Air Filtration and Indoor Air Quality

air filtration

What is Air Filtration 

Your central HVAC (Heating, Ventilation, Air Conditioning) system has an air filter. Do you know what kind of air filter you have? And what that filter actually does? And whether your air filtration system makes a significant difference in lowering your indoor AQI (Air Quality Index) values so you and your family breathe easier in your home?

To clear the air about your questions about air filtration and its relation to air quality, this post examines:

  • What an air filtration system does
  • How MERV is measured
  • What CADR measures
  • The difference between air filtration and air purification
  • The types of air filters
  • Why an air filtration system upgrade can help lower indoor AQI
  • Whether you need to change or upgrade your air filtration system and the best way to proceed to upgrade

What Air Filtration Does

Central air filtration systems are installed on the return air side of your HVAC system, either at the indoor unit or the return air filter grill. The primary job of air filters is to keep your air and cooling system running smoothly and optimally. But air filters also do improve the overall cleanliness of your indoor air quality by removing airborne particulates and reducing the AQI in your home. These particulates include outside air pollution, pollens, skin particles, hair, smog, pet dander, smoke and household dust.

Not all filtration systems remove all particulate matter, however.  The degree to which they remove these particulates also varies. To determine exactly what an air filter removes and how effective it is in removing them, you need to know the air filter:

  • Type
  • MERV rating

To understand the variance of cleaning capabilities among air filter types, let’s first define MERV.

What MERV Measures

As defined by the Environmental Protection Agency (EPA),  MERV stands for Minimum Efficiency Reporting Value. It measures how much matter passes through an air filter, based on a scale of 1 to 20. Higher rated air filters capture smaller-sized particles. Consequently, the higher the MERV rating, the fewer particles pass through the air filter and circulate throughout your house. The following chart indicates the particles you can expect filters can capture with varying MERV ratings. 

MERV RatingParticles Captured
1-4Particles greater than 10 micrometers; this includes bugs, dust mites, and household debris. Most window air conditioners fall within this range.
5-8Captures particles greater than 3 micrometers. This includes mold, pet dander and aerosol sprays. This is the baseline for permanently installed residential air conditioners.
9-12Captures particles greater than 1 micrometer. This includes most dust and common pollutants. Hospital laboratories and superior residential air conditioners fall within this range.
13-16Captures particles greater than 0.3 micrometers. This includes bacteria, droplets from sneezing, smoke and most other sources of pollution. This level of filtration is used in patient and surgery areas of hospitals.
17-20Captures virtually all particles. This includes viruses and the finest particles of smoke, dust, and other debris. This level of filtration is typically only used in clean rooms for the manufacture of electronics or scientific experiments.


Generally, MERV ratings higher than 9 are for industrial, scientific and medical use. Some air filters for home use may, however, a rating that is 9 or even higher. So you might think, the “best” air filter should have a MERV rating around 8. However, that is not necessarily the case. It depends on the application. A professional HVAC contractor can provide the best advice. But you should know the type of air filter that is best to keep your HVAC system running efficiently, as well as whether you might need to consider a different heating and cooling application to fit your needs and those of your family. 

Again, depending on the application, you might also want to consider something called Clean Air Delivery Rate (CADR).

What CADR Measures

CADR was developed by the Association for Home Appliance Manufacturers (AHAM). It measures not only the efficiency with which three kinds of particles—smoke, pollen and dust—are removed from filtered air, but also the rate at which clean air is delivered to your living space. Without getting into the math, CADR essentially measures particles that might remain in the air in a given sized room. 

So what does CADR mean to you in determining what is the right rating for your home application? AHAM recommends “the 2/3 Rule.” In other words, you want an air filter with a CADR rating in which at least two-thirds of the particles are removed from a room of a given size. For example, say you want to be sure an air filter can effectively remove tobacco smoke from a 10-foot by 12-foot room, or 120 square feet. Two-thirds of 120 square feet is 80. So if you’re replacing a filter in an air conditioning unit for that room, look for a CADR of at least 8.

Because CADR is a measurement of particulates removed according to room size, whole house air filters do not have CADR ratings. Consult a certified HVAC contractor if you have any questions about the ability of a filter to remove certain kinds of particles in your home application.

