Continuous emission monitoring systems (CEMS) were historically used as a tool to monitor flue gas for oxygen, carbon monoxide and carbon dioxide to provide information for combustion control in industrial settings. They are currently[when?] used as a means to comply with air emission standards such as the United States Environmental Protection Agency’s (EPA) Acid Rain Program, other federal emission programs, or state permitted emission standards. Facilities employ the use of CEMS to continuously collect, record and report the required emissions data.

The standard CEM system consists of a sample probe, filter, sample line (umbilical), gas conditioning system, calibration gas system, and a series of gas analyzers which reflect the parameters being monitored. Typical monitored emissions include: sulfur dioxide, nitrogen oxides, carbon monoxide, carbon dioxide, hydrogen chloride, airborne particulate matter, mercury, volatile organic compounds, and oxygen. CEM systems can also measure air flow, flue gas opacity and moisture.

In the U.S., the EPA requires a data acquisition and handling system to collect and report the data. SO2 emissions must be measured in pounds per hour using both an SO2 pollutant concentration monitor and a volumetric flow monitor. For NOx, both a NOx pollutant concentration monitor and a diluent gas monitor are required to determine the emissions rate (lbs/mmBtu). Opacity must also be monitored. NOx measuring is not a current requirement, however if monitored, a CO2 or oxygen monitor plus a flow monitor should be used. In monitoring these emissions, the system must be in continuous operation and must be able to sample, analyze, and record data at least every 15 minutes and then averaged hourly.

note: CAMS= Continuous Atmosphere Monitoring Systems