Understanding Volatile Organic Compounds (VOCs) and TVOC Measurements

Indoor air quality is increasingly recognized as a crucial factor in our health and well-being. Among the many pollutants that can affect the air we breathe indoors, Volatile Organic Compounds (VOCs) stand out due to their wide presence and potential health impacts. 

However, understanding VOCs and interpreting the data from VOC sensors can be challenging. This complexity arises partly because of the limitations of the low-cost sensor technology (MOX sensors) used to measure them and partly due to a general lack of clear information on the subject.

In this article, we’ll break down the complexities of VOCs and TVOC (Total Volatile Organic Compounds) measurements, making it easier to understand how they influence the air quality in your home or workplace.

VOCs encompass a broad range of organic compounds that easily evaporate at room temperature. They can have different chemical structures, volatility, and toxicity levels. Some VOCs, such as formaldehyde or benzene, are known to be harmful at certain concentrations, while others may be relatively less toxic or even non-toxic. For example, harmful VOCs include BTEX chemicals like formaldehyde, benzene, and toluene, which can be emitted from building materials, furniture, and household products. Long-term exposure to these harmful VOCs can cause health issues such as respiratory irritation, allergies, headaches, dizziness, and even asthma attacks.

On the other hand, some VOCs are harmless and naturally occurring, like ethanol and acetone, which are emitted by plants, trees, and certain foods. While these don’t generally pose health risks, they can still contribute to the overall amount of VOCs in indoor air.

VOCs are emitted from various everyday items, including furniture, paints, varnishes, cleaning products, cosmetics, and even from cooking and human breath. In enclosed spaces like homes or offices, VOCs can accumulate and degrade the quality of fresh air.

Therefore, VOCs can include very harmful but also completely harmless gases.

Here are some more resources with a comprehensive overview of VOCs:

- EPA's Guide to VOCs

- Berkeley Lab on VOCs

Given the diversity of VOCs, measuring their individual concentrations requires advanced analytical devices like gas chromatographs or mass spectrometers, which are large, expensive, and impractical for everyday consumer use. Instead, most consumer-grade indoor air quality monitors use low-cost TVOC sensors, which provide a single measurement that aggregates the concentrations of various VOCs into a single value.

The term Total Volatile Organic Compounds (TVOC) refers to the total concentration of multiple airborne VOCs present simultaneously in the air. This aggregated value is typically expressed in micrograms per cubic meter (ug/m3), milligrams per cubic meter of air (mg/m3), parts per million (ppm), or parts per billion (ppb).

To better understand TVOC, consider the classification of VOCs based on their boiling points:

Instead of measuring each VOC individually, TVOC sensors aggregate their concentrations into a single number. This is because the sensors are designed to provide a general idea of the presence of VOCs rather than precise, individual measurements.

Most TVOC sensors used in consumer devices are Metal-Oxide (MOx)-based.

But what exactly is a MOx sensor?

A MOx sensor consists of a heated metal oxide surface that changes its electrical resistance based on the oxygen content on its surface. Oxidizing gases like NOx increase resistance (providing more oxygen than ambient air), while reducing gases like VOCs decrease it (consuming oxygen by being combusted on the metal oxide surface). The sensor’s output is a value representing the resistance of the MOx material, which reacts to a broad spectrum of VOCs without distinguishing between them. MOx sensors are therefore broadband-sensitive.

In the lab, MOx sensors are calibrated to a specific gas or a mix of gases. However, in the field (out in the real world), the situation is more complicated.

When a MOx sensor is used in the field, it encounters a wide variety of different gases (VOCs and more). Since it was calibrated for a specific gas or gas mix, it may not measure other VOCs effectively. This is because the sensor can't tell which specific VOC is causing the reading to change; it's reacting to all the VOC at once.

For example, in the case of TVOC sensors in ATMO devices that are calibrated for ethanol - ethanol will have a stronger signal compared to formaldehyde. See the illustration below of how a MOx sensor reacts to different VOCs:

Because of these limitations, MOx sensors are better for providing a general idea of VOC levels rather than precise measurements.

It's important to understand these limitations. When using the sensor, you need to watch for changes or spikes in the readings and try to understand what's causing them. Sometimes, the cause is easy to identify, for example, when someone uses a cleaning agent, but other times, the reason might be unclear. 

In short, while MOx sensors can be helpful, they aren't perfect, and you need to interpret their readings carefully based on what's happening around you.

For more detailed information on the workings and limitations of MOx sensors, please refer to this support article: TVOC Sensor: Functionality, Limitations, and Calibration

Given the limitations of MOx sensors, many manufacturers now use an index-based measurement to convey TVOC levels. The VOC Index focuses on the relative change in VOC concentrations over time, rather than absolute values.

The idea is that an index-based measurement focuses on the relative change due to accumulation of VOCs in the air. The Sensirion SGP41 TVOC sensor utilized in Atmocube, our indoor air quality monitor, defines improving air quality with numbers below 100 and worsening air quality with numbers above 100 in a VOC index scale which ranges from 1 to 500:

• It constantly learns from the sensor data of the past 24 hours.
• VOC Index = 100 represents the average conditions based on the past.
• Values between 100 and 500 represent VOC events (more intense compared to average of the past).
• Values between 1 and 100 represent clean-air or low-VOC events (less intense compared to average of the past).

Based on the above, the TVOC Index values can be interpreted as:

Note: TVOC values in parts per million (ppm) are still available via Sensirion's conversion formula: Sensors Thresholds

The VOC Index describes the current VOC status in a room relative to the sensor’s recent history. Imagine you have a super-sensitive nose. It can tell when the air in a room smells different from what you're used to. When you walk into a room, your nose compares the current smell to what it remembers from outside. If the room smells stronger or different, your nose notices that change.

Assuming that we are entering a room from outside, our nose will take the air composition outside the room as an offset (baseline) and provide us with feedback if it recognizes higher or lower levels of gases when entering the room. The VOC Index performs similarly using the moving average over the past 24 hours (the learning time) as the baseline. When the VOC sensor samples the air, it compares it to this recent memory. If the air has more or fewer VOCs, the VOC Index changes to show that difference.

Although the human nose is a great gas detector, it fails  to detect odorless gases or those found at low concentrations. In this respect, TVOC sensors add great value to indoor air quality applications by monitoring most VOCs at the same time.

Continuous VOC monitoring can help identify issues that affect indoor air quality, allowing for targeted interventions that can ultimately lead to improvement. This research study found the Sensirion SGP41 TVOC sensor utilized in Atmocube effectively detected VOC solvents and NOx gases with good response times.

Facility managers and homeowners alike can quickly identify and address sources of pollution, ultimately improving indoor air quality and safety. Consider the following use case:

• A person with asthma noticed through their portable quality monitor, Atmotube PRO, that VOC levels in their home were consistently rising in the afternoon. Concerned about the impact on their health, they investigated the cause and discovered that their cleaning service was using aggressive chemicals during those times. The monitoring system allowed them to identify this pattern and work with the cleaning staff to switch to gentler products, significantly improving the air quality in their home and reducing their symptoms.

• A study conducted in two high schools in Southern Italy revealed elevated levels of total volatile organic compounds (TVOCs) in various areas, including classrooms and bathrooms. The VOCs, such as benzene, toluene, ethylbenzene, and xylenes (BTEX), primarily originated from indoor sources like cleaning agents and building materials. To mitigate these risks, the study recommended improving ventilation, using low-VOC products, and implementing continuous air quality monitoring. These measures were essential in reducing exposure to harmful VOCs and enhancing indoor air quality for students and staff.