Many monitoring projects call for monitoring of Volatile Organic Compounds (VOCs). VOCs is a collective term given to organic compounds, which have high vapour pressure and become gases at ambient pressure and temperatures.
VOCs are commonly monitored as they are a key contributor in photochemical smog. Many are hazardous to human health, with several classified as carcinogenic. The key sources of VOCs are industrial processes (especially those involving solvents) vehicle emissions, evaporative loss from petrol storage and even natural sources like forest fires.
What is BTEX?
Often, people choose to measure a group of VOCs, collectively known as BTEX, comprising benzene, toluene, ethylbenzene and xylene (often expressed as total xylenes (m+p)). The BTEX suite has grown in popularity, as they are straightforward to monitor together, and provide a well-rounded picture of VOCs and are present in most urban areas.
BTEX is monitored for both ambient applications, and industrial health and safety applications.
Benzene (C6H6) is a clear, colourless and flammable liquid with a sweet petrol-like smell. Benzene is found in ambient air as a result of burning fuels, such as coal, petrol and wood. Benzene is common in unleaded fuel, where it is added as a substitute for lead, allowing smoother running.
Benzene concentrations in fuel were once as high as 20%, but are now reduced to <1% in many countries, due to harmful health impacts. The World Health Organisation (WHO) and International Agency for Research on Cancer (IARC) classify benzene as a group one carcinogen . Prolonged exposure to high concentrations of benzene causes leukaemia and impacts red and white blood cells.
Less severe health impacts can occur at lower concentrations, causing headaches, nausea, drowsiness and even unconsciousness. The WHO has not set a standard for ambient benzene concentrations, stating that there is no safe level of exposure. Many countries use an annual average standard of 3.6 µg m-3.
Toluene (C7H6), also known as methylbenzene, is a colourless liquid, with a strong, solvent-like smell. Toluene is inexpensive and simple to produce, and is widely used in industrial processes as a solvent. Significant amounts of toluene are used in industrial processes worldwide, with over US$20 billion generated from toluene sales in 2013.
In non-industrial uses, toluene can be found in petrol as an octane booster and in glues, solvents and resins.
Ethylbenzene (C6H5CH2CH3), is a colourless liquid, with a petrol-like aroma. Ethylbenzene is widely used in industrial processes for the manufacture of styrene, which is then used for polystyrene manufacture. Ethylbenzene is also present as a solvent in inks, dyes and in petrol.
Xylene (C8H10) is the term used to describe the three isomers of dimethyl benzene; m-xylene, p-xylene and o-xylene. Usually concentrations of each are added together as total xylenes. Xylene is refined from crude oil, and is a clear, greasy liquid.
Xylene is widely used in production of plastic bottles and polyester clothing and as a solvent with a range of applications from circuit board cleaning to thinning paints and varnishes.
Why measure BTEX?
BTEX is a useful suite to measure. In typical urban environments, there are a lot of VOCs present, emitted by everything from paint to vehicle exhaust. Typical GC-MS analysis can resolve many VOCs, and some of these may be below detection limits of the analysis – but you’ll probably still pay for them!
It’s often more sensible to be selective, and monitor BTEX only, and use the 4 components as indicators of VOC concentrations as a whole.
BTEX gives a great indication of VOC emissions from a range of sources. Benzene for instance, is commonly present in petrol, vehicle exhaust and burning of solid and liquid fuels. In an urban environment, there’s usually benzene present from these sources. It gives a great indication of the dominance of petrol vehicles in the vehicle fleet, and is useful as an indicator for home heating emissions from solid fuels.
The remainder of the BTEX suite are found in petrol, but their largest sources are industrial emissions. In residential areas bordered by industrial land use, the BTEX suite is useful for assessing the impact of industry on other land uses. Industries typically measure BTEX at their fence line, to minimise their VOCs emissions and their impact on surrounding communities.
Many also use BTEX monitoring to measure the exposure of their staff to VOCs. Commonly this is done with sorbent tubes attached to a person’s clothing, but it’s also possible and practical to do this using ambient measurement.
In areas with significant manufacturing industries, especially those using glues, solvents and dyes, then higher concentrations of toluene, ethylbenzene and xylenes may be present. Monitoring BTEX will allow quantification of these, and can be used to identify health and safety concerns, demonstrate compliance with local regulations and even assess if there is a leak in storage or processing infrastructure.
BTEX Standards and Guidelines
A range of exposure standards for BTEX are in use around the world. The Texas Effects Screening Model is often used to detect concentrations above 200 µg m-3. A range of standards for occupational health and safety exposure are freely available from National Institute for Occupational Safety and Health (NIOSH).
For example, here is the benzene standard. These standards are intended for health and safety applications, but they can be used to assess ambient concentrations when you need to get an idea of concentrations.
How is BTEX measured?
BTEX is commonly measured by a range of methods, ranging in cost, from very inexpensive, to very expensive. The method you choose is dictated by the goals of your monitoring project – simple projects will use cheaper methods, with lower resolution.
Commonly, passive sampling methods are used for simple projects. Tubes, or badges are deployed in the field, which contain a medium which absorbs VOCs, at a known rate. Depending on the concentrations expected, they may be exposed for as little or as long as a week or a month. The samples are then processed in a lab, using acid digestion before being analysed using Ion Chromatography or Inductively Coupled Plasma Mass Spectrometry (ICP-MS).
Often 2 samples are analysed per site, and the average of these are used. The advantages of passive sampling is that it’s cheap and straightforward, but data resolution is limited. Common methods are 3M badges or sorbent tubes. The USEPA uses standard methodology for these techniques, published as Compendium Method TO-17.
BTEX can be measured in real time, with high resolution techniques such as Gas Chromatograph Mass Spectrometer instruments (GC-MS). These sophisticated instruments are able to measure compounds and elements at the ion level to distinguish both the presence and concentration of various compounds.
These instruments have fast measurement time, and typically can measure at resolutions as short as 30 minutes. However, they are expensive to operate, calibrate and maintain. Typical systems need temperature controlled environments, and some even require helium as a carrier gas, which is becoming increasingly expensive worldwide. Standard methods are available for the use of GC-MS, particularly from the European Union.
How can Aeroqual help you measure BTEX?
Aeroqual can provide sensors to measure the components of BTEX individually.
Aeroqual VOCs sensors are available for our handheld and fixed monitors, as well as our AQM65 continuous ambient air quality monitoring station.
Aeroqual sensors use PID technology, and are calibrated using isobutene. Aeroqual can provide a range of correction factors, to convert your readings into the VOC of your choice. Correction factors are available for specific VOC.
In some applications, a standard calibration gas mixture of BTEX, instead of isobutene, can be used to further increase the accuracy of your data. You could use one of our handheld monitors for spot checks around an industrial facility, or for indicative readings in urban areas. A fixed monitor can be used in an area prone to high concentrations, such as a storage facility, and can be used to generate an alarm.
If you’re looking to monitor other criteria pollutants, like O3, CO, NOx, NO2, SO2 and Particulate Matter (PM) alongside your BTEX measurement, then an AQM 65 is an ideal solution.
Contact us to see what options could suit you for your BTEX applications.