In the chemicals industry, there are some substances which are more pervasive than others—most often for their properties as solvents or precursors to important manufactured materials.
BTEX chemicals—the colloquial shorthand for benzene, toluene, ethylbenzene, and the three configurations of xylene—are aromatic hydrocarbons used by the tens of millions of tons worldwide. They play a part in almost all aspects of chemical synthesis, particularly plastics and fuels. But how safe are they? What’s the story behind these ubiquitous chemicals?
Composed of six carbons and six hydrogens in a perfect ring, benzene is the poster child for organic chemistry, specifically as the basis of aromatic compounds. Its main use is as an intermediate for making other chemicals, such as ethylbenzene, cumene, cyclohexane, and nitrobenzene.
In addition to being extremely flammable, benzene is highly hazardous to people and the environment, and is the most harmful of the BTEX group. It may cause cancer, genetic defects, and infertility, and it may be fatal if swallowed or inhaled. Australian Workplace Exposure Standards (WES) dictate that benzene exposure should not exceed 1 part per million over an eight-hour workday, however the recommended ambient concentrations in air and water sources are magnitudes lower. Benzene should not be detected in drinking water above a concentration of 1 part per billion, or in air above 3 parts per billion, to minimise the potential for cancerous, mutagenic, or reprotoxic effects.
Toluene, also known as methylbenzene, is an aromatic compound like benzene, but with a substituted methyl group attached. It is regularly used as a solvent for the manufacture of paints, rubber, pharmaceuticals, and other feedstock chemicals.
Toluene is considered to be a less harmful alternative to benzene, however there are still inherent risks associated with its usage. Toluene and similar volatile solvents have been linked to occupational asthma, even from seemingly safe levels of exposure, and WES recommends airborne exposure does not exceed 50 parts per million over an eight-hour day. Australian drinking water guidelines advise that toluene concentration should not exceed 800 parts per billion, however changes in water taste and smell can be detected at as little as 25 ppb. Environmental air quality guidelines suggest that toluene concentrations not exceed 100 ppb.
Ethylbenzene is most notable for its use in the production of polystyrene, as a precursor to the styrene molecule. It’s also found as a solvent in the production of synthetic rubber, paints, glues, and insecticides, as well as automotive and aviation fuels. It consists of a benzene ring with an ethyl group substituted for one of the hydrogens.
Ethylbenzene is highly flammable, and at high levels of exposure can cause dizziness, irritation of the eyes and respiratory system. Exposure in the workplace should not exceed 100 parts per million over an eight-hour day. Air quality guidelines do not specify a limit for ethylbenzene in the wider environment, however it is recommended that concentrations in water not exceed 300 parts per billion.
Also known as dimethylbenzene, xylene consists of a benzene ring with two substituted methyl groups, which can be in one of three configurations. Para-xylene—where the methyl groups sit opposite each other around the aromatic ring—is the most commonly used of these isomers. Para-xylene is most often used as a feedstock for making polyethylene terephthalate (PET) plastic, while ortho-xylene is extremely useful in the manufacture of pharmaceutical products, plasticizers, and dyes as a precursor to phthalic anhydride.
Xylene is often considered the least inherently hazardous of the BTEX category, however it can still be harmful if inhaled and irritating to eyes and skin. WES guidelines state that daily workplace exposure should not exceed 80 parts per million. Concentration in drinking water is recommended as no higher than 600 parts per billion, and ambient air concentrations are advised to not exceed 200 ppb.
Oil spills and contamination from industrial activities are potent sources of BTEX exposure in everyday environments and carry risk of significant health effects, from respiratory irritation to asthma to even cancer.
Apart from industrial sources, cigarettes contain significant levels of these chemicals, with toluene being one of the most common. High levels of emissions have also been found to come from vehicle exhaust and petrol stations. While studies have shown that these incidental emissions do not exceed ‘safe’ levels in air or drinking water over a short period of time, the safest level of exposure over a lifetime is as little as possible to minimise risks of dizziness, drowsiness, asthma, or more serious conditions like cancer.
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