What a Continuous Emission Monitoring System Does

A continuous emission monitoring system (CEMS) is equipment that measures air pollution coming from industrial smokestacks, vents, and exhaust points in real time. Instead of taking samples once a month or once a quarter, CEMS runs constantly and sends data to regulators automatically. The system watches for pollutants like nitrogen oxides, sulfur dioxide, particulate matter, and carbon dioxide — the specific pollutants depend on what the facility produces and what the Clean Air Act requires for that industry.

Facilities that must install CEMS include coal-fired power plants, large refineries, cement kilns, waste incinerators, and other major industrial sources. The Environmental Protection Agency (EPA) sets the rules for what gets measured, how often data must be reported, and how accurate the equipment must be. State environmental agencies then oversee whether individual facilities are following those rules.

Key Takeaways

  • CEMS measures pollution continuously rather than through periodic sampling, and the data flows automatically to state and federal regulators.
  • The EPA requires CEMS at large industrial sources like power plants and refineries, but not at smaller facilities or offices.
  • A CEMS installation includes analyzers that measure specific pollutants, a data acquisition system that records and transmits readings, and calibration equipment to keep measurements accurate.
  • Facilities must maintain CEMS equipment, run quality assurance tests regularly, and report any downtime or equipment failures to regulators.
  • CEMS data is public and available through state environmental agency websites, so anyone can see what a facility is emitting.

The Main Components of a CEMS

A working CEMS has three essential parts. First is the analyzer — the instrument that actually measures pollutant concentrations in the exhaust stream. Different analyzers detect different pollutants; a facility measuring nitrogen oxides uses a different analyzer than one measuring sulfur dioxide. The analyzer sits in or near the smokestack and pulls a sample of exhaust continuously.

Second is the data acquisition and handling system (DAHS), which receives the signal from the analyzer, records it, performs calculations, and stores the data. The DAHS also flags problems — if the analyzer goes offline, if readings spike unexpectedly, or if calibration drifts too far. Third is the calibration equipment, which includes reference gases and automated systems that check whether the analyzer is reading correctly. Facilities must run these checks on a schedule set by the EPA, typically daily or weekly depending on the pollutant.

The system also includes sample lines that carry exhaust from the stack to the analyzer, a probe that sits inside the stack to collect the sample, and sometimes a conditioning system that removes moisture or cools the sample before it reaches the analyzer. All of these parts must work together reliably, which is why CEMS installations are expensive and require trained technicians to maintain.

What Pollutants CEMS Measures

The EPA does not require every facility to measure every pollutant. Instead, the agency specifies which pollutants must be monitored based on the type of facility and the Clean Air Act regulations that explore to it. Coal power plants typically monitor nitrogen oxides (NOx), sulfur dioxide (SO₂), carbon dioxide (CO₂), and sometimes mercury. Natural gas power plants usually monitor NOx and CO₂. Refineries monitor multiple pollutants including volatile organic compounds (VOCs) and particulate matter.

Waste incinerators and cement kilns have their own pollutant lists. Some facilities also monitor opacity — how much light the exhaust blocks — as a way to track particulate matter indirectly. The specific requirements appear in the facility's air permit, which is a public document that anyone can request from the state environmental agency.

How CEMS Data Gets Reported and Used

CEMS data flows in two directions. First, the facility's DAHS transmits data to the state environmental agency, usually through a find online system. Most states require hourly or daily submissions, though the exact schedule varies. Second, the EPA collects data from state agencies and makes it available to the public through the Enviromapper database and state-specific websites.

Regulators use CEMS data to check whether a facility is staying within its permitted emission limits. If a facility exceeds its limit, the state agency investigates and can issue a violation notice, require corrective action, or impose penalties. Environmental groups and the public also use CEMS data to track pollution from specific facilities and to identify which sources are the largest emitters in a region.

The data is typically reported as hourly averages or rolling averages over longer periods — 30 days, for example — depending on the pollutant and the regulation. A single high reading does not automatically trigger a violation; regulators look at whether the facility meets its limits over the averaging period specified in its permit.

