Fenceline data tells you what is at the property line.
It does not tell you who is being exposed.

Fenceline air monitoring is the practice of measuring air pollutant concentrations at the property boundary of an industrial facility, rather than at the stack or inside the plant. The goal is to characterize what nearby communities are actually exposed to as pollutants migrate off site. The most common federally required example is the EPA refinery benzene rule, which uses passive samplers placed around the perimeter of every petroleum refinery in the United States. Fenceline data is one signal in a broader exposure picture, not a complete health risk assessment by itself.

In the United States, petroleum refineries are required to conduct fenceline benzene monitoring under 40 CFR Part 63 Subpart CC, the EPA Petroleum Refinery Sector Rule. Beyond that federal requirement, some state and local programs add their own fenceline obligations for chemical plants, ports, rail yards, and other large emitters. In California, facilities in AB617 designated communities may be subject to enhanced monitoring under the Community Air Protection Program. Outside refineries, fenceline monitoring is often voluntary or driven by consent decree, settlement, or community pressure.

The EPA refinery fenceline benzene rule is the part of 40 CFR Part 63 Subpart CC that requires every petroleum refinery in the United States to monitor benzene concentrations at its property boundary using passive samplers. Samples are collected on a two-week cycle and analyzed by an accredited laboratory. The rule sets an action level of 9 micrograms per cubic meter as a rolling annual average. If the action level is exceeded, the refinery must investigate the root cause and take corrective action. EPA publishes the data publicly, which has changed how communities and regulators see refinery emissions.

AB617 is the 2017 California Community Air Protection Program, codified at Health and Safety Code section 44391.2. It directed the California Air Resources Board to establish a statewide strategy for reducing air pollution exposure in the most burdened communities. CARB designates AB617 communities based on cumulative exposure burden, vulnerability, and air quality concerns. Each designated community gets a community air monitoring plan, a community emissions reduction plan, or both. As of 2026, more than a dozen communities have been designated under the program, and the work emphasizes cumulative impact rather than single-pollutant compliance.

Fenceline monitoring tells you the concentration of a pollutant at the property line over time. A health risk assessment translates concentrations into expected health outcomes for the people who live, work, and go to school nearby. That translation requires dispersion math, exposure duration assumptions, age sensitivity factors, and population data at the census-tract level. LSARS picks up where the monitoring data hands off and runs the same EPA AirToxScreen and CA-OEHHA 2015 methods that air districts and OEHHA reviewers expect, validated against CARB HARP2 to five significant figures.

For petroleum refineries under the EPA rule, benzene is the required species because it serves as a marker for the broader BTEX (benzene, toluene, ethylbenzene, xylenes) family. Many refineries also measure additional volatile organic compounds, hydrogen sulfide, and 1,3-butadiene either voluntarily or under state requirements. AB617 community air monitoring networks typically measure a wider list, including PM2.5, black carbon, NO2, ozone, and air toxics like formaldehyde and acrolein. The exact list is set in the community air monitoring plan for each designated AB617 community.

Two communities can be exposed to the same concentration of an air toxic and experience very different health outcomes, because age distribution, baseline disease burden, housing quality, and access to care all matter. LSARS bundles CDC Social Vulnerability Index 2022 and American Community Survey 2023 demographic data at the census-tract level. These layers feed directly into the cumulative impact view alongside the monitored concentrations and the calculated cancer risk and hazard index. The result is a single picture of who is exposed, where they live, and what the math implies for their health.

A community air monitoring network is a coordinated set of air quality sensors deployed across a defined community, usually a designated AB617 community in California or a similarly burdened area elsewhere. Networks may mix regulatory-grade reference monitors with lower-cost sensors to get spatial coverage. The data is typically published on a public dashboard and used to track progress against a community emissions reduction plan. LSARS is sensor-agnostic on the ingest side and focuses on the next step: turning the network data into HRA-grade exposure and risk estimates with SDOH context attached.