can remove particles down to 3.0 microns in size.
• MERV 9-12 Air Filters: High quality air filters that can remove particles down to 1.0 microns in size.
• MERV 13-16 Air Filters: Highest quality in standard filters. Removes particles down to 0.3 microns.
• MERV 17-20 Air Filters: Also known as HEPA air filters (High Energy Particulate Air). These filters surpass the standard MERV rating by removing 99.97% of airborne particles 0.3 micron and larger. While often used in hospitals, they cause too much air resistance to be installed in a standard HVAC system without making modifications first.

There are a number of other things you can do to help clear the air in your home in addition to installing a quality air filter and changing it regularly, including:

• Limiting or removing carpets and rugs.
• Keeping pets outdoors.
• Reducing Volatile Organic Compounds (VOCs) in your home.
• Use quality vacuum cleaner equipped with a HEPA filter.
• Install a room air purifier equipped with a HEPA filter.

In addition, certain houseplants can improve air quality including philodendron, peace lily, dracaena, and snake plant.

Further Information

• Changing the Air Filter in Your Home (video)
• Choosing Air Filters for Your Home (video)
• How to Find the Air Filter for the HVAC System in Your Home (article)
• Guide to Air Cleaners in the Home (EPA)

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