-More likely to have been measured
-Calculations based on available data on concentration and flow rate of emission
-Unavailability of analytical techniques of many chemicals limits this approach
-Total hydrocarbon analysis can be used to set up upper limit to the estimate
-Emission factor specific to some point sources (e.g. reactor vent for ethylene dichloride production) are available and should be used if monitoring data are not available
-When these approaches are not possible, use mass balance calculations, engineering calculations, design data