Due to energy concerns, the majority of air in buildings is recirculated rather than replaced with fresh
air. This means you are breathing in air that was already in someone else’s lungs. Cleaning the air is
important, but with all the options available, it can be difficult for property owners and managers to
know the best strategy. In this article, we will describe different types of air cleaners and help you select
the right one for your application.
Indoor air contaminants can be classified into two main categories: gases and particles. Examples of
gases are volatile organic compounds (VOCs) and odors. Examples of particles are smoke, mold spores,
and pollen grains. Some air cleaners remove particles, others remove gases, and some remove both. If
you have a particular indoor air quality concern, it is important to know if it is a particle or a gas. If it’s a
gas such as chemical odors from a recent renovation project, more outdoor air ventilation should be
considered as an alternative to air cleaning.
The HVAC System
Air cleaners can be incorporated into a building’s heating, ventilating, and air-conditioning (HVAC)
system, or just be a portable, stand-alone unit. When incorporated into the HVAC system, clean air is
delivered throughout the zone served by the air handling unit, rather than to a single room. Another
option is an individual air cleaner, which is often referred to as an “air purifier”. Portable air cleaners
often have a higher level of filter efficiency but deliver a lower volume of clean air.
There are only a few different technologies that you will find in air cleaning. The most common method
is media filtration. These are typically pleated filters that physically remove particles, but not gases.
Media filters come in a wide range of efficiency, which is best measured using the MERV 1 standard. The
higher the MERV number (ranging from 1-16), the better. Media filtration is used in central HVAC
systems as well as portable air cleaners.
The next method for air cleaning is activated carbon, which is specifically used to remove gases and
odors from the air—it has a minimal effect on particles. A wide range of gaseous contaminants are
attracted to the activated carbon and adsorb to the surface. Over time the activated carbon becomes
saturated and must be replaced. The removal efficiency is a function of the amount of activated carbon,
so don’t expect good removal if there is only a light dusting of activated carbon on a filter.
Another air cleaning method is ionization. These work by giving particles an ionic charge which leads
them to agglomerate together and fall out of the air via gravity or to be attracted to oppositely charged
surfaces. Particles are removed from the air, but not necessarily the room. Some units incorporate an
oppositely charged collector plant and are called electrostatic precipitators. Some ionizers produce
ozone, a hazardous lung irritant. Ionizer-based air cleaners generally remove particles but not gases.
Yet another air cleaning strategy is oxidation. This is typically done using ozone or hydroxyl radicals,
which are often created via photocatalytic oxidation (PCO). Oxidation is primarily used to remove gases
from the air by breaking them down in a reaction process. However, the same oxidants that break down
1 Minimum Efficiency Reporting Value, as described in ASHRAE Standard 52.2. gases can also cause oxidative stress in humans. Many researchers caution against using oxidation based air cleaners, such as this recent paper: Unintended Consequences of Air Cleaning Chemistry.
The final air cleaning method we will cover is ultraviolet light (UV). The sun sends us wavelengths of
light that we cannot see with our eyes. A certain range of UV wavelengths referred to as UVC is able to
deactivate many harmful microorganism. Therefore, UVC can be incorporated into air cleaning when
there is concern over airborne viruses and bacteria. Because UVC light can damage eyes, it should be
mounted on the upper wall of a room using care to prevent reflection down into the room.
Alternatively, it can be enshrouded within a portable air cleaner that takes care to prevent UVC leakage.
If your primary concern is a particle such as respiratory droplets during a pandemic, then we would
recommend starting with high-efficiency media filtration in your centralized HVAC system. This tried
and true method delivers great results throughout the space. If you have specialty concerns, you may
want to consider some of the other technologies mentioned in this article, but tread carefully. And
always remember, it is usually best to identify and remove air quality contaminants at their source
rather than relying on air cleaning. Have Indoor Science evaluate your building and establish a plan of
action for having cleaner air!