---
title: "Continuous Emissions Monitoring System (CEMS)"
description: "A Continuous Emissions Monitoring System (CEMS) is the suite of instruments that measures stack emissions in real time. A typical industrial CEMS measures opacity, particulate matter, NOx, SOx, CO, O₂, moisture and gas flow. CEMS data is the primary basis for environmental-compliance reporting under most jurisdictions' emission permits."
canonical_url: "https://sylio.co/glossary/cems"
last_updated: "2026-06-28T02:29:34.404Z"
---

A **Continuous Emissions Monitoring System (CEMS)** is the suite of instruments that measures stack emissions in real time. A typical industrial CEMS measures [opacity](/glossary/opacity), [particulate matter](/glossary/particulate-matter), [NOx, SOx, CO](/glossary/nox-sox-co), O₂, moisture and gas flow. CEMS data is the primary basis for environmental-compliance reporting under most jurisdictions' emission permits.

## CEMS quality assurance

CEMS instruments are governed by quality-assurance frameworks:

- **EU** — [EN 14181](/glossary/en-14181-en-13284) (QAL1, QAL2, QAL3 and AST)
- **US** — EPA Reference Method 6, 7, 19 etc. plus Part 75 CEMS requirements
- **National regulators** — various local specifics

## How cleaning intersects with CEMS data

Operators see fouling-driven degradation of [ESP](/glossary/electrostatic-precipitator) or [baghouse](/glossary/baghouse) performance in near-real-time on the CEMS trace. A rising opacity baseline, more frequent excursions, or trended particulate increase all indicate worsening collection. Active [sonic-horn](/glossary/sonic-horn) cleaning that defends collection efficiency shows up on CEMS as flatter, lower, more predictable traces.

## Related terms

- [Opacity](/glossary/opacity)
- [Particulate matter](/glossary/particulate-matter)
- [NOx / SOx / CO](/glossary/nox-sox-co)
- [EN 14181 / EN 13284](/glossary/en-14181-en-13284)