In previous articles we have discussed the health risks of indoor pollution, the need for adequate ventilation and the desirability of having air quality monitors and air purifiers in our homes. However these benefits are only obtained if we have regular maintenance of our air cleaning equipment and air conditioning units. And most important is the use of correct air filters.
WHAT THE SCIENTISTS TELL US
Types of Air Filters
There are mainly two types of air filters for particle collection.
– Fibrous Media Filters
There are two methods for evaluating this type of filter. a) Test Method specified in The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) Standard 52.2. This method evaluates the removal efficiency for particle sizes 0.3 to 10 µm in diameter. The results are reported as Minimum Efficiency Reporting Value (MERV).MERV numbers range from MERV 1 to MERV 16,based on average removing efficiency across 3 particle sizes in ranges 0.3 -1 µm, 1 – 3 µm, and 3- 10 µm. The higher the filter rating the higher the filter’s removal of particles efficiency. Filters with a MERV 13 and above ratings require at least 50 % removal efficiency for 0.3 – 1 µm particles (These are the size of particles that are the most harmful to human health). MERV 11 filters must achieve at least 20 % removal of these particles. The Environmental Protection Agency (EPA) recommends furnace filters with at least MERV 13 rating . High Efficiency Particulate Air (HEPA) filters are generally equivalent to MERV 16 and offer the highest available particle removal efficiency of fibrous media air filters. Usually with a removal efficiency of 99 % or higher. Click here to see details on my recommendation for a very efficient HVAC air filter.
b) When the air filters are used in a portable air cleaning unit, the filter’s efficiency is measured by the unit’s Clean Air Delivery Rate (CADR). The CADR is a measure of a portable air cleaner’s delivery of relatively clean air, expressed in cubic feet per minute (cfm)
– Electrostatic Precipitation Filters (ESP)
ESP’s use a high voltage wire to charge incoming particles which are then collected on charged plates inside the air cleaner. These units are mostly used in whole house ventilation systems at the main air entrance duct. They are considered “green” technology due to their lower energy usage and high efficiency. In selecting an ESP based air cleaner it is important that the unit selected does not intentionally emit ozone. Air cleaners that intentionally emit ozone are not approved for use indoors where humans are present. The California Air Resources Board mandates device testing for ozone production as per Underwriters Laboratory (UL) Standard 867. However, there are no national standards that set a limit on allowable amount of ozone that can be produced by these units. More information is available in an article published by the EPA and listed here under References. 
There are other air cleaning systems that do not use air filters, such as Ultraviolet germicidal irradiation (UVGI), Ionizers, Catalytic Oxidation, Plasma, and others. However these are for specialized applications such as for use in Hospitals or Laboratory Clean Air Rooms, etc. However, for home use there is one other system that merits consideration.
Whole House Electronic Air Cleaners. These systems are used at the entrance of the home HVAC system. By including an ESP system together with a Fibrous Media Filter they promise removal of 98 % of airborne particles. These systems interface with the existing HVAC unit in the home. Therefore installation by a professional HVAC technician is recommended. Furthermore, some of the units are available with controls independent of the HVAC system. This allows the owner to operate only the air cleaning unit with a lower energy consumption, compared to when the HVAC system is operational .These systems can therefore be run for a longer time insuring better air quality.
Air filtration does not replace the need to control pollutants at their source and the need to ventilate . Please note that filter and portable air cleaning unit’s efficiency is measured when the Air Conditioning Unit, the Furnace or the Portable Air Cleaning Unit are operating. Therefore in selecting an air filter system other factors such as operating costs, ease of maintenance and noise need to be considered.Adding an Air Purifier will provide further insurance that you have a healthy indoor environment.
 Residential Air Cleaners EPA402-F-09-002 –July 2018 www.epa.gov/iaq
 Darius Ciuzas, et al. (2016)