Total Volatile Organic Compounds (TVOC) are a group of compounds containing carbon that have a high vapour pressure present in the air. There are abundant VOC emitted from everyday activities among which some are colourless and some odourless and some are toxic. TVOC monitoring is an efficient way to detect exposure to toxic VOCs. This article covers information on total volatile organic compounds, their sources in the ambient air, characteristics, health and environmental impact, possible corrective measures, need for TVOC monitors as well as different methods of TVOC monitoring.
What is TVOC?
Volatile organic compounds (VOC) are a group of compounds containing carbon that have a high vapor pressure i.e. easily convert into vapor or gases and have low water solubility. There are thousands of VOCs in the environment however, some may have short- and long-term adverse health effects when exposed to. The principal VOCs of interest include benzene, toluene, formaldehyde, ethylbenzene (BTEX), xylene, acetaldehyde, ethylene glycol, methylene chloride, acetone, styrene, etc.
There are a large number of VOCs present in the environment making it difficult to continuously monitor them simultaneously. Hence, a common term TVOC (Total Volatile Organic Carbon) is used representing the total concentration of VOCs present in the air. Such a common term of measurement makes it easier to determine the overall presence of VOC and is cheaper than measuring individual VOCs.
TVOC in Atmosphere
We are in continuous contact with several products that contain and/or emit VOCs like disinfectants, paints and varnishes, solvents, semiconducting materials, rubber, vehicle exhaust, fuel, ink, adhesives, etc. Each VOC has its characteristic signature. For example, due to combustion, vehicle exhaust emits ethene while broadleaf trees emit isoprene. Different VOCs have different reaction rates and hence, have a large range of transport distances ones emitted. VOCs that have low creativity remain in the atmosphere for large durations of time are known as Persistent Organic Pollutants (POPs) and some VOCs are known as Hazardous Air Pollutants (HAPs) because of their toxicity.
Below given are some common VOCs found in urban atmospheres with their characteristic sources and approximate lifetime in the ambient air.
|Compound||Atmospheric lifetime (approx)||Characteristic Sources|
|Ethane||1.5 months||Natural gas, biomass burning|
|Acetylene||15 days||Vehicular emission, biomass burning|
|Methanol||12 days||Plants, VOC oxidation|
|Propane||11 days||Liquified Petroleum Gas (LPG), Natural gas|
|Benzene||10 days||Industrial emissions, vehicle emissions, biomass burning|
|iso/n-Butane||5 days||Vehicular emissions, LPG|
|Ethanol||4 days||Plants, biofuel|
|iso/n-Pentane||3 days||Vehicle emissions, gasoline evaporation|
|Toluene||2 days||Solvents, vehicle emissions|
|Ethene||1 day||Vehicle emissions|
|Formaldehyde||1 day||VOC oxidation, biomass burning|
Longer-lived species such as ethane and propane are less reactive and therefore slower to form secondary products such as ozone. But other VOCs are large contributors of ground-level ozone, smog, and particulate matter in the atmosphere through photochemical reactions.
VOCs are released naturally in the atmosphere from forests, oceans, termites, wetlands, and volcanoes.
Anthropogenic sources of VOC include:
- vehicular emissions
- production, storage, and burning of fossil fuels such as gasoline, wood, coal, or natural gas
- Industrial processes in the chemical, petrochemical, automotive, and pharmaceutical sectors,
- Petroleum handling
- Oil refining
- Bio decomposition
- Painting and dry cleaning operations
- Materials such as paint, varnishes, sealants, adhesives, cleaners, disinfectants, furniture, pesticides, air fresheners, cosmetics, deodorants, printing inks, flooring, etc
- Tobacco Smoke
Health & Environmental Impact of TVOC
Not all VOCs are harmful to human health, its impact majorly depends on the chemical composition, amount of exposure, and its dilution. Benzene, formaldehyde, diesel exhaust, styrene, and perchloroethylene are known or suspected carcinogens and their exposure can pose serious health risks. Toluene, ethylbenzene, o-, m-, and p- xylenes have been found to develop adverse health impacts on respiratory and neurological effects.
