MEMORANDUM

DATE: May 19, 2014

SUBJECT: Risk Assessment to Determine a Health-Based Emission Limitation for Acid 

Gases for the Clay Ceramics Manufacturing Source Category

FROM: Mark Morris

Health and Environmental Impacts Division

Office of Air Quality Planning and Standards

Ted Palma

Health and Environmental Impacts Division

Office of Air Quality Planning and Standards

Chris Sarsony

Health and Environmental Impacts Division

Office of Air Quality Planning and Standards

TO: Docket EPA-HQ-OAR-2013-0290

Section 112(d)(2) of the Clean Air Act (Act) mandates that emission standards be promulgated 

for sources of hazardous air pollutants, and that the standards “shall require the maximum degree 

of reduction in emissions of the hazardous air pollutants subject to this section (including a 

prohibition on such emissions, where achievable) that the Administrator, taking into 

consideration the cost of achieving such emission reduction, and any non-air quality health and 

environmental impacts and energy requirements, determines is achievable for new or existing 

sources in the category or subcategory to which such emission standard applies...” These

standards are referred to as Maximum Achievable Control Technology (MACT) standards.

Section 112(d)(4) of the Act states that, “With respect to pollutants for which a health threshold 

has been established, the Administrator may consider such threshold level, with an ample margin 

of safety, when establishing emission standards under this subsection.” In other words, for 

hazardous air pollutants (HAP) with a health threshold, standards may be promulgated under a 

process different from that otherwise specified in Section 112(d), and these standards are referred 

to as health-based emission limitations (HBEL). This memorandum documents a risk 

assessment performed to determine an HBEL for acid gases for the Clay Ceramics 

Manufacturing source category.
2

Summary

The purpose of the assessment was to determine a level of emissions that would ensure 

health thresholds for acid gases are not exceeded (with an ample margin of safety) at any facility 

in the source category. To determine this level, unit emissions (one ton per year) of hydrochloric 

acid (HCl) were assigned to each facility in the source category, and this level was used as the 

emissions input into EPA’s Human Exposure Model (HEM3). Using the unit emissions along 

with emission release parameters for each facility, HEM3 provided estimates of ambient 

concentrations (and noncancer hazard indices based on those concentrations) per unit emissions 

near the facilities, based on meteorological data near each facility, and at the census block 

centroid locations used as surrogates for human exposure. Using the highest modeled chronic 

concentration for any facility, the unit emissions were scaled up such that an emissions level 

equivalent to a target-organ-specific hazard index (TOSHI) of one (to one significant figure)

could be estimated. This level is 600 tons per year (tpy) of HCl-equivalent emissions, with the 

equivalency based on the chronic Reference Concentration (RfC) values for the acid gases, and 

is equivalent to 420 tpy per year of hydrofluoric acid (HF) or 4.5 tpy of chlorine (Cl2). Because 

the 600 tpy level of HCl-equivalent emissions is based on the chronic RfC values of the acid 

gases, the limit ensures that no chronic threshold for any acid gas would be exceeded. To assess 

the potential for acute health effects and environmental effects from HCl, HF, and Cl2, separate 

HEM3 runs were performed for each pollutant at the emissions levels discussed above for each 

pollutant.

Facilities Assessed

Eight facilities are estimated to be in the Clay Ceramics source category and were 

included in this assessment. These facilities are listed in Table 1.

Table 1. List of Modeled Facilities

Company Name Facility Name Address City State County

Marazzi Group 

S.p.A.

Marazzi, Sunnyvale 

Facility

359 Clay Road Sunnyvale TX Dallas

Dal-Tile Corp. Dal Italia 3801 Dal-Tile Road Muskogee OK Muskogee

Mohawk Industries Dal-Tile El Paso Plant 12001 Railroad 

Drive

El Paso TX El Paso

Marazzi Group 

S.p.A.

Monarch Ceramic 

Tile, Inc.

