the equipment manufacturer. Information on fuel composition (i.e., sulfur and ash content) may  be obtained from the fuel supplier.  Emissions from boilers and heaters are estimated based on the amount and type of fuel burned by  those sources over the time period being analyzed. Emissions for each pollutant are calculated by  multiplying the fuel consumption rate by the emission  factor specific to each pollutant. This  technique is repeated for each boiler or heater in order  to obtain total emissions for these  stationary combustion sources.  As shown in  Equation A5-1  ( Total Pollutant Emissions for Boilers and Heaters ), the total fuel  consumption is multiplied by the emission factor to estimate emissions from the boiler or heater.  The accuracy of this methodology depends upon the ability to determine an appropriate emission  factor for each pollutant. The following section describes how to obtain EPA-approved emission  factors for each combustion source.  EPA provides emission  factors for a wide range of boilers, but provides little emission  information specific to heaters. Therefore, for the purpose of airport emissions inventories it can  be assumed that the emission  factors provided for boilers are applicable to heaters of a  comparable size and fuel type.  Emission factors for most stationary combustion sources found at airports are found within  Chapter 1,  External Combustion Sources,  of EPA’s  Compilation of Air Pollution Emissions  Factors  (i.e., AP-42) 20 . These emission factors take into consideration several elements that have  a significant impact on pollutant emissions, including:  fuel type, fuel sulfur content, fuel ash  content, boiler size, boiler type, and pollution control equipment. The emission factors calculated by obtaining the uncontrolled emission factor based on fuel type, boiler size and boiler  23$_{ }$ 20 EPA, AP-42, Fifth Edition,  Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and  Area Sources,  http://www.epa.gov/ttnchie1/ap42/.  E = F x EF  Total pollutant emissions from boilers and heaters may be estimated by applying  Equation A5-1  ( Total Pollutant Emissions for Boilers and Heaters ).  Equation A5-1. Total Pollutant Emissions for Boilers and Heaters  APPENDIX A5$_{ }$ A5.2.1.1  Methodology  A5.2.1.2  Emission Factors  Where:  E = total emissions of pollutant from the stationary source for a given time period, expressed  in pounds.  F = total amount of fuel consumption for a given time period; liquid fuels should be expressed  in terms of thousand gallons, natural gas as million cubic feet, and coal as tons.  EF = emission factor of pollutant (i.e., CO, SO$_{2}$, PM$_{10}$, PM$_{2.5}$, NO$_{x}$, or VOC), expressed in pounds  per thousand gallons (for liquid fuels), million cubic feet (for natural gas), or ton of fuel (for  coal). 