CEMS - continuous emission monitoring systems) are used to monitor industrial smoke stacks in order for the company to maintain compliance to Environmental Protection Agency standards.

CEMS are all of the equipment used to monitor and report emissions, including the probes, gas analyzers, and computers.

Examples of substances which CEMS measure and report to the EPA are:

• NOx- Oxides of Nitrogen
• CO- Carbon Monoxide
• CO2- Carbon Dioxide
• SO2- Sulfur Dioxide
• NH3-Ammonia
• Hg - Mecury
• PM - Particulate Matter

Industries obtain CEMS from vendors such as American Ecotech.

Sources: 

American Ecotech- http://www.americanecotech.com/Air_Monitoring_Equipment/Air_monitor_gas/CEMS.aspx

Wikipedia- http://en.wikipedia.org/wiki/continuous_emissions_monitoring_system

EPA- http://www.epa.gov/ttn/emc/cem.html