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EPA in Illinois

Questions and Answers: National Air Toxics Assessment (NATA)

National Air Toxics Assessment (NATA)

What was the highest risk exposure number your model generated? What was the lowest risk exposure number? Has the level you calculated been independently validated through real-world cancer data?

The National Air Toxics Assessment (NATA) provides risk estimates that are a “snapshot” in time. EPA’s most recent NATA is based on emissions data from 2014. We do not have sufficient information to characterize risks for prior years. For example, even though EPA conducted NATAs for prior years (1996, 1999, 2002, 2005, and 2011), it’s not meaningful – and sometimes misleading – to compare these assessments to the 2014 NATA, because we have improved the NATA source inventory, made modeling changes, revised background calculations, and updated some health benchmarks.  

The lack of detailed emissions information for this facility for prior years– is a significant limitation with being able to assess past risks. But we do know that current emissions have changed since 2014 in light of the additional controls put in place in July 2018.  We are in the process of gathering the detailed emissions information we need to assess current risks for people in Willowbrook.

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What is the difference between the National Air Toxics Assessment and the ATSDR letter?

EPA’s National Air Toxics Assessment, or NATA, is a nationwide screening tool. Its purpose is to help air quality agencies determine if they need to look closer at particular areas, pollutants, or types of pollution sources to better understand risks to public health.

NATA estimates potential risk from long-term exposure to 180 different pollutants called “air toxics.” It estimates potential risk across the entire U.S., at the census tract level.

The ATSDR letter is reporting the results of what is known as a “health consultation.” ATSDR conducts health consultations to provide information on potential environmental hazards in a community, and whether these hazards may adversely impact health. EPA’s Region 5 asked ATSDR to answer the question, “If modeled and measured ethylene oxide concentrations represent long term conditions, would they pose a public health problem for people living and working in Willowbrook?”  The health consultation letter reports ATSDR’s preliminary answer to that question.

Neither NATA nor the ATSDR letter is considered a full risk assessment, and neither can tell any one person if they are going to get cancer. But both identified ethylene oxide in the air in the Willowbrook area as an issue that needs to be addressed. EPA is going to conduct a more refined risk assessment in the Willowbrook area to help us understand current ethylene oxide levels in the air and whether those levels need to be lower.

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The 2014 National Air Toxic Assessment identified DuPage county as having two of the worst census tracts in total cancer risk. Why are we just finding out about this now? 

In August of 2018, EPA released its 2014 National Air Toxics Assessment or NATA. It is called the 2014 NATA because it is based on 2014 air emission levels.

For the 2014 NATA, EPA used new estimates of the cancer potency of ethylene oxide that were issued in December of 2016 and not available for the previous version of NATA (2011). This means that in the 2014 NATA, more areas show elevated risks caused by ethylene oxide than in the 2011 NATA. This does not necessarily mean there is more of this compound in the air in these places than before.

As EPA was finishing the 2014 NATA, it was working on a parallel track with Illinois EPA and Sterigenics to reduce its emissions. In June of 2018, Sterigenics proactively submitted a construction permit application for additional controls to reduce EtO emissions. After review from the EPA and Illinois EPA, Illinois EPA issued the permit on June 26, 2018, and Sterigenics installed emissions controls at the facility in July.

The EPA, Illinois EPA, and Sterigenics are in the process of conducting additional analyses to understand:

  • actual concentrations of EtO in the air now that the controls are in place,
  • current health risks based on those concentrations, and
  • any further measures we need to take to reduce emissions.

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My family currently lives in an area colored on the NATA map as having the highest risks from ethylene oxide, but I believe that is only as current as of 2014?

Correct, the current NATA released in 2018 is based on estimates of emissions in 2014. Sterigenics installed additional emissions controls in August 2018 to reduce ethylene oxide emissions at the facility.

The colors in the NATA map are based on estimates of risks, which are a combination of emissions, how those emissions spread in the air, and exposure to people.

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What other towns have toxic levels of cancer causing chemical from Sterigenics? How toxic is the air in Westmont/ Oak Brook?

The National Air Toxics Assessment (NATA) is a screening tool, intended to help U.S. EPA and state, local and tribal air agencies determine if areas, pollutants or types of pollution sources need to be examined further to better understand risks to public health. NATA provides broad estimates of the risk of developing cancer and other serious health effects over census tracts across the country. It does not estimate any person’s individual risk. For example, the 2014 NATA estimated health risks from all toxic air pollutants in census tracts in the Westmont area are approximately 50-in-1 million.
To see NATA results for other areas, go to the 2014 NATA map application at:

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What is a healthy level to gauge the 50-in-a-million Westmont air by?

In general, based on EPA’s National Air Toxics Assessment (NATA), the U.S. EPA estimates that the average cancer risk across the U.S. population, specifically due to air toxics, to be 30 in a million.

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Why did US EPA inform Illinois Gov. Rauner and Sterigenics in 12/2017 about cancer risks by Sterigenics , while public informed  9 months later?

NATA takes many months to put together – it estimates risks nationwide and includes emissions information for tens of thousands of facilities, along with all types of mobile sources.  We work closely with our state regulatory partners as NATA is developed to ensure that the information it contains is accurate.  Part of that involves us contacting state and local environmental agencies and facilities to verify the emissions information they have reported to our National Emissions Inventory.  It was in the process of these verification activities that the U.S. EPA contacted the State and Sterigenics.

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You use modeled data to determine the amount (I assume in µg/m3) of EtO for any given tract in your map.  That quantity then is used to calculate the risk as n in a million.  I want to find that quantity for each tract for Illinois DuPage and Cook counties near the Sterigenics facility.  Can you provide any support to obtain that information?

You are correct that we use modeled exposure concentrations to estimate risks. The NATA mapping application does not contain these exposure concentrations but, rather, includes what are called ambient air concentrations. Exposure concentrations are calculated from the outdoor ambient air concentrations by adjusting for how much time people spend at certain locations (like their home, school, work, etc.) and how the indoor concentration may be lower than the outdoor ambient concentration. Although not available in the mapping application, the exposure concentrations for each pollutant are available on the NATA website at:, under the section called 2014 “Modeled Ambient Concentrations, Exposures and Risks”.

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Is there a NATA map available from the late 80s that would show cancer risk using the latest 2014 assessments? Particularly regarding Willowbrook, Illinois and the surrounding area, including Darien, IL.

The first NATA was done for the year 1996, and has been done since then for the years 1999, 2002, 2005, 2011, and 2014. Although data for all these versions of NATA are available, we do not recommend comparing the results between the years. We continue to improve NATA’s methods. We have improved the NATA source inventory, made modeling changes, revised background calculations, and updated some health benchmarks. That’s why it’s not meaningful – and sometimes it’s even misleading – to directly compare the 2014 NATA with previous assessments.

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