Whenever you breathe inhale through your nose and perceive a scent, that’s your olfactory system reacting to a gaseous chemical in the air.
Many scents we perceive are caused by molecules which contain carbon and hydrogen atoms, generally referred to as organic molecules. Some molecules have pleasant scents, some do not. Some have no scent at all. Some can be beneficial to your health while just a tiny amount of others can be dangerous, even fatal.
Welcome to the fascinating world of Volatile Organic Compounds!
What is a VOC?
Volatile Organic Compounds, or VOCs, are a class of molecules which contain primarily carbon-carbon and carbon-hydrogen bonds, though they can - and frequently do - contain other atoms.
Generally, they have high vapor pressure and low boiling points (volatile) and poor solubility in water (organic).
These properties mean that they pass from solid or liquid to gas states easily, meaning they are emissions into the atmosphere. From here, their effects on the environment are complex and can contribute to everything from air pollution to climate change, depending on the specific VOC in question as well as factors like where it was emitted, the time of the emission, and even factors like the weather.
The most prevalent VOC-emission is methane, CH4, making up around half of all VOC emissions, all over the world, every year. Methane is well-known for its role in contributing to climate change. Despite this prevalence however, VOC emissions are not uniform across all areas. For instance, the second most prevalent global emission, isoprene (C5H8), dominates VOC emissions in forested areas. There are estimated to be thousands to tens-of-thousands of VOCs. In human-dominated environments, VOCs come from human activities and products. Activities like cooking, cleaning, smoking, transportation, and industry will all emit a unique blend of VOCs.
People and animals also release VOCs, as do our devices and furniture. With all these emissions, indoor environments - where we spend up to 90% of our time - VOCs can build up to potentially harmful concentrations.
VOCs and IAQ
Indoor Air Quality (IAQ) is affected by the presence of certain kinds of VOCs. In high enough concentrations, over long periods of time, we can say that this is a leading cause of Sick Building Syndrome (SBS). Dangerous chemicals, such as formaldehyde (HCHO) are released from wooden furniture, floorboards and other fixtures which can lead to headaches, nausea, and irritation of the eyes, skin or nose. Benzene, released from solvents, paints, adhesives and combustion is a well-known carcinogen. Acetone, a common organic solvent used in cleaning products, can attack the nervous system in high enough concentrations and given time.
These are just a few examples of the thousands of potential VOCs and what they can mean for our health.
How do we measure VOCs?
As different VOCs have different chemical structures, measuring each individual kind of molecule is a challenging task, which scientists have been figuring out for centuries. Shortcuts can be taken, however, and there exist methods for determining multiple VOCs at once. These kinds of measurements are known as Total Volatile Organic Compounds or TVOCs.
A modern TVOC sensor makes use of a layer of a material known as a metal oxide (or MOx). Oxygen gas, which makes up ~21% of normal air, reacts and covers the surface of the MOx. This oxygen is then able to react with target gases that pass over the sensor, including a wide variety of VOCs. When these VOCs react with the oxygen on the MOx, an electrical current results which can be detected by a computer. This signal is proportional to the concentration of TVOCs, allowing an accurate determination.
This information can be used to determine what and where the sources of VOCs are in a given indoor environment, how strong they are, and how long they stick around in the air. From this, we can get to the work of solving a piece of our IAQ puzzle, allowing all to breathe just a little bit easier.
- EPA - What are volatile organic compounds (VOCs)?
- Earth System Science Data - The Global Methane Budget
- BBC News - Cutting methane gas 'crucial for climate fight'
- European Geosciences Union - Estimates of global terrestrial isoprene emissions using MEGAN (Model of Emissions of Gases and Aerosols from Nature)
- Allen H. Goldstein (University of California) - Know and unexplored organic constituents in the Earth's Atmosphere
- Building Green - We Spend 90% of Our Time Indoors. Says Who?
- EPA - Report to Congress on Indoor Air Quality / Volume II
- EPA - Indoor Air Facts No. 4 / Sick Building Syndrome
- World Health Organization - Concise International Chemical Assessment Document 40
- World Health Organization - Exposure to Benzene: A major public health concern
- CDC Wonder - Acetone: ATSDR Fact Sheet
- uRADMonitor - Metal Oxide VOC Sensors