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Emissions Inventories

IDEM maintains an emissions inventory for hazardous air pollutants (HAPs), also referred to as air toxics, which are substances that are known or suspected to cause cancer and other serious non-cancer health effects. Section 112 of the federal Clean Air Act (CAA) contains requirements for the reduction of air toxics from human sources and provides an initial list of 189 compounds, which U.S. EPA has modified to the current list of 187 regulated air toxics. IDEM’s HAPs emissions inventory includes a list of the sources of these pollutants in Indiana, along with the estimated amount of the pollutants each source emits over a certain timeframe. Categories of inventoried sources include “point” sources, “area” sources and “mobile” sources. Following are descriptions:

  • Point Sources:
    • Point sources include stationary sources that are major sources of emissions such as power plants, steel mills, and factories. U.S. EPA defines a major source as emitting 10 tons or more of a single air toxic or 25 tons or more of a combination of air toxics. These sources may release air toxics from equipment leaks, when materials are transferred from one location to another, or during discharge through emission stacks or vents.
  • Area Sources:
    • Area sources include stationary sources with emissions below major thresholds and include facilities that emit less than 10 tons per year of a single air toxic or less than 25 tons per year of a combination of air toxics. Though individual emissions from small area sources are often relatively low, the combined, collective impact from area sources can be a significant concern, particularly where many area sources are located in close proximity. Examples of area sources include emissions from common consumer and commercial products (such as household cleaners and lawnmowers), gas stations, small surface coaters, dry cleaners, and other smaller emission sources.
  • Mobile Sources:
    • Mobile sources include on-road and off-road gasoline and diesel engines, aircraft, commercial marine vessels, and railroad emissions. U.S. EPA currently provides area and mobile source emissions estimates and provides data on its National Emissions Inventory (NEI) website.

How IDEM Uses the HAPs Emissions Inventory

IDEM and U.S. EPA use IDEM’s HAP emissions inventory for several purposes including but not limited to studies, the development of state and national air toxics models, and U.S. EPA risk and technology reviews. HAPS inventory data may also be useful for identifying potential sources for certain HAPS identified by Indiana’s Air Toxics Monitoring program.

How IDEM Acquires HAPs Emissions Data

In Indiana, sources of air toxics are not required by law to report HAPs emissions data to IDEM. However, IDEM utilizes several resources to ensure Indiana’s HAPs Emissions Inventory is as complete and accurate as possible:

  • Voluntary Reports from Indiana Sources:
    • Certain major sources are required by law to report to IDEM their emissions of criteria pollutants (including carbon monoxide, lead, nitrogen dioxide, volatile organic compounds associated with ozone, particulate matter [PM10 and PM2.5] and sulfur dioxide). Although they are not required to report HAP emissions, sources that are required to report criteria air pollutants will sometimes voluntarily include HAP emission estimates.
  • U.S. EPA’s National Emissions Inventory:
    • The National Emissions Inventory (NEI) is a comprehensive and detailed estimate of air emissions of both criteria and hazardous air pollutants from all air emissions sources. U.S. EPA issues the NEI every three years (triennially) based on emission estimates and emission model inputs provided by IDEM, as well as supplemented data that U.S. EPA develops.
  • U.S. EPA’s Toxics Release Inventory (TRI)
    • U.S. EPA’s Toxics Release Inventory (TRI) Program includes estimations of releases and waste management activities reported annually by certain industries that are subject to the reporting program. TRI includes stack and fugitive air releases as reported by facilities for a large number of HAPS.
  • U.S. EPA National Air Toxics Assessment (NATA):
    • The NATA inventory is U.S. EPA’s most recent risk modeling inventory. It is built on the NEI, which U.S. EPA also issues every three years and incorporates state Quality Assurance (QA) revisions.

The geographic area of IDEM’s HAPs inventory is statewide. The point sources included in the HAPs inventory are coded to a certain geographic point using county, city, address, zip code, and Universal Transverse Mercator (UTM), or latitude and longitude coordinates. Area and mobile source estimates are compiled at the county level and apportioned accordingly in modeling studies.

The HAPs inventory has been evolving since its inception in the mid-1990s. Initial HAP inventories were more limited in scope due to limits on available knowledge, information, and resources. Improvements in HAP emissions estimating techniques and available information have resulted in improved estimates today, but quality data is still lacking in many cases. Changes in criteria pollutant reporting requirements have resulted in a complete HAP inventory every three years.