How Residential VOC Sensors Work and How They Are Affected By Bipolar Ionization

Most people know that when a smell is not good, it’s usually not good for your health, especially when it’s pungent to your nose or eyes, makes you light-headed or gives you a headache.  What are we really smelling?  Often it’s a VOC, or volatile organic compound.  Odor, however, is not a requirement for gas to be a VOC.  There are two main requirements for a gas to be a VOC:

1. It contains carbon (element C) as part of its chemical structure, which qualifies it to be “organic”.

2. It has a low boiling point, which means it easily evaporates at room temperature.  This makes it “volatile”.  You can even “see” VOCs when you see the air distorted through vapors coming off a container of gasoline.  

There are also some carbon-containing gases that aren’t VOCs (like carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate). (What is the definition of VOC?)  Still, the VOC category covers a huge range of chemicals from simple molecules that contain a few atoms such ethylene to more complex, containing tens of atoms.  The reason we should be monitoring them is for our health, because they can have detrimental long and/or short health effects from breathing them.  Chronic health effects include damage to liver, kidney and the central nervous system. Some VOCs are also proven to be carcinogens. (Q&A: Why It's Important to Monitor VOCs)

Air quality instrument companies have recognized the need to have VOC monitors for the home.  Nail polish remover or rubbing alcohol are probably the most easily recognized VOCs, but there are dozens more, from candle emissions, to cleaning and personal care products, to cooking odors, that qualify as well.  How exactly do these monitors work?

There are several main types of VOC sensors:  metal oxide semiconductor sensors (MOS), photoionization detectors (PID), electrochemical sensors (EC), infrared sensors (IR) and gas chromatography-mass spectrometry (GC-MS).  In order to include a reasonably-priced VOC sensor in residential air quality monitors, most consumer product manufacturers have embraced MOS, so we’ll focus on those for the purpose of this article.  They operate based on the principle of change in resistance of a metal oxide layer when exposed to reactive gasses (ideally VOCs). The sensor consists of a heating element and a metal oxide layer. The metal oxide layer naturally adsorbs oxygen from the air, forming a layer on the surface that is depleted of electrons.  When VOCs come into contact with the heated metal oxide layer, they either steal the oxygen or otherwise release electrons back into the MOS, causing a change in the electrical resistance of the MOS. This change in resistance is proportional to the concentration of VOCs and can be measured to determine the VOC level.  

The following two snapshots come from an animation of how VOCs “steal” oxygen and change the electrical resistance of the circuit.  (VOC Sensors: A Comprehensive Guide to Their Applications and Maintenance)

Different metal oxides work differently to adsorb oxygen and interact with VOCs, and manufacturers are also able to increase selection and sensitivity of the VOCs by altering the chemical composition (combining 2 or more metal oxides), performing surface modification (adding noble metals like gold, silver, platinum or palladium as catalysts to the surface), and/or altering the microstructure of the MOS surface (more surface area = more sensitivity).  (Road Map of Semiconductor Metal-Oxide-Based Sensors:A Review)  MOS sensors are constantly undergoing development and improvements, however, they have certain drawbacks that limit their accuracy.  Following are some of these limitations.

Humidity and temperature:  Environmental humidity and temperature changes can significantly alter MOS sensor output unless compensation is applied.  High relative humidity can potentially reduce the sensitivity of the MOS sensors because adsorption of water molecules on the metal oxide layer would reduce chemisorption of oxygen and the reaction between water molecules and adsorbed oxygen molecules could lead to a decrease in baseline resistance of the sensor.  (Laboratory Evaluation of VOC Sensors: Laboratory Setup and Testing Protocol)  For this reason, other sensors are often included in the product in order to sense the ambient temperature and humidity and make corrections. 

Cross-sensitivity: Some MOS sensors are affected by “non-target” gasses like CO, NO, H2, H2S, NH3 and CH4 and these can alter the measurement of VOCs.  (Long-term field calibration of low-cost metal oxide VOC sensor: Meteorological and interference gas effects)  Below is a depiction of an MOS reacting to nitrogen dioxide (NO2), which is a pungent, reddish-brown gas that is a common air pollutant and a component of smog. NO2 is not a VOC but since it changes the charge composition at the surface of the MOS, the sensor will see it as a VOC. (Low-temperature operating ZnO-based NO2 sensors: a review)

The two foremost reasons we would urge caution when using a home air quality monitor to measure VOCs in your home are: 

1. Inability of MOS sensors to distinguish one VOC from another.  The MOS interacts differently with each VOC.  This is fine when only one VOC is present in the air, but indoor air can be a complex chemistry!  Therefore, with multiple VOCs, they do not “know” the effects that each VOC is applying to the electrical circuit.  Here is an image from a popular MOS manufacturer, Sensiron (MOX = MOS):

In these two situations, the MOS has a higher “total VOC” or tVOC reading in situation 1 because ethanol reacts more strongly with the sensor.  However, situation 2 registers lower tVOCs, even though it is arguably more dangerous to human health because tolulene has higher toxicity and potential for causing severe health effects than ethanol.  Therefore, the tVOC level can be misleading. 

2. Bipolar ionization affects MOS sensors. Since MOS sensors rely on the transfer of electrons to change the conductivity/resistivity of the base metal during the detection of VOCs, they are also affected by ions in the air, because ions are positively or negatively charged molecules.  This means that since the “electron depletion zone” at the surface of the MOS is lacking electrons, negatively-charged ions that come near the sensor will be attracted to it.  If these ions donate their electrons to the MOS sensor, the electrical conductivity is increased and the sensor thinks that VOCs are causing the change.  Therefore, the sensor gives a false reading of VOC presence.  Our bipolar ionizers produce a number of species of negative ions including O2- (superoxide ion), OH- (hydroxide ion), and HCO3− (bicarbonate ion), which are NOT VOCs.  These are helpful ions that can deactivate microbes and help to clear small particulates (PM2.5).  However, we’ve observed that air quality monitors misread the presence of these ions (especially when there’s a high density such as near an air conditioning vent when using 1 or more whole home units) as high VOCs. 

Here are some real-world observations that show these limitations:

• Casino Case: A high-end MOS-based TVOC monitor displayed 0.0 ppb in a smoke-heavy casino environment—demonstrating the disconnect between sensor output and actual VOC conditions.
  
  

• Aerospace Test: Despite a $15,000 allocation for GC‑MS and PID-based VOC detection in search of an intermittent odor, a leading aerospace company could not isolate any VOC—even with professional-grade tools.

For these reasons, we wanted to help our clients understand the limits of VOC monitoring, especially with MOS and even with more high-end equipment.  MOS sensors cannot distinguish between or among VOCs, some of which are more reactive or potentially dangerous than others.  The sensors are also affected by temperature and humidity, meaning that if your home interior climate varies a great deal, VOC measurement can be affected.  Finally, use of bipolar ionization–a technology that benefits your air quality–can be misinterpreted by these sensors as increased VOCs.  If your air quality monitor includes a tVOC reading, you can use it to track relative changes to try to figure out how your air is changing, but keep in mind these principles of how it works so that you can also take into consideration other factors that might be affecting the reading.

Photo by Pablo Escobar on Unsplash