834 Rickwood 

Road

Florence AL Lauderdale

Florim Ceramiche 

S.p.A

Florim USA 300 International 

Blvd

Clarksville TN Montgomery

Kohler Co. Kohler, Spartanburg 

Plant

4000 South Pine 

Street

Spartanburg SC Spartanburg

Kohler Co. Kohler, Brownwood 

Plant

4601 Hwy 377 

South

Brownwood TX Brown

Kohler Co. Kohler, Wisconsin 

Plant

444 Highland Drive Kohler WI Sheboygan

Emissions and Release Parameters 
3

As discussed above, unit emissions (1 tpy) of HCl were assigned to each facility, and this 

level was used as input into HEM3 to estimate the maximum ambient chronic concentration (at 

census block receptors or other populated receptors) for each facility. The emissions of the 

facility with the highest modeled ambient chronic concentration were scaled up such that they 

were equivalent to a noncancer TOSHI of one. The resulting emissions level is 600 tpy of HClequivalent emissions, which is equivalent to 420 tpy of HF or 4.5 tpy of Cl2. A HEM3 run was 

made for each pollutant’s emissions level to estimate pollutant-specific acute concentrations and 

to create model output files needed to estimate environmental risks. No emissions multiplier was 

used in HEM3 to account for short-term emissions being higher than the annual tpy level. The 

resulting acute modeled estimates reflect 1-hour emissions being simply the annual emissions 

level divided by 8760 hours per year.

Facility information and emission release parameters used in HEM3 were obtained mostly from 

Information Collection Request (ICR) data collected in 2008 through 2011. Where there were 

data gaps, the data were supplemented with information from various data sources, including an 

earlier 1997 ICR and title V permits. For missing release parameters, conservative default values 

(e.g., the 25th percentile of stack height, exit velocity, and temperature) were calculated based on 

the data from other facilities. Where it was determined that kilns were shut down, inactive, or 

area sources (true area or synthetic minor), they were not included in the data. For facilities with 

multiple stacks, the facilitywide emissions level was divided evenly by the number of stacks. 

Where multiple kilns share the same stack, the emissions from those kilns were modeled as being 

emitted from the single stack.

Dispersion Modeling and Estimating Inhalation Exposures

Inhalation exposure concentrations and potential health risks were estimated using 

HEM3, an inhalation risk model that incorporates the American Meteorological Society/EPA 

Regulatory Model (AERMOD) dispersion modeling system (AERMOD version 13350). The 

AERMOD model is a state-of-the-science Gaussian plume dispersion model that is preferred by 

EPA for modeling point, area, and volume sources of continuous air emissions.1 The model is 

used to estimate annual average ambient concentrations through the simulation of hour-by-hour 

dispersion from the emission sources into the surrounding atmosphere, and can also be used to 

estimate short-term ambient concentrations, with the shortest averaging time being one hour. 

Further details on AERMOD can be found in the AERMOD user’s guide,

2 and further details on 

HEM3 can be found in the user’s guides for HEM3.3 For this assessment, annual concentrations 

were estimated to assess chronic health impacts and the potential for environmental effects, and 

1-hour concentrations were estimated to assess acute health impacts. Unless otherwise stated in 

this document, the modeling methodologies used for this assessment are the same as those used 

in air toxics assessments performed for the Risk and Technology Review (RTR) program.

 1 US EPA, 2005. Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat 

and Complex Terrain) Dispersion Model and Other Revisions; Final Rule. 40 CFR Part 51. 

http://www.epa.gov/EPA-AIR/2005/November/Day-09/a21627.htm. 2 US EPA, 2004. User’s guide for the AMS/EPA regulatory model – AERMOD. EPA-454/B-03-001. 

http://www.epa.gov/scram001/7thconf/aermod/aermodugb.pdf.

3 http://epa.gov/ttn/fera/human_hem.html#guide.
4

The first step in the application of the HEM3 modeling system is to predict ambient 

concentrations at locations of interest. The AERMOD model options employed are summarized 

in Table 2 and are discussed further below.

