How Accurate Are IAQ Monitors (and How Accurate Do They Need to Be)?

When choosing an indoor air quality monitor, accuracy always comes up in discussion. Of course, if you want to understand the quality of your indoor air, you’ll want to make sure you get the most accurate and reliable equipment. However, the accuracy of IAQ monitors can be a complicated topic, and there are many different considerations behind that one line on the spec sheet. In this article, we’ll explore accuracy in-depth, as well as all the other factors that are just as important to consider when choosing the most suitable IAQ solution for your project.

Understand Different Types of Air Quality Monitors

When discussing “accuracy”, it’s important to make sure we are comparing apples with apples. It’s unreasonable to expect a weight scale in our bathroom to produce readings as accurate and reliable as a delicate scale used in science labs - and a similar logic applies to air quality monitors.

While there isn’t a universal standard for different types of IAQ monitors, we can think of three categories of monitors based on their use case - reference-grade IAQ monitors, personal IAQ monitors, and commercial IAQ monitors.

Reference-grade IAQ monitors are high-quality, highly accurate instruments that are used in healthcare settings, scientific research, and environmental monitoring, such as the beta attenuation mass (BAM) monitor. These monitors can be extremely expensive and usually are not the most user-friendly devices to use for day-to-day cases.

Personal IAQ monitors, on the other hand, can measure a lot more air quality parameters within one device. They are usually quite affordable, and most come with user-friendly applications as well as smart home integrations.

Commercial air quality monitors are similar to personal IAQ monitors in that they also measure a set of different parameters, usually including particulate matter, VOCs, CO2, ozone, etc., but these monitors are designed to work with the built environment, providing advanced connectivity, security, and automation capabilities which are usually not of a concern for personal IAQ monitors.

Although not as accurate as reference-grade monitors, some well-crafted personal and commercial IAQ monitors can still be quite accurate and reliable. But how do you tell the good ones apart from the less reliable ones? The answer lies in examining the sensor type, which we'll discuss next.

What’s Under the Hood? Understanding Sensor Types

There are many different types of sensors that measure the same parameter, and depending on the exact technology being used, the accuracy of the sensor also differs. Below we will walk you through some of the most popular types of sensors in the market today for the common air quality parameters we measure, and discuss the accuracy levels for each type.

Particulate Matter Sensors

Except for BAM monitors which are only used for scientific research or environmental monitoring, most sensors that measure particulate matter fall into either of these two categories: infrared, and laser diffraction. If you are interested in exactly how these types of sensors work, feel free to nerd out with us in this blog post about the 3 types of particle detectors, but the TLDR is that laser diffraction sensors are a lot more accurate than infrared sensors and infrared sensors are not able to pick up smaller particles as well as laser diffraction sensors.  As sensor technology advances, infrared sensors have become less common.

CO2 Sensors

Most CO2 sensors fall into one of three categories: non-dispersive infrared sensors (NDIR), electrochemical sensors, and metal oxide semiconductor (MOS) sensors. NDIR sensors measure specific lightwaves to detect CO2 levels, electrochemical sensors measure conductivity to assess CO2 presence, and MOS sensors use the resistivity of metal compounds to determine gasses in the air. 

Each of these kinds of CO2 sensors comes with its pros and cons, but, in short, NDIR is by far the most suitable for offices and other commercial settings. These sensors are highly accurate at about 1000 ppm, which reflects CO2 levels that are commonly found in enclosed spaces. MOS sensors, on the other hand, are inaccurate until about 2000 ppm, at which point occupants may already be suffering adverse effects.

Both MOS and electrochemical sensors are also susceptible to cross-sensitivity, meaning that gasses other than CO2 can trigger the device. NDIR sensors do not suffer from this issue, as only CO2 can absorb the light emitted by the sensor.

VOC Sensors

MOS sensors are typically used for continuously monitoring TVOCs. The best MOS sensors heat a thin film of metal oxide nanoparticles to about 300°C, at which point oxygen particles are absorbed on the surface and react with the target gasses. This reaction releases electrons, which alters the electrical resistance of the metal oxide layer. 

Like CO2 sensors that use MOS technology, these VOC sensors are susceptible to cross-sensitivity, which can affect their accuracy. However, in this case, there are little or no cost-effective alternatives available, due to the complicated nature of VOCs. 

Ozone Sensors

Ozone monitors use either MOS or electrochemical sensors, each of which we’ve discussed above. However, MOS ozone sensors are prone to cross-contamination with nitrogen dioxide, which can negatively affect accuracy levels. Electrochemical sensors are the most accurate method of reading indoor ozone levels. They are less susceptible to cross-contamination, more accurate at reading ozone levels typically found indoors, and record levels more consistently over time. 

The Big Question: How Accurate Is Accurate Enough?

Now that you understand the basics, you're better equipped to make informed decisions when comparing the accuracy of air quality monitors. However, it's important to recognize that for air quality monitors used in commercial or home environments, you don’t necessarily need the most accurate ones in the world.

