Air Purifiers

What to Look For in an Air Purifier—Important Criteria

To make this decision, you need to know how to compare air purifiers. Then you can choose an air cleaner most suitable for your needs.

The best air purifier has five important factors working together:

1. Safe technology
2. Capacity and efficiency
3. Good air flow rate and 'maximum coverage'
4. Inert construction materials
5. Acceptable sound level

6. Cost
7. Installation requirements
8. Ease of use
9. Ease and frequency of maintenance
10. Appearance

1. Safe Technology

In our discussion of Air Purifier Technologies we explain the relative safety of a good filtration air purification system, over other types of air cleaners.

In summary, combined true medical grade HEPA and activated carbon filtration air purifiers are safer and more effective in removing a broader range of pollutants than electrostatic, ionic or ozone air cleaners.

The best air purifiers are safe for the human body's delicate, intricate systems.

In contrast, ionizers and ozonators both produce ozone in varying amounts, and therefore we do not recommended them. Electrostatic machines which charge particles can also produce some ozone, and the negative charge on the particles can be problematic.

If an air purifier is advertised as being "silent", it is probably one of the above three types which we do not recommend.

If you see "no filters needed", then you know the equipment is either putting something into the air (such as with ionizers and ozone producers mentioned above); or electrostatic plates are charging and attracting particles to itself.

2. Capacity and Efficiency

Particle Removal

• • air flow rate through the cleaner
• • the efficiency of its particle capture mechanism
• • the mass of the particles entering the device
• • the degradation rate of the efficiency of the capture mechanism caused by loading
• • the bypass of air around the collection mechanisms
• • how well the air leaving the device is mixed with air in the room before re-entering the device
• • the size of the space to be cleaned

Removal of Gaseous Pollutants

Adsorption or 'capture' on solid sorbents such as activated carbon is the most frequently used process for removing chemicals, gases and odors from indoor air. The performance of solid sorbents is dependent on several factors, including:

• • the air flow rate through the sorbent
• • the concentration of the pollutants
• • the presence of other gases or vapors (such as humidity)
• • the physical and chemical characteristics of both the pollutants and the sorbent (weight, polarity, size, and shape)
• • the configuration of the sorbent in the device
• • the quantity of sorbent used and the sorbent bed depth

Why is Dwell Time Important?

Type of Pollutants Removed

The best air purifiers are not only safe, they are effective and efficient in removing a broad spectrum of pollutants – from pollens to perfumes!.

Since both airborne chemicals and particles aggravate, and in some cases, may even trigger allergies and asthma, it is vital that an air cleaner for respiratory conditions provides filtration for both.

That means the filtration for particles and biological contaminants (dust, pollen, mold, bacteria, viruses, etc.) must be true, medical grade HEPA which traps 99.97% of particles 0.3 microns in diameter; and smaller and larger particles with even greater efficiency . This is the type of filtration used for medical and scientific clean rooms. And this is the only type of fine particle filtration that we feature.

Positioning of the Filters

And finally an important factor in the efficiency of the air purifier is the positioning of the filters. There must be no air bypass around the filters, putting back pollutants back into the air.

3. Good Air Flow Rate and 'Maximum Coverage'

Although there is no universally accepted method for comparing air-cleaning devices, CADR (clean air delivery rate) is a measurement that is often used. The CADR is the product of the unit efficiency and the air flow rate, and is measured in CFM (cubic feet per minute). For example, an air cleaner with a CADR of 250 would reduce pollutants to a level where it can provide 250 cubic feet of clean air each minute.

Number of Air Exchanges

For normal residential use, a minimum of two air exchanges should take place every hour in a room that is being cleaned. Your air purifier should have the capacity to do this on the lowest fan speed.

When the pollution level is higher, you want to be able to increase the number of air exchanges per hour by increasing the fan speed. For example, in a home office where there are laser printers, photocopiers or other similar electronic equipment in operation, we recommend that the air be cleaned every 12 minutes, or 5 times an hour.

Or you may have a visitor in your home who is a smoker, or who wears excessive perfume (or both!). Or smoke from a local wildfire or from a neighbor's wood burning stove could enter your home. In these cases of higher indoor air pollution, being able to increase the number of air exchanges will quickly reduce the irritants before they cause any negative effects to the members in your household.

In other words, it is important to choose a CFM rating that has the capacity to clean the air in unusually high pollution. Then it will be well suited to meet your everyday needs on a low setting, as well as to clean the air when there is an unusual or emergency situation.

Manufacturer's Specifications

In addition, the quoted CADR is usually based on the unit operating on the highest fan speed and two air exchanges per hour unless indicated otherwise. In practice, a low setting is recommended for normal, 24 hour operation. On a higher fan speed the air moves more quickly through the air cleaner. CFM on low setting is usually about ¼ or less of the rate of the high setting.

