Stack testing, also known by its many other names such as isokinetic source sampling, fixed-source emission testing, flue gas testing and isokinetic stack sampling, aims to collect a representative sample of the gas inside a chimney (otherwise known as a smokestack or just "stack") to be analyzed so that we know how much air pollutants are being emitted into the environment.  

One of the most important parts of getting a representative sample is the need to sample isokinetically. The gas we exhaust from the flue gas stream in the chimney/stack must be at the same ("iso") velocity ("kinetic") as the velocity of the flue gas stream itself. And because the temperatures and velocities at different spots in the chimney/stack are all different and changing constantly, this makes sampling isokinetically a challenging task.  

A smokestack is an example of a point source and stationary (fixed) source of air pollutants (as opposed to area/fugitive sources such as dust from unpaved roads or mobile sources such as vehicles). Pollutants include dioxins and furans, "dust" (i.e., particulate matter, solid particulates), Mercury (Hg), Volatile Organic Compounds (VOCs) and greenhouse gases causing anthropogenic climate change like Carbon Dioxide (CO2).

Data from the stack test is usually reported to the regulatory authorities (Environmental Protection Agencies, Ministries of Environment etc.) as is to demonstrate compliance with emission standards/limits. But it is also used in emission inventories whereby people try to find out the total amount of air pollutants discharged into the atmosphere. Data collected from stack testing is also used in Relative Accuracy Test Audits (RATA) to check the quality of data from continuous emission monitors (CEMS/PEMS).