Our aim in measuring air quality is to make informed decisions about our indoor environments in order to protect occupants, employees, and ourselves better. The objective is to gauge the overall quality of the air and direct appropriate actions, rather than achieving pinpoint precision in measuring a specific pollutant. You absolutely need an accurate and reliable air quality monitor in order to do a good job, but going anywhere beyond that level could easily be an overkill.

More Risk Factors for “Accuracy”: Installation Location and Placement Density 

Even if you choose an air quality monitor with all the right sensors, you are still not guaranteed to get an accurate reading of the air quality inside your space. Installation location and placement density are two often overlooked factors that could have a major impact on the “accuracy” of your data. In this context, when we discuss "accuracy", we aren't solely referring to the specification of the sensor or device itself. Instead, we're emphasizing how accurately the data represents the entire indoor environment.

Some people imagine IAQ monitors having a ‘range’, like an internet router, but this is not actually the case. They work by measuring the air that comes in contact with them: if that air is not representative of the overall quality of the air in the space, the data will not be accurate or reliable.

Think of an extreme example - if you install an air quality monitor right next to your air vent, the data you are getting is more reflective of the qualify of your supply air (which is usually clean and fresh), rather than what your occupants are breathing on the other side of the room.

To make sure your air quality monitor is giving you accurate readings of the air quality in your space, we recommend considering these factors:

Installation Location

• Place IAQ monitors in your most frequently occupied spaces.
• Install monitors within the ‘breathing zone’, which is around 3-6 feet (0.9-1.8 meters) up from the floor.
• Place monitors at least 16 feet (5 meters) from operable windows, air filters and diffusers, or at least half of the space away from a window, and closer to a return than a diffuser.

Placement Density

• The more IAQ monitors you have strategically placed throughout a space, the better your overall IAQ picture is, based on their combined readings. 
• Place at least one monitor in each type of occupiable space with similar sizes, if not one in each room.
• Make sure to have monitors on all floors to get a comprehensive view of the building.
• For large open spaces, make sure to place enough monitors to cover the entire space. 
• Building certifications like WELL and RESET provide guidelines for monitoring density that can be used as a reference. For example, RESET recommends one monitor every 5382 ft² (500 m²).

The bottom line is that air changes as it travels, and is affected by thousands of factors that leave one side of a room breathing a different combination of particles and gasses to the other.

Rather than one hyper-accurate monitor, multiple monitor setups are by far the best way to ensure that you are getting correct readings of your entire workplace. 

What Else Should You Consider When Selecting IAQ Monitors for Your Building?

Besides accuracy, there are many other factors you should consider when choosing an air quality monitor for your building. While this is a huge topic to be covered in depth in another blog, we provided some guiding questions below to help you get started.

1.  What parameters do you need to measure? 
   Every space has its unique needs regarding which parameters should be monitored. In a commercial project, it's often recommended to monitor PM2.5, PM10, TVOC, CO2, ozone, temperature, and relative humidity. However, it's essential to determine which parameters are most crucial to you and research the relevant sensor technologies.
2. How and where do you want to access the data? 
   Are you interested in viewing the data through a cloud-based dashboard, or would you prefer it entirely offline? Do you need to send the data to your building management system (BMS)? Understanding how and where you want to access the data will help narrow your search for air quality monitors. Not all monitors offer offline data access or can send data to the BMS. If you opt for cloud access, ensure you assess the security protocols of the air quality monitor against your needs and requirements.
3. How do you plan to analyze the data? 
   Data becomes valuable when you can analyze it and derive insights. Unless you have an in-house team of data scientists for interpreting IAQ data, you'll likely need a robust software platform that provides more than mere raw data points for analysis. Consider what metrics matter to you beyond standard real-time data and time-series graphs, and evaluate your IAQ monitor vendors for their data analysis capabilities.
4. What is your long-term maintenance plan? 
   Regardless of what you may hear, every IAQ sensor experiences drift over time, affecting its accuracy. It's essential to consider how you plan to maintain your monitors in the long run. Poorly maintained IAQ monitors can yield inaccurate readings, potentially leading you astray in your efforts to enhance the indoor environment.

Striking the Right Balance in IAQ Monitoring

Choosing the right IAQ monitor with reliable accuracy involves understanding not only the different types of monitors and sensors available but also considering factors such as installation location and placement density. Ultimately, the goal of monitoring indoor air quality is to provide a safe, healthy, and comfortable environment for building occupants. To achieve this, it's essential to strike a balance between accuracy and practicality. 

It's important to remember that while accuracy is essential, it's not the sole determinant of a successful IAQ monitoring program. Factors such as data analysis, long-term maintenance, and appropriate actions taken in response to the data are equally important. Only by considering these factors and understanding the unique needs of the building can building managers and workplace leaders effectively address the indoor air quality needs of the occupants and contribute to a healthier indoor environment.