How to Calculate Appropriate Room Size

Let's say you are interested in a specific air purifier and you want to know what size room it will clean efficiently.

EXAMPLE 1.

And let's say the air purifier has a CADR of 400 CFM on the highest fan speed. You want to be able to exchange the air 5 times in an hour which is (60/5) = 12 minutes per air exchange:

Maximum Room Size:

= (Minutes per air exchange x CFM) / 8 ft ceiling
= (12 x 400) / 8
= 600 sq.ft.

This is the maximum room size, since the rated CADR is lower than the actual clean air delivery rate.

Again, the manufacturers base their recommended maximum room coverage on only two air exchanges per hour, which results in their recommendation of a 1500 square foot room for a machine with a rated CADR of 400 CFM.

For extremely high pollution, smaller size spaces per air cleaner are recommended. And if your ceiling height is more than 8 feet, the square footage of the room needs to be smaller to accomplish the same air cleaning with the same air purifier.

The smaller the space for a given air purifier, the quicker it will exchange the air, and the less likely you will suffer negative effects from the airborne pollutants you are removing.

EXAMPLE 2.

Let's say an air purifier you are interested in has a rated CADR of 200 CFM on high. With 5 air exchanges:

Maximum Room Size:

= (Minutes per air exchange x CFM) / 8 ft ceiling
= (12 x 200) / 8
= 300 sq.ft.

Again, the manufacturers base their recommended maximum room coverage on only two air exchanges per hour, which results in their recommendation of 750 square feet for a machine with a CADR of 200 CFM.

Choosing a Machine for Your Space

Let's say you know the size of the room you want to air clean and you want to know how powerful a machine you need. Using the same measurements as Example 1, we'll assume the room is 600 sq.ft. and you want to exchange the air 5 times per hour:

Required CADR:

= room size in cu.ft / minutes per air exchange
= (600x8) / 12
= 4800 / 12
= 400 CFM

In the same way, for a room size of 300 sq.ft. you will need a machine that has at least 200 CFM on its highest fan speed.

Fill Factor

Sometimes it is helpful to consider the amount of furniture and contents in an area to be cleaned. This is called the 'fill factor'. If 30% of the room is full, then the air purifier only needs to clean 70% of the room, so less power may be required of the air cleaner. On the other hand, contents in the room may also impede overall air circulation, and therefore may in fact reduce the number of air exchanges per hour. For normal residential use, we do not use the fill factor in our calculations.

4. Inert Construction Materials

The best air purifiers have steel housing with powder coated, baked on enamel paint, rather than plastic housing which could off-gas and cause irritation.

5. Acceptable Sound Level

The intensity of sound is measured in decibels (dB). Each level higher in the decibel scale is actually 10 times louder. The smallest audible sound (near total silence) is 0 dB. A whisper is about 15 dB. Normal conversation is about 60 dB. A jet engine or rock concert is one trillion times the smallest sound, at 120 dB.

Acceptable air purifier sound levels range from less than 50 on low settings to 75 on high settings. Sound level is another reason why it is good to calculate the required capacity of an air purifier based on low settings for general use. The high settings can be used for more intense, short term pollution.

6. Cost

There are three categories of expense related to an air filtration system: purchase cost, maintenace cost and operating cost.

In general, high efficiency air purifiers with high air flow rates and effective particle and chemical capture systems cost more to purchase. They also have far greater value because they work! And they may cost less to operate and maintain.

Maintenance costs vary depending on the device. Electrical consumption costs for room units are usually negligible.

7 and 8. Installation and Ease of Use

Installation of room units is generally just a matter of checking the cord for any damage and if all looks well, just plugging it in to the electrical outlet, and it's installed!

It is very important to read the manufacturer's operating manual BEFORE turning on the machine.

Central air purification units need to be professionally ducted in to an existing HVAC system.

To avoid electrical and mechanical hazards, it is important that a buyer ascertain that the unit is listed with Underwriters Laboratories (UL), CSA or another recognized independent safety testing laboratory.

9. Ease and Frequency of Maintenance

Are pre-filters easily cleaned and replaced?

Activated carbon filters generally are good for up to two years, depending of course upon the environment and the type of pollutants that are adsorbed. True medical grade HEPA filters can last up to 5 years, again depending upon the amount and type of pollutants that are trapped.

For larger machines, it is helpful to have the units on wheels/casters for easy maneuverability. Smaller units may offer optional casters.

10. Appearance

If your primary concern is your health and the health of your family, appearance may be the least important feature of your air purifier.

At the same time, it is important for you to be aware of what a unit looks like before you buy. And the machine has to fit in with your personal environment in order for you to use it consistently and be satisfied.