Some Air Cleaners Release Harmful By-Products. Now We Have a Way to Measure Them.

In the wake of COVID-19 and widespread wildfires, demand skyrocketed for air cleaners, machines that could remove potentially harmful particles from the air in a home. Manufacturers responded by producing a wider variety of air cleaner devices designed for single rooms.

The main purpose of an air cleaner is to, well, clean the air, making it safer and more comfortable to breathe. Yet in the process, some of these devices may also release new unwanted chemicals as a by-product. In large quantities these chemicals can be harmful, but it's unclear how much these devices release into the air. The first step in addressing this problem is to find a way to reliably measure these unwanted by-products.

That measurement solution has just been created. A new standard test method was published last week. The National Institute of Standards and Technology (NIST) led the development of the new test, which lays out a consistent way to measure and compare a wide range of air cleaner by-products. This test will allow manufacturers to make improvements in their devices and paves the way for consumers to make apples-to-apples comparisons and for emission limits of key by-products such as ozone.

What Kind of Air Cleaners Produce Unwanted Chemicals?

Not all air cleaners are the same. There are many pollutants floating around in indoor air - pet dander, cooking smoke, chemical fumes from cleaning products, and even tiny microorganisms. Different air cleaners target different kinds of pollution. Some air cleaners use chemistry or energy to remove these pollutants from the air, and those processes can add pollutants of their own.

A fibrous filter is the most common type of air cleaner, but it typically will not remove chemicals. "Most air cleaners are designed to remove relatively large particles rather than small molecules," explained Dustin Poppendieck, an indoor air quality researcher at NIST, "so a filter alone might not be enough to remove all things of concern in the air."

Some air cleaners are designed to actively kill airborne viruses or neutralize small harmful molecules. Methods include shining ultraviolet light through room air, ionizing the air with electricity, or passing the air over catalysts that will react with pollutants and break them down into less harmful substances like water and carbon dioxide.

These active air cleaners do remove pollutants from the air, but they can also add new chemicals of their own. For example, ultraviolet light can create ozone, which is dangerous in high concentrations.

In addition to detecting ozone, the new test method looks for two other potential pollutants, formaldehyde and ultrafine particles, to get an overall idea of how the cleaner affects the air.

"If you see formaldehyde or ultrafine particles in the air after turning on an air cleaner, then you know some unwanted air chemistry is happening," explained Poppendieck. "When you have unwanted new molecules, they can chemically react with the air to either get bigger or smaller. If they break apart to get smaller, the parts can break up into formaldehyde. If they get bigger, they form what air quality experts call 'ultrafine particles.'"

How Does the Test Work?

To perform the test, researchers run an air cleaner for four hours in a sealed room that contains a set of specific chemicals. They then test samples of the air from the room for those three pollutants: ozone, formaldehyde and ultrafine particles.

Ozone and formaldehyde absorb UV light. So, the researchers determine the concentration of these chemicals by shining UV light through the sample and measuring how much of that light is absorbed at specific wavelengths.

Ultrafine particles are measured through a process called scanning mobility particle sizing (SMPS). First, the testers pass the air through an X-ray field. This gives any particles in the air a predictable electric charge, enabling them to be pushed around by electric fields. Larger particles have a stronger charge and are pushed more forcefully. The testers then use electric fields to remove the larger particles from the air, leaving only the small particles they're interested in. At that point, the particles are too small to easily count, so the testers send the particles through a cool steam bath. The particles absorb the water and swell to a size that's large enough to scatter laser light. By counting the number of times the laser is scattered, the testers can count the number of ultrafine particles that were in the sample.

How Will the Test Be Used?

The new test method was published by ASTM International, a standards development organization. NIST played a critical role in proposing the new standard, bringing stakeholders together, and conducting the fundamental research over four years. The new test method is a consensus standard, meaning it was created with the input of air quality researchers, air cleaner manufacturers, testing laboratories and the general public. This deliberation can be a long process, but it's important to create standards that everyone can agree on to ensure that the measurement is fair and trustworthy.

A standardized test levels the playing field. If everyone measures these chemical by-products the same way, then it becomes much easier to compare air cleaners made by different companies, and it becomes easier for companies to evaluate their own products and make improvements. This test also provides a clear way to fairly test the devices to set limits for these by-products for safety reasons.

As experts on measurement science, NIST provides the scientific basis for standards like this one that support other government agencies and address evolving concerns about new technologies.

"I'm excited that it's finally coming out and will start impacting the world," said Poppendieck. "Now that these by-products can be reliably measured, we can be more confident that air cleaners are beneficial to the air we breathe."