Benzene, toluene, and xylene (BTEX) are aromatic hydrocarbons that are commonly found together in fossil fuels like oil and gasoline. Each has its own unique chemical formula and structure:

- Benzene (C6H6) is composed of six carbon atoms bonded together in a ring with one hydrogen atom attached to each carbon. This ring structure is what gives benzene and other aromatic compounds their name. Benzene has a boiling point of 80.1°C and evaporates easily at room temperature.

- Toluene (C7H8) contains a methyl group (CH3) attached to one of the carbon ring positions in benzene. This makes toluene slightly less volatile than benzene, with a boiling point of 110.6°C.

- Xylene is the name given to three isomer compounds with the formula C8H10 - ortho-xylene, meta-xylene, and para-xylene. In each, a methyl group is attached to one of the carbon atoms in a benzene ring. Xylenes have boiling points between 138-144°C, making them less volatile than benzene and toluene.

Because they readily evaporate at environmental temperatures, Benzene, Toluene, And Xylene compounds are commonly found as air pollutants in both indoor and outdoor settings where petroleum products are used, stored, or transported. Their light aromatic rings also make them highly soluble in non-polar organic solvents.

Exposure Pathways and Health Effects

As a result of their chemical and physical properties, human exposure to BTEX usually occurs through inhalation of vapors or ingestion of contaminated water or food. Dermal contact may also contribute to exposure in occupational settings.

Short-term, high-level exposure to benzene, toluene, and xylene via inhalation can cause dizziness, nausea, eye/skin/respiratory irritation, and in more severe cases, depression of the central nervous system leading to unconsciousness. Long-term or repeated exposures are linked to more serious health impacts:

- Benzene is a established human carcinogen associated with leukemia and other blood cancers. It can also damage the immune and central nervous systems.

- Toluene exposure has been tied to effects on the developing fetus as well as impacts on the nervous system like fatigue, confusion, and headaches. It is sometimes used as an intentional inhalant.

- Xylenes may cause similar neurological symptoms as toluene as well as irritation of the skin, eyes, nose, and throat. Long-term exposure has been linked to impacts on the liver, kidneys, and lungs.

Environmental Occurrence and Regulations

Fossil fuel production, distribution, and combustion operations are major sources of BTEX releases into the air, water, and soil. Due to their widespread use, gasoline stations, industrial solvents, paints, and adhesives are also significant environmental polluters. Once in the environment, benzene, toluene, and xylene can seep into groundwater from leaky storage tanks or move through the air via evaporation and wind currents.

To protect public health, numerous regulations have been adopted governing BTEX levels in various media:

- Drinking water standards limit benzene to 5 parts per billion and toluene/xylenes to 1 part per million each.

- Indoor and ambient air quality standards cap annual benzene levels at under 1 microgram per cubic meter.

- Gasoline specifications restrict benzene content to less than 1% by volume nationally. Some states and Canada have set even lower limits.

- The EPA designates benzene, toluene, and xylene as hazardous air pollutants that require strict control technology on industrial emission sources.

- Workplace exposure limits aim to keep 8-hour time-weighted average concentrations of BTEX well below recommended occupational exposure limits.

- Leak detection and spill prevention programs are mandated at underground storage tank facilities to reduce soil and groundwater contamination risks.

When properly managed according to regulations, exposures to BTEX from environmental sources can generally be kept at levels not expected to cause harm to public health. Continued monitoring and pollution controls remain important due to their widespread distribution and toxicity.

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