Maintenance and Quality Assurance Requirements

CEMS equipment must stay accurate, so the EPA requires facilities to perform regular quality assurance tests. These include daily zero and span checks, where the analyzer is exposed to a known zero-concentration gas and a known high-concentration gas to verify it reads correctly. Facilities also run linearity checks at least quarterly, testing the analyzer with multiple reference gases across its full measurement range.

Every three years, an independent auditor must perform a relative accuracy test audit (RATA), comparing CEMS readings to measurements taken by a certified reference method. If the CEMS drifts too far from the reference method, the facility must recalibrate or repair the equipment. Facilities must also report any downtime — periods when CEMS is not operating — to regulators, and the EPA has rules about how much downtime is allowed before it affects compliance.

Maintenance records, calibration logs, and quality assurance test results must be kept for at least five years and made available to regulators on request. Many states now require facilities to submit these records electronically as part of their regular reporting.

When CEMS Is Required and When It Is Not

The EPA requires CEMS at large industrial sources that emit significant quantities of regulated pollutants. This includes fossil fuel power plants, large refineries, cement plants, steel mills, waste incinerators, and some chemical manufacturers. The specific threshold depends on the pollutant and the regulation; for example, the Acid Rain Program requires CEMS at power plants that burn coal or oil, while the Greenhouse Gas Reporting Program requires CEMS at some facilities that emit large amounts of CO₂.

Smaller facilities, offices, hospitals, and most commercial buildings do not have CEMS. Instead, they may report emissions through other methods — stack testing once per year, emission factors based on fuel use, or material balance calculations. The facility's air permit specifies which monitoring method applies. If you work at a facility and are unsure whether CEMS is required, the permit is the authoritative document; it lists all monitoring requirements and is available from your state environmental agency.

Finding CEMS Data for a Specific Facility

CEMS data is public. To find emissions from a specific facility, start with your state environmental agency's website — most states have a searchable emissions database or a facility lookup tool. You can also search the EPA's Enviromapper tool at www.epa.gov/enviro, which pulls data from multiple sources and lets you search by facility name, location, or pollutant.

The data typically shows hourly or daily averages, sometimes going back several years. You can see whether a facility exceeded its permit limits, how often equipment was down for maintenance, and how emissions have changed over time. Some states also publish annual compliance reports that summarize CEMS data and note any violations. If you cannot find what you are looking for online, you can file a public records request with your state environmental agency asking for CEMS data, quality assurance records, or compliance reports for a specific facility.

Frequently Asked Questions

Does CEMS measure all the pollution a facility produces?

CEMS measures only the pollutants specified in the facility's air permit, which are usually the ones regulated under the Clean Air Act. A facility may produce other pollutants that are not monitored by CEMS. The permit specifies which monitoring method applies to each pollutant, so some may be measured by CEMS while others are tracked through stack testing or calculations.

What happens if a CEMS breaks down?

Facilities must report downtime to regulators and typically have a limited window — often 30 days per year — during which CEMS can be offline without affecting compliance. If downtime exceeds the limit, the facility may be in violation. During extended downtime, the facility usually must use an alternative monitoring method, such as stack testing, to show it is still meeting its permit limits.

Can I see CEMS data from my neighborhood?

Yes. Search your state environmental agency's website or the EPA's Enviromapper tool using the facility name or address. The data shows what pollutants are being measured, how much is being emitted, and whether the facility is meeting its limits. If you find violations or have concerns, you can contact your state environmental agency or file a complaint with the EPA.

How accurate is CEMS?

The EPA sets accuracy standards for CEMS, typically requiring measurements to be within 10 to 20 percent of a reference method, depending on the pollutant. Facilities must run regular calibration checks and annual audits to verify accuracy. If a CEMS fails an audit, the facility must repair or replace it and may have to resubmit data for the period when it was out of calibration.

Who pays for CEMS installation and maintenance?

The facility owner pays for all CEMS costs — equipment, installation, calibration, maintenance, and staff training. These costs are typically passed along to customers through higher prices for electricity, fuel, or products. The EPA does not provide funding for CEMS, though some state programs offer grants or low-interest loans for pollution control equipment.