--- title: "BTEX / speciated VOC Sensors & Limit Values | Aethair" description: "BTEX (benzene, toluene, ethylbenzene, and xylene) monitoring for refinery fenceline and fuel sites. Benzene limits, GC methods, and placement." url: https://aethair.io/parameters/btex/ --- Measurements · Gases & VOCs # BTEX / speciated VOC Also known as benzene, toluene, ethylbenzene Speciated volatile organics (benzene, toluene, ethylbenzene, xylenes, and chlorinated VOCs) from gas chromatography. [Schedule a call](https://calendly.com/d/ctdd-mkr-v9v/aethair-demo-website) [Configure BTEX / speciated VOC Monitoring](https://aethair.io/build/?params=btex) At a glance ppb Gases & VOCs 1 way to measure it - **Thiamis** 1 external instrument 6 standards set limits - [EPA Refinery Sector Rule](https://aethair.io/standards/epa_refinery_fenceline/) - [EPA EPA HON](https://aethair.io/standards/epa_hon_fenceline/) - [AQMD SCAQMD 1180](https://aethair.io/standards/scaqmd_1180/) - [NIOSH NIOSH REL](https://aethair.io/standards/niosh_rel/) - [OSHA OSHA benzene](https://aethair.io/standards/osha_benzene/) - [CAL Cal/OSHA PEL](https://aethair.io/standards/cal_osha_pel/) What it is ## What is BTEX? BTEX is the collective name for four closely related volatile organic compounds (benzene, toluene, ethylbenzene, and xylene) that occur together in petroleum products and are among the most health-critical VOCs. Unlike a total-VOC (TVOC) reading, which condenses everything into a single figure, BTEX refers to these species measured individually, because their toxicities differ so sharply that lumping them together would hide the most dangerous one. They are released by fuel storage and handling, refineries and petrochemical plants, vehicle emissions, solvents, paints, and adhesives, and they are the priority compounds in [fence-line monitoring](https://aethair.io/solutions/perimeter-monitoring/) around oil and gas operations. Where a TVOC number tells you that chemical load is elevated, BTEX tells you whether the load includes the carcinogen that matters most: benzene. ### Health and operational effects Benzene is an established human carcinogen linked to leukemia, with no exposure regarded as free of risk, and it is what drives speciated BTEX monitoring. Toluene, ethylbenzene, and xylene are less acutely hazardous but cause neurological, respiratory, and irritant effects and add to the overall burden. Because benzene can be a small part of a much larger VOC mixture, a high TVOC reading may or may not signal a benzene problem, and only speciation resolves the difference. Ethylbenzene and benzene also carry longer-term concerns that keep them on regulatory priority lists. For refineries, fuel terminals, and communities near oil and gas operations, BTEX, and benzene in particular, is a regulatory and public-health parameter rather than a housekeeping one. Limits ## BTEX / speciated VOC Limit Values The published values, by averaging period; each links to the standard that sets it. Annual average Δc, fenceline [**9 µg/m³** Refinery Sector Rule](https://aethair.io/standards/epa_refinery_fenceline/ "benzene; Δc is the highest minus the lowest 14-day fenceline sample, averaged over 26 periods; above it, root cause analysis and corrective action") [**9 µg/m³** EPA HON](https://aethair.io/standards/epa_hon_fenceline/ "benzene; the rule also sets 1,3-butadiene 3, vinyl chloride 3, ethylene dichloride 4, chloroprene 0.8, and ethylene oxide 0.2 µg/m³") 8-hour TWA (PEL) [**1 ppm** OSHA benzene](https://aethair.io/standards/osha_benzene/ "benzene") [**1 ppm** Cal/OSHA PEL](https://aethair.io/standards/cal_osha_pel/ "benzene (see also section 5218)") 1-hour, notification threshold [**8 ppb** SCAQMD 1180](https://aethair.io/standards/scaqmd_1180/ "benzene; toluene 1,300 ppb and xylenes 5,000 ppb") TWA [**0.1 ppm** NIOSH REL](https://aethair.io/standards/niosh_rel/ "benzene; carcinogen") STEL [**1 ppm** NIOSH REL](https://aethair.io/standards/niosh_rel/ "benzene") [**5 ppm** OSHA benzene](https://aethair.io/standards/osha_benzene/ "benzene, 15-minute") [**5 ppm** Cal/OSHA PEL](https://aethair.io/standards/cal_osha_pel/ "benzene") Action level [**0.5 ppm** OSHA benzene](https://aethair.io/standards/osha_benzene/ "benzene, 8-hour TWA") - **Refinery Sector Rule** 9 µg/m³: benzene; Δc is the highest minus the lowest 14-day fenceline sample, averaged over 26 periods; above it, root cause analysis and corrective action - **EPA HON** 9 µg/m³: benzene; the rule also sets 1,3-butadiene 3, vinyl chloride 3, ethylene dichloride 4, chloroprene 0.8, and ethylene oxide 0.2 µg/m³ - **OSHA benzene** 1 ppm: benzene - **Cal/OSHA PEL** 1 ppm: benzene (see also section 5218) - **SCAQMD 1180** 8 ppb: benzene; toluene 1,300 ppb and xylenes 5,000 ppb - **NIOSH REL** 0.1 ppm: benzene; carcinogen - **NIOSH REL** 1 ppm: benzene - **OSHA benzene** 5 ppm: benzene, 15-minute - **Cal/OSHA PEL** 5 ppm: benzene - **OSHA benzene** 0.5 ppm: benzene, 8-hour TWA BTEX compounds are reported in ppb, or sometimes µg/m³. Benzene carries the strictest values (occupational limits under OSHA and NIOSH and, for refineries, fenceline action levels for ambient benzene), while the other three compounds have their own, less stringent limits. The applicable values appear in the table on this page. A TVOC figure cannot be compared against