Written and maintained by CASRAI Editorial Board
Last updated
Who this is for: researchers, environmental laboratory staff and students who need to understand what a continuous emission monitoring system does and how its data are checked. This is a general explainer, not legal or compliance advice.
Short answer
Continuous emission monitoring is the use of a continuous emission monitoring system (CEMS) to measure a gas or particulate concentration or an emission rate from a stack on an ongoing basis, rather than in occasional tests. U.S. EPA defines a CEMS as the total equipment necessary for that determination. Because the data can be used for compliance decisions, the system is checked on a fixed schedule against calibration and reference-method tests.
Definition and scope
EPA’s Emission Measurement Center describes a CEMS as “the total equipment necessary for the determination of a gas or particulate matter concentration or emission rate.” The analyzer measurements are combined with a conversion equation, graph or computer program so that results are expressed in the units of the applicable emission limit or standard (EPA EMC, read 2026-11-08).
CEMS are required under certain EPA rules, mainly to determine continuous compliance or to identify exceedances. Each rule’s subpart names the reference methods used to verify accuracy and precision. This page covers the general concept and, as a worked example, the Acid Rain program monitoring rules in 40 CFR Part 75; other programs have their own requirements.
For wider context on U.S. environmental data sources see Envirofacts and the Toxics Release Inventory; for the engineering discipline behind emission control see what is environmental engineering.
How a CEMS is used in practice (40 CFR 75.10)
Section 75.10 of 40 CFR Part 75 sets general operating requirements for units in the Acid Rain program. It specifies which quantity is measured with which kind of monitor:
| Quantity | Monitoring approach named in 75.10 |
|---|---|
| Sulfur dioxide (SO2) | SO2 CEMS plus a flow monitor (reported as ppm, scfh and lb/hr) |
| Nitrogen oxides (NOx) | NOx-diluent CEMS: a NOx monitor plus an oxygen or carbon dioxide diluent monitor, giving a NOx emission rate in lb/mmBtu; NO2 must be accounted for |
| Carbon dioxide (CO2) | CO2 CEMS with a flow monitor, or a fuel carbon-content calculation (Appendix G), or an O2-based option (Appendix F) |
| Opacity | Continuous opacity monitoring system (COMS) |
| Heat input rate | Determined in mmBtu/hr for each hour in which fuel is combusted |
Operating rules in the same section: the monitors must operate at all times the affected unit combusts any fuel, except during calibration, quality assurance, maintenance, repair and recertification. Gas monitors complete a sampling cycle at least every 15 minutes, and an hourly average requires a valid data point in each quadrant of the hour. Opacity is sampled every 10 seconds and averaged over 6-minute periods. If these conditions are not met the hour is invalid, and missing-data substitution applies under subpart D. Data from a backup monitor can be used only when the primary monitor is not operating or is out of control (40 CFR 75.10, read 2026-11-08).
CEMS, COMS and PEMS
- CEMS measures gas concentrations (and, with a flow monitor, mass emission rates) directly.
- COMS measures opacity, the extent to which the stack gas blocks light; Part 75 requires it for opacity monitoring.
- PEMS (predictive emission monitoring system) estimates emissions from process or control-device operating parameters instead of measuring the pollutant. EPA notes it is still studying PEMS for NOx compliance use (EPA EMC).
Two sets of EPA quality documents are often confused. Performance specifications such as PS 2 (40 CFR Part 60, Appendix B) evaluate whether a CEMS is acceptable at installation or soon after. Appendix F to Part 60 covers the ongoing quality-control and quality-assurance procedures and data-quality assessment for CEMS used in continuous compliance. Part 75 has its own quality assurance appendix, summarised next.
How the data are checked: Part 75 Appendix B
Appendix B to Part 75 defines the quality-assurance tests that keep CEMS data valid. The figures below are those stated in the appendix (read 2026-11-08):
| Check | Frequency | Key criterion | Section |
|---|---|---|---|
| Daily calibration error test | Daily | Out of control if error exceeds 5.0% of span (SO2, NOx), 1.0% CO2 or O2 (diluent monitors), or 6.0% of span (flow). A passed test validates data for 26 clock hours | 2.1.1, 2.1.4, 2.1.5 |
| Linearity check | Once per QA operating quarter; never more than four successive calendar quarters without one | Checks the analyzer response across its range | 2.2.1, 2.2.3(f) |
| Relative accuracy test audit (RATA) | Semiannual as standard; annual if relative accuracy is 7.5% or better; never more than eight successive calendar quarters without one | Semiannual frequency applies when relative accuracy is above 7.5% up to 10.0% | 2.3.1.2 |
| Bias test | After each passed RATA of SO2, NOx and flow monitors | A failed bias test leads to a bias adjustment factor and is not by itself an out-of-control condition | 2.3.2, 2.3.4 |
The pattern is worth noting: the daily test is cheap and frequent, the RATA compares the CEMS with an independent reference method and is the heavy periodic audit, and the bias test decides whether a correction factor is needed.
Where it matters for environmental laboratories
Emission-testing laboratories and stack-testing teams meet CEMS mainly through RATAs, where reference-method results are compared with the CEMS readings, and through the calibration gases and records that support the daily and quarterly checks. Researchers using CEMS-derived emissions data should note which rule produced the data, because validity rules (for example the hourly-validity and missing-data provisions above) differ by program.
Common misunderstandings
- “Continuous” does not mean every instant. Under 75.10 the data are reduced to hourly averages from cycles of at least one reading every 15 minutes (opacity: 6-minute averages).
- A CEMS is more than the analyzer. EPA’s definition covers all equipment needed to get results in the units of the standard, including the conversion step.
- A passed daily test is not permanent validation. It validates data for 26 clock hours.
- A failed bias test is not an out-of-control event. It triggers an adjustment factor.
- Part 75 is not every program. The specifics above come from Part 75; other EPA rules cite their own performance specifications and reference methods.
FAQ
What does CEMS stand for?
Continuous emission monitoring system: the total equipment needed to determine a gas or particulate concentration or emission rate on a continuing basis.
What is the difference between CEMS and PEMS?
A CEMS measures the pollutant directly. A PEMS estimates emissions from operating parameters of the process or control device.
How often is a CEMS checked under Part 75?
Daily calibration error tests, a linearity check each QA operating quarter, and RATAs semiannually or annually depending on relative accuracy, with a bias test after each passed RATA for SO2, NOx and flow monitors.
Sources
- U.S. EPA Emission Measurement Center, Continuous Emission Monitoring Systems, read 2026-11-08.
- 40 CFR 75.10, General operating requirements, read 2026-11-08.
- 40 CFR Part 75 Appendix B, Quality Assurance and Quality Control Procedures, read 2026-11-08.
