What continuous emissions monitoring is and why facilities use it

Continuous emissions monitoring (CEM) is a system that measures air pollutants flowing out of a smokestack, vent, or exhaust point in real time, 24 hours a day. Instead of taking a snapshot of emissions once or twice a year, a CEM system collects data constantly and sends it to regulators automatically. Facilities that burn fuel, manufacture chemicals, generate power, or process metals are typically required to install and operate these systems under federal and state air quality rules.

The Environmental Protection Agency (EPA) sets the standards for what gets measured, how accurately it must be measured, and how often the data must be reported. States then enforce those standards through their own air quality agencies — often called a Department of Environmental Quality, Department of Air Quality, or similar. A facility's CEM system becomes part of its operating permit, and regulators can inspect the equipment and the data it produces at any time.

The reason regulators require CEM instead of periodic testing is straightforward: a single test on a Tuesday afternoon does not tell you what a facility is emitting on a Saturday night or during a production surge. Real-time monitoring catches violations as they happen and creates a permanent record that both the facility and the public can review.

Key Takeaways

  • Continuous emissions monitoring systems measure pollutants like nitrogen oxides, sulfur dioxide, and particulate matter every few seconds and report the data to state regulators automatically.
  • The EPA sets minimum accuracy standards for CEM equipment, and facilities must calibrate and maintain their systems according to a strict schedule or face penalties.
  • Data from CEM systems is public record in most states and can be accessed through your state air quality agency's website or the EPA's national database.
  • Facilities that fail to operate their CEM system correctly — or that report false data — face civil penalties, permit revocation, and in some cases criminal charges.

What pollutants are measured and how the equipment works

The pollutants measured depend on the type of facility and its permit. The most common are nitrogen oxides (NOx), sulfur dioxide (SO₂), carbon dioxide (CO₂), particulate matter (PM), and opacity (how much light a plume blocks). Some facilities also measure mercury, hydrogen chloride, or volatile organic compounds. The permit specifies which pollutants must be monitored at each emission point.

A typical CEM system has three main parts: a probe or inlet that sits in the exhaust stream, an analyzer that measures the concentration of the pollutant, and a data acquisition system that records and transmits the results. For gas pollutants like NOx and SO₂, the analyzer uses infrared absorption or chemiluminescence — the gas passes through a chamber and the instrument reads how much light it absorbs or emits. For particulate matter, the system may use light scattering or beta radiation to measure the mass of particles in the exhaust.

The system takes a reading every 10 to 30 seconds, depending on the regulation. Those readings are averaged into hourly values, which are then reported to the state. If an hourly average exceeds the permit limit, the facility must document why and may be required to notify regulators within 24 hours. The data is stored for at least five years and must be made available to the EPA and the public upon request.

Calibration, maintenance, and quality assurance requirements

A CEM system is only useful if it is accurate. The EPA requires facilities to perform three types of quality checks: daily zero and span checks, quarterly linearity tests, and annual relative accuracy tests. A zero check confirms the analyzer reads zero when exposed to clean air. A span check confirms it reads correctly when exposed to a known concentration of the pollutant. A linearity test checks accuracy across the full range of concentrations the system might encounter.

The annual relative accuracy test (RATA) is the most rigorous. A third-party technician brings reference equipment to the facility and runs it side by side with the CEM system for a full day. If the CEM system's readings differ from the reference equipment by more than a set percentage — usually 10 to 20 percent depending on the pollutant — the system fails and must be repaired or replaced. The results of the RATA are submitted to the state and become part of the public record.

Facilities must also maintain a log of all maintenance, repairs, and downtime. If the CEM system is down for more than four hours in a day, the facility must report it. Extended downtime can trigger an investigation and may result in a violation if the facility cannot show the downtime was unavoidable. Some permits allow facilities to use backup monitoring methods or to estimate emissions during short outages, but the rules are strict and the burden is on the facility to prove compliance.

How CEM data is reported and made public

Facilities report their CEM data to the EPA through a system called the Clean Air Markets Division (CAMD) database or through their state air quality agency, which then forwards it to the EPA. The data is submitted monthly or quarterly, depending on the permit. Large facilities — particularly power plants and refineries — often report continuously through an automated system that uploads data in real time.

The EPA publishes this data on its website and makes it searchable by facility name, location, and pollutant. Most state air quality agencies also maintain their own public databases where you can look up a specific facility's emissions history. Some states provide tools to map emissions by county or to compare facilities side by side. The data is typically available with a lag of a few months, since it takes time for facilities to compile and submit it and for regulators to review it for errors.