a benzene limit, because it does not isolate benzene, which is exactly why speciated measurement is required. Methods and guidance [EPA Method 325](https://aethair.io/standards/epa_method_325/) [BAAQMD 12-15](https://aethair.io/standards/baaqmd_12_15/) How it is measured ## How BTEX / speciated VOC Is Measured **Gas chromatography** is the defining method: it separates the individual BTEX compounds and quantifies each, usually with photoionization or mass-spectrometry detection, achieving the ppb sensitivity that benzene monitoring demands. Automated online gas chromatographs provide continuous, near-real-time speciation for fence-line and process work. **Photoionization detectors (PIDs)** measure total VOC and flag when aromatics rise, but they cannot separate benzene from the rest, so they complement rather than replace speciation. **Passive sorbent tubes** collect a time-averaged sample for later laboratory analysis, widely used for perimeter surveys. An instrument on Thiamis 1 A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account. ### Placing BTEX monitoring For fence-line work, arrange instruments or passive samplers around the facility boundary, concentrating on the sides facing the community and the main emission sources, and always record wind speed and direction so a plume can be traced back and attributed to its origin. Continuous analyzers are sited downwind of storage tanks, loading racks, and process units where fugitive emissions are most likely. Pairing BTEX with a PID that tracks total VOC gives both the specific and the general picture. ### Every device and option that measures BTEX / speciated VOC 1 | Device / option | Type | Base platform | Measures | | - | - | - | - | | [Defiant Technologies](https://aethair.io/sensors/defiant-technologies-specialty-air-analyzers/) · FROG-5000, FROG-4000 | Specialty Air Analyzers | Thiamis | BTEX / speciated VOC | [Configure BTEX / speciated VOC Monitoring](https://aethair.io/build/?params=btex) Where it is monitored ## Solutions That Monitor BTEX / speciated VOC Each one links to the solution page showing what that industry measures it for. ### [Oil & Gas / Petrochemical](https://aethair.io/solutions/oil-gas/) Often added Fence-line VOC, H₂S, and methane leak detection Article Library ## Reading on BTEX / speciated VOC ### [Industrial Perimeter Air Quality Monitoring: Action Levels](https://aethair.io/resources/perimeter-air-quality-monitoring-guide/) What industrial perimeter monitoring requires at fenceline and construction sites: DER-10 CAMP action levels, EPA fenceline rules, and continuous records. ### [OSHA Air Quality Standards 2026: PELs & 29 CFR 1910.1000](https://aethair.io/resources/osha-air-quality-standards-compliance/) OSHA air quality standards for indoor and outdoor work: permissible exposure limits, Table Z-1, 29 CFR 1910.1000, and the heat and wildfire smoke triggers. [All articles](https://aethair.io/resources/) Related ## More in Gases & VOCs [Carbon Dioxide (CO₂)](https://aethair.io/parameters/co2/) [VOC (Total)](https://aethair.io/parameters/voc/) [Hydrogen Sulfide (H₂S)](https://aethair.io/parameters/h2s/) [Carbon Monoxide (CO)](https://aethair.io/parameters/co/) [Methane (CH₄)](https://aethair.io/parameters/ch4/) [Oxygen (O₂)](https://aethair.io/parameters/o2/) [Sulfur Dioxide (SO₂)](https://aethair.io/parameters/so2/) [Nitric Oxide (NO)](https://aethair.io/parameters/no/) [Nitrogen Dioxide (NO₂)](https://aethair.io/parameters/no2/) [Ammonia (NH₃)](https://aethair.io/parameters/nh3/) [Combustible (LEL)](https://aethair.io/parameters/lel/) [Chlorine (Cl₂)](https://aethair.io/parameters/cl2/) Questions ## BTEX / speciated VOC FAQ What is BTEX and why single out these four compounds? BTEX stands for benzene, toluene, ethylbenzene, and xylene, four aromatic VOCs that occur together in petroleum and solvents. They are grouped because they travel together and are the most health-relevant VOCs in fuel-related emissions, but they are measured individually because benzene is far more hazardous than the others. Singling them out lets each be judged against its own limit. How is BTEX different from a TVOC reading? TVOC is a single summed response to all volatile organics present and cannot say which compounds make it up. BTEX identifies and quantifies the four aromatic species separately, so it can reveal benzene specifically. A high TVOC flags that something is off-gassing; BTEX tells you whether that something is the carcinogen of concern. Why is fence-line BTEX monitoring carried out? Communities near refineries and oil and gas sites are exposed to fugitive emissions that can include benzene, and regulators increasingly require monitoring at the facility boundary to show that exposure stays within action levels. Fence-line BTEX, read with wind data, gives both the concentration and the source direction. It is a compliance and community-assurance measurement. How do I monitor BTEX? BTEX speciation needs a gas chromatograph, so either an external online GC is connected to a Thiamis gateway, or passive sorbent tubes are analyzed in a laboratory for a time-averaged result. Pairing this with a PID for total VOC and with wind data gives a complete fence-line picture. Use the configurator to combine BTEX with VOC and the meteorological parameters your site needs.