Air Filtration is Not Air Purification

People sometimes confuse air filtration and air purification. The basic difference is that air filters trap certain dust and dirt particles, while an air purifier uses ionization to remove particles most air filtration systems cannot contain. While your heating and cooling application always has some type of air filter, you may not necessarily need an air purifier. However, if you live in an area with heavy pollution and/or someone in your family has respiratory issues, you may breathe easier with the added protection of an air purification system. Consult your certified HVAC service technician if you are interested in adding an air purifier. Keep in mind that it does not replace your air filter or perform the same function to keep your HVAC system running efficiently. 

Types of Air Filters 

There are five types of air filters:

  1. Fiberglass

Fiberglass air filters are disposable, must be changed every three months, and are the most affordable. But they also have the lowest MERV rating of 1-4. They are primarily designed to protect your air conditioning system rather than screening dust and pollutants from the air you breathe indoors. This type of air filter does little to nothing to help people with respiratory problems.

  1. Washable

Washable air filters also have a MERV rating of 1-4. While washable filters generally cost more than disposable filters, they can be removed, cleaned and reused.  Recommended cleaning is every three to six months, depending on HVAC system usage and manufacturer specs. Key to the proper use of any washable air filter is to make sure it dries completely after cleaning before reinstalling. A damp filter can cause mold growth and bacteria buildup.

  1. Pleated

Made of polyester fabrics or cotton folds, pleated air filters are effective in filtering larger dust and other pollutants. The MERV rating usually ranges between 5 to 16, with the higher-rated, higher performance filters having more pleats. They can come in 1” up to 5” pleated filters and should be changed every 3-6 months depending on HVAC system usage. 

  1. Electrostatic

Electrostatic air filters use static electricity to trap dust and airborne particles and prevent them from circulating in your home. Most electrostatic air filters can successfully remove large particles, dust, pollen, and carpet fibers, but do a poor job of capturing small particles and mold spores. MERV ratings generally range from 1 to 4, though some electrostatic filters are rated as high as 9. Some filters of this type are washable and some electrostatic filtration also contain pleated or media filtration for maximum efficiency. Electrostatic air filters should be cleaned every six months. 

  1. HEPA

High Efficiency Particulate Air (HEPA) filters are effective in screening up to 99.97% contaminants such as dust, pollen, mold, bacteria and any airborne particle with a size of 0.3 microns. Consequently, HEPA filters are recommended for people with allergies and respiratory issues. These filtration systems should be changed every 6 months.

Is My Air Filtration System Good Enough?

Any kind of air filtration system requires periodic changing or cleaning to ensure efficient operation. But do you need to change your current air filtration system for a better one?

You can certainly achieve “better” performance and air quality with regular maintenance. If you want to improve indoor AQI, you could upgrade your filter to a higher rated MERV or CADR. If you do upgrade, have your HVAC specialist check the system in a month or two, as initial operation may collect higher than normal amounts of matter in doing a thorough cleaning. Then you can return to a normal maintenance schedule.

Another way to upgrade your system is to install a thermostat with an “Air Circulation” mode. This allows you to run the system fan to circulate air at periodic intervals, for example 15 minutes every hour. More moving air means more air passing through your filter, which means more particles are trapped, making your indoor air cleaner.

It also may be beneficial to purchase an AQI monitor for your home. The monitor can help warn you if your indoor air is getting dirtier, in which case you can run the Air Circulation mode more frequently.

Consult the Air Filtration Experts—Ongaro and Sons 

Even if you are confident that you can change air filters on your own, it’s still a good idea to have a certified HVAC specialist perform regular maintenance on your system. It’s particularly a good idea to consult with a knowledgeable expert if you are considering changing or upgrading your air filtration system. An expert can help you weigh the pros and cons of different air filtration systems to determine what is the best application that best fits your home and family needs.

Installing or maintaining an air filtration system requires professional skill and knowledge. Call the heating and cooling specialists at Ongaro and Sons. We are Trane Comfort Specialists certified in high-efficiency HVAC systems and indoor air quality. As we like to say, “Ongaro and Sons puts the V back into HVAC.”