Because the data is public, environmental groups, researchers, and concerned residents can track whether a facility is complying with its permit limits. If the data shows repeated violations, it can trigger a state inspection or an EPA enforcement action. Some states also require facilities to post their CEM data on their own websites or to notify nearby residents when emissions exceed certain thresholds.

Violations, penalties, and enforcement

A violation occurs when a facility's CEM data shows that emissions exceeded the permit limit for one or more hours, or when the facility fails to operate or maintain the CEM system correctly. Common violations include operating the system without proper calibration, failing to report data on time, reporting false or incomplete data, or exceeding the permit limit for a pollutant.

Penalties for CEM violations vary by state and by the severity of the violation. A facility that misses a quarterly calibration might receive a warning or a civil penalty of a few thousand dollars. A facility that operates without a functioning CEM system for weeks, or that falsifies data, can face penalties of tens of thousands of dollars per day of violation. In rare cases — particularly when a facility knowingly submits false data — criminal charges can be filed against the company and its officers.

Beyond financial penalties, a facility with a pattern of CEM violations may lose its air permit, be required to install additional pollution control equipment, or be ordered to shut down operations until it comes into compliance. State air quality agencies also use CEM violations as grounds to deny permit renewals or to impose stricter limits in the next permit cycle.

How to find and interpret CEM data for a specific facility

To find CEM data for a facility near you, start with your state air quality agency's website. Most states have a searchable database where you can enter a facility name, address, or permit number and retrieve years of emissions data. If your state does not have an online tool, you can contact the agency directly and request the data under your state's public records law.

The EPA's CAMD database (available at ampd.epa.gov) covers power plants and large industrial facilities nationwide. You can search by facility name or location and read raw data or view charts and summaries. The EPA also publishes annual reports on emissions trends by state and by industry sector.

When you look at CEM data, pay attention to the units (pounds per hour, tons per year, parts per million), the permit limit, and the number of hours the facility exceeded that limit. A single exceedance might be due to a startup or shutdown, which some permits allow. Repeated exceedances, or exceedances during normal operating hours, suggest a compliance problem. Also check whether the facility reported any downtime or maintenance issues that might explain gaps in the data.

Differences between CEM and periodic stack testing

CEM is continuous, but it is not the only way regulators monitor emissions. Many smaller facilities — those that emit below certain thresholds or that operate only seasonally — are required to conduct periodic stack testing instead. A stack test is a one-time measurement performed by a certified technician using portable equipment. The facility schedules the test, notifies the state, and the test typically takes a full day.

Stack testing is less expensive than installing and maintaining a CEM system, but it provides only a snapshot of emissions on the day the test is performed. A facility could be in compliance during the test and out of compliance the rest of the year. For that reason, the EPA requires CEM for facilities that emit large quantities of pollutants or that operate continuously. Smaller or intermittent sources can usually use stack testing, though some states are moving toward CEM for more facilities as the technology becomes cheaper.

Some facilities use both: they operate a CEM system for their primary emission point and conduct periodic stack tests for secondary points or as a backup quality check. The permit specifies which method applies to each emission point at each facility.

Frequently Asked Questions

Can I see the CEM data for a facility in my neighborhood?

Yes. Most state air quality agencies publish CEM data online, and the EPA's CAMD database covers large facilities nationwide. You can search by facility name or address. If the data is not available online, contact your state air quality agency and request it under your state's public records law — they are required to provide it.

What does it mean if a facility's CEM system was down for several days?

It means the facility was not monitoring emissions during that time. Depending on the permit and the reason for the downtime, this may or may not be a violation. If the downtime was due to equipment failure and lasted more than a few hours, the facility must report it to the state. Extended or repeated downtime can trigger an inspection or enforcement action.

How accurate does a CEM system have to be?

The EPA requires CEM systems to be within 10 to 20 percent of a reference measurement, depending on the pollutant and the type of facility. This is verified through an annual relative accuracy test performed by a third-party technician. If the system fails the test, it must be repaired or replaced before the facility can continue operating.

What happens if a facility's CEM data shows it exceeded its permit limit?

The facility must report the exceedance to the state, usually within 24 hours. Depending on how often exceedances occur and why, the state may issue a warning, conduct an inspection, or file an enforcement action. A single exceedance during a startup or equipment malfunction may be excused; repeated exceedances during normal operation typically result in penalties.

Can a facility dispute its CEM data if it thinks the system is wrong?

A facility can request that the state review its CEM system's accuracy and can conduct its own quality checks. However, the burden is on the facility to prove the system is faulty. If the annual relative accuracy test shows the system is within acceptable limits, the data stands. Facilities can also request a variance or a temporary permit modification if they can demonstrate that equipment failure caused an exceedance.