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How is the Performance of an Air Purifier Measured?

air quality measuring

(Air quality can be measured with hand held instruments like this one)

Introduction

Consumers who have decided to take the step of purchasing an air purifier are faced with a wide range of choices. How are they to know which air purifier is the best choice? A few different ways to measure purifiers' effectiveness have cropped up. This article will discuss the different ways that air purifiers are ranked.

Whole House or Building Systmes - MERV (Minimum Efficiency Reporting Value)

The National Air Filtration Association, which describes itself as "a group of over 600 air filter distributors, manufacturers and engineers," uses this ranking system, devised by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) along with the American National Standards Institute (ANSI). Here is how the ANSI/ASHRAE test is performed:

  1. An aerosol generator introduces particles into the air. These particles fall into twelve different size categories.
  2. The affected air passes through the filter. Instruments measure how much of each particle size is present in the air before and after it goes through the filter.
  3. This process is repeated six times.
  4. The worst result of the six is recorded and used to calculate the purifier's efficiency. Ron Wilkinson of MC2 Market & Competitive Convergence explains why this is advantageous: "Because the rating represents the worst-case performance, specifiers can use it to assure performance in applications where a maximum particle count must be maintained over the filter’s entire life."
  5. Finally, based on the purifier's performance in the test, the device is assigned a Minimum Efficiency Reporting Value (MERV) of 1-16, with 1 indicating the least effectiveness and 16 the most. The MERV is always stated along with the airflow rate that was used in the test. Only seven specific airflow rates are approved to be used in this type of test.

The ANSI/ASHRAE Standard is a reasonably good way to determine the quality of large air purifiers. According to the Environmental Protection Agency, "Medium efficiency filters with a MERV of 5 to 13 are reasonably efficient at removing small to large airborne particles." A higher MERV than that range can only be achieved by air purification systems that could not easily be installed in the typical residence.

Portable Air Purifiers - CADR (Clean Air Delivery Rate)

This method has been developed by the Association of Home Appliance Manufacturers (ANSI). The goal of this method, in the words of AHAM, is to measure the effectiveness of "portable household electric room air cleaners ... regardless of the particle removal technology utilized." The ANSI method works in the following manner:

  1. The portable air purifier is placed in a testing chamber of 1008 ft3.
  2. The amount of contaminants in the room is measured before and after.
  3. The reduction in contaminants is compared to their natural rate of decay. The results are used to provide a final score, or Clean Air Delivery Rate (CADR), for dust and for pollen and smoke, based on a measurement using cubic feet per meter. The EPA explains CADR's rankings: "For example, if an air cleaner has a CADR of 250 for dust particles, it may reduce dust particle levels to the same concentration as would be achieved by adding 250 cubic feet of clean air each minute."

The AHAM tests are useful in comparing different portable air cleaners with CADR ratings to one another. The CADR rating is used by two important organizations outside of AHAM: the EPA and Consumers Union, publisher of Consumer Affairs magazine, which independently reviews air purifiers. In the feud between Consumers Union and The Sharper Image over the Ionic Breeze, the courts rejected Sharper Image's claim that CADR was a poor way of measuring the efficacy of air cleaners.

Limitations of CADR Testing

However, not everyone is so enthused about the use of AHAM-ANSI. The fact that AHAM is a trade association of appliance manufacturers raises warning bells, since it means that the makers of the products are testing themselves rather than using independent standards. Ed Sherbenou, operator of the website "Air Purifier Power," has decidedly mixed feelings on CADR rankings. While acknowledging their usefulness, he also raises some objections:

Ultimately Sherbenou suggests that buyers consider CADR only as one factor among others.

Beyond these objections, it must also be noted that CADR ratings are not useful for larger air purification systems that target the whole home, as the AHAM-ANSI test was designed specifically with portable systems in mind.

Particle Counters

Particle counters are relatively small devices that are used to measure the density of particles in a room, as well as their size. There is less to say about these, as they are a tool rather than an organized method of rating purifiers. They work by emitting a laser, and the counter detects and categorizes the particles that pass through it. They are an accurate way, when used systematically, of measuring a purifier's impact on a room's air quality.

Conclusion

No method of measuring the performance of an air cleaner is perfect. Larger systems have the basically reliable ASHRAE Standard, but these are not useful for portable models. The AHAM's CADR rating can be useful in evaluating a purifier, but it can also be misleading and biased. Therefore, potential buyers should look beyond any rating given to a device and consider other things as well; for instance, finding out whether the purifier is effective against very particles and gasses that cause health problems.

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