Sources of BTEX
BTEX refers to the chemicals benzene, toluene, ethylbenzene and xylene.
These compounds occur naturally in crude oil and can be found in sea water in the vicinity of natural gas and petroleum deposits. Other natural sources of BTEX compounds include gas emissions from volcanoes and forest fires.
The primary man-made releases of BTEX compounds are through emissions from motor vehicles and aircrafts, and cigarette smoke. BTEX compounds are created and used during the processing of petroleum products and during the production of consumer goods such as paints and lacquers, thinners, rubber products, adhesives, inks, cosmetics and pharmaceutical products.
BTEX compounds are among the most abundantly produced chemicals in the world.
Common exposure to BTEX
The most common sources of exposure to BTEX compounds are from breathing contaminated air, particularly in areas of heavy motor vehicle traffic and petrol stations, and through cigarette smoke. Exposure to BTEX from water contributes only a small percentage of the total daily intake, compared with inhaled air and dietary sources.
Health standards for BTEX
Public health guidelines for BTEX are available for drinking water in the Australian Drinking Water Guidelines (ADWG). These guidelines are based on the amount of a chemical that can be ingested every day over a lifetime without adverse effect.
Benzene is a known carcinogen (cancer causing). The ADWG specify that it should not be detected in drinking water at more than 1 part per billion (ppb) †. The remaining chemicals (toluene, ethylbenzene and xylenes) are not recognized as carcinogenic and their drinking water health guidelines are much higher—between 300 and 800 ppb.
In air, different types of guidelines are available for both ambient and occupational settings. In Queensland, the Environmental Protection (Air) Policy 2019 specifies guideline values for benzene, toluene and xylenes in air to ensure protection of human and environmental health.
The ambient air quality objectives to protect human and environmental health are 3 ppb for benzene, 100 ppb for toluene and up to 200 ppb for xylene—based on the ambient concentrations of these chemicals being averaged over a yearly period.
BTEX chemicals occur naturally in underground water sources. So to ensure these levels don’t rise above environmental and human health standards, the use of BTEX in the fraccing process has been strictly regulated, including a ban on adding these chemicals to fraccing fluid.
The following environmental and health standards for BTEX in fraccing fluids ensure that BTEX chemicals are not at a level that will contaminate drinking water or impact on groundwater dependant plants and animals:
- Benzene – 1 ppb*
- Toluene – 180 ppb#
- Ethylbenzene – 80 ppb#
- m-Xylene – 75 ppb#
- o-Xylene – 350 ppb#
- p-Xylene – 200 ppb#
†As guidance, 1 ppb is equivalent to 1 micro gram per litre, or a teaspoon of liquid in an Olympic size swimming pool.
*Australian Drinking Water Guidelines
# Australia and New Zealand Environment Conservation Council (ANZECC) Guidelines for Fresh and Marine Water Quality.
More detailed information on BTEX is available in the research paper A short primer on benzene, toluene, ethylbenzene and xylenes (BTEX) in the environment and in hydraulic fracturing fluids . This paper has been prepared by Dr Frederic Leusch and Dr Michael Bartkow from Griffith University’s Smart Water Research Centre.
Estimated daily intakes of BTEX (µg/d)
Daily intakes are given in microgram per person, per day (µg/d)—a microgram is one millionth of a gram.
|Source of exposure||Benzene||Toluene||Ethylbenzene||Xylenes|
|Air breathing||90 – 1,300||2 – 12,000||2 – 3,600||70 – 2,000|
|Cigarette smoking||1,800||2,000||40a||Up to 190a|
|Food||Up to 250||Up to 64||NA||NA|
a Assuming 5 cigarettes a day,b Assuming two litres a day
Outdoor levels of benzene range from 0.06 ppb in remote rural areas to 107 ppb in industrial centres with a high density of motor vehicle traffic. The percentage of benzene in unleaded petrol is approximately 1 to 2 per cent. Levels of up to 3,000 ppb of benzene have been measured in air at petrol stations.
Driving a car for one hour per day is estimated to add 40 micrograms (µg) of benzene to a person’s daily intake. Spending a little under two minutes a week to refuel a car at the petrol station leads to an additional estimated daily intake of 10 µg.
Benzene has been detected at levels as high as 154 ppb in indoor residential air. Benzene can also occur in foods and drinks. Research in the United Kingdom has found benzene concentrations in soft drinks as high as 28 ppb.
The largest source of toluene release is during the production, transport, and use of petrol, which contains about 5 to 8 per cent toluene. In remote areas, the concentrations of toluene in air is quite low, however levels from 1.3 to 6.6 ppb are common in suburban and urban areas, with levels as high as 350 ppb in areas of high traffic density.
Concentrations at a petrol station can be as high as 2,400 ppb. Refilling a car can add significantly to daily toluene intake.
Toluene is also used in common household products such as paints, adhesives and nail polish. Toluene is occasionally detected in drinking water supplies, but generally the levels are below 3 ppb.
Ethylbenzene is a common compound in ambient air, primarily resulting from industrial activities and vehicle emissions. Petrol contains about 1 to 2 per cent ethylbenzene.
At urban sites, ethylbenzene concentrations range from 0.1 to 83 ppb. Levels found at rural sites are generally less than 0.46 ppb.
Ethylbenzene is also found in products such as pesticides, solvents, paints, varnishes, automotive products, adhesives, and fabric and leather treatments.
The compound is rarely detected in drinking water and the levels of ethylbenzene in uncontaminated groundwater are generally less than 0.1 ppb.
Xylene is a compound primarily released from industrial sources and motor vehicle exhausts. In urban and industrialised areas, xylene levels have been measured at up to 178 ppb. Petrol contains 7 to 10 per cent of xylenes. It is also used as a solvent.
Typically, background levels of xylene in ambient air are around 0.23 ppb. In suburban areas it can be around three times higher.
BTEX in groundwater
The BTEX compounds are found naturally in crude oil, coal and gas deposits and therefore they can be naturally present at low concentrations in groundwater near these deposits. The BTEX levels can range from 0.1 ppb to 100 ppb in typical groundwater.