Initial symptoms of breathing TVOC include irritation to eyes, nose, and throat, headache, fatigue, dizziness, nausea and vomiting, difficulty in breathing, loss of coordination, allergic skin reactions, visual disorders, and memory impairment, etc. It can also aggravate existing respiratory diseases such as bronchitis and pneumonia. Long term exposure to VOC may lead to chronic respiratory diseases, kidney and/or liver damage, problems of the nervous system, and can cause cancer.
VOCs result in the formation of secondary air pollutants such as ozone, smog and fine particulate matter which in turn have their own adverse effects on human health and environment.
VOCs in the atmosphere react with another primary pollutant NOx, in the presence of sunlight to form ozone (O3) and fine particulate matter (PM2.5). The accumulation of O3, PM2.5, and other air pollutants in the atmosphere results and smog formation and visibility reduction.
Possible corrective measures
The primary action is TVOC monitoring i.e. to measure how much of TVOC concentrations you are exposed to. In addition to this following corrective measures can be taken:
- Avoid staying in poorly ventilated areas for long durations.
- Promote the use of alternative fuels and improve quality fuels with high emission rates.
- Avoid keeping your vehicle idle for long.
- Avoid areas with traffic congestions. Breathing such polluted air daily also leads to health effects in the long run.
- Limit outdoor activities and close windows and vents during times of high air pollution.
- Also, avoid or limit using VOC- containing products such as organic cleaning solvents.
Measurement methods of TVOC monitoring
Different working principles for TVOC monitoring in the ambient environment are photoionization detection (PID), flame ionization detection, and semiconductor.
Photoionisation detection (PID)
TVOC monitor working on the principle of PID uses high-energy photons to detect the VOC levels. The air sample drawn into the TVOC monitor is exposed to UV light of a specific output that excites the VOC molecules present in the air sample. The molecules become electrically charged as it ionizes everything that has an ionization energy less than or equal to the UV lamp output. The ions produce an electric current proportional to the concentration of TVOC and are measured by the detector.
Flame ionization detection (FID)
TVOC monitors based on the principle of FID works similar to the PID based TVOC monitors. FID uses a hydrogen flame to ionize all the hydrocarbons present in the sample air. The ions produced are proportional to the TVOC concentration in the sample air which is detected by the detector.
When a metal oxide semiconductor-based hydrogen sulfide monitor is exposed to an air sample, the VOC molecules react on the metal oxide surface of the sensor and dissociate into charged ions that alter the resistance of the film. Further, this interaction is measured as a signal and is converted to the gas concentration. However, the energy consumption of such VOC monitors is higher when compared to others.
Among all the above principles of TVOC monitoring, monitors based on PID are typically found to be preferred for ambient air monitoring as they yield more accurate TVOC concentrations and are inexpensive in comparison with the others.
Oizom’s working principle for TVOC monitoring
Oizom’s ODOSENSE is the real-time odour emission tracking solution. It continuously detects, measures and monitors the odourful gaseous contaminants including hydrogen sulfide, ammonia, sulfur dioxide, methyl mercaptan, TVOC, formaldehyde, methane, and weather parameters like temperature, humidity, wind speed, and wind direction. The sensor that measures TVOC work on the principle of photoionisation detection (PID). Odosense is a proactive approach to measure real-time odour emissions. This makes it an ideal choice for landfill sites, wastewater treatment facilities, fertilizers, paper-pulp industries and soil-treatment sites, etc.
Reasons why TVOC monitoring is important
- VOCs are a group of organic compounds that are easily vaporized. There are abundant VOC emitted from everyday activities among which some are colorless and some odorless.
- Exposure to toxic VOCs may lead to severe health issues such as chronic respiratory diseases, damage to the kidney, liver, and/or central nervous system, and even cause cancer.
- VOCs in the atmosphere are major contributors to the formation of secondary air pollutants ground level ozone, fine particulate matter (PM2.5), and smog which adversely affect human health and the environment.
- TVOC monitoring is an efficient way to detect exposure to toxic VOCs.
- Real-time monitoring of TVOC levels helps in determining their source as well as formulating an action plan to control VOC emissions.