Table 2. AERMOD Version 13350 Model Options

Modeling Option Selected Parameter

Type of calculations Hourly ambient concentration

Source type Point

Receptor orientation

Polar (13 rings and 16 radials)

Discrete (census block centroids and additional residential 

receptors)

Terrain characterization Actual from USGS 1/3 arc second NED data

Building downwash Not included

Plume deposition/depletion Not included

Urban source option Yes

Meteorology 1 year (2011) NWS from nearest site processed with 

AERMET 133501

1 Meteorological data processed with the pre-public release version of AERMET 13350.

See http://www2.epa.gov/fera/download-human-exposure-model-hem for a complete listing of 

meteorological stations.

The HEM3 estimates chronic and acute ambient concentrations at the geographic 

centroids of census blocks (using the 2010 Census), and at other receptor locations that can be 

specified by the user. For chronic exposures, the model accounts for the effects of multiple 

facilities when estimating concentration impacts at each block centroid, and includes only census 

blocks with at least one resident (i.e., locations where people may reasonably be assumed to 

reside rather than receptor points at the fenceline of a facility). Ambient concentrations for all 

averaging times (e.g., 1-hour, annual) are calculated using modeled one-hour concentrations at 

each block centroid. Possible future residential use of currently uninhabited areas is not 

considered. Census blocks, the finest resolution available in the census data, are typically 

comprised of approximately 50 people or about ten households.

For the facilities included in this assessment, census block locations were evaluated to 

determine if they are representative of the residential locations within the blocks. For one 

facility, a block centroid was located on facility property, and this block was moved to a location 

that better represents the residential locations associated with the block. There was also one 

census block with no apparent population, and this block was removed. The census block 

adjustments are listed in Table 3.
5

Table 3. Revised Census Block Centroid Locations for the Clay Ceramics Source Category

Centroid Revisions

Block ID NEI ID New 

Latitude

New 

Longitude

Comment

481130181041091 32.771973 -96.568415

010770109005008 Block removed – no 

apparent population

One year of meteorological data (2011) was used for each facility. These data were 

processed using EPA’s AERMET (version 13350) program, which is the meteorological data 

preprocessor for AERMOD. The ADJ_U* non-Default BETA option that adjusts the surface 

friction velocity (u* or ustar) for low wind speed stable conditions was used. The AERMINUTE 

program (version 11325) is a data preprocessor for AERMET, and was used to process 1-minute 

wind data to generate hourly average winds for input to AERMET. The EPA’s AERSURFACE 

tool (version 13016) was also used to process land cover data to determine the surface 

characteristics for use in AERMET.

Chronic noncancer health hazards are expressed by comparing a chronic exposure to a 

reference level as a ratio. The “hazard quotient” (HQ) is the estimated exposure divided by a 

reference level (e.g., the RfC). For a given HAP, exposures at or below the reference level 

(HQ≤1) are not likely to cause adverse health effects. As exposures increase above the reference 

level (HQs increasingly greater than 1), the potential for adverse effects increases. For a typical 

risk assessment where multiple pollutants are co-emitted, we aggregate noncancer HQs of HAPs 

that act by similar toxic modes of action, or (where this information is absent) that affect the 

same target organ. This process creates, for each target organ, a TOSHI, defined as the sum of 

hazard quotients for individual HAPs that affect the same organ or organ system. Because we 

performed HEM3 model runs for each acid gas individually, we did not aggregate HQ values of 

different acid gases. Of course, multiple acid gas pollutants are emitted at Clay Ceramics

facilities, but the 600 tpy level of HCl-equivalent emissions ensures that a TOSHI of one is not 

exceeded as long as the HCl-equivalent emissions do not exceed 600 tpy. 

Acute Health Risk Screening

For the assessment of potential health risks from acute exposures to the acid gases, we 

performed a screening assessment using conservative assumptions that in combination 

approximate a worst-case exposure. The result of this screening process is that either the facility 

being assessed poses no potential acute health risks (i.e., it “screens out”), or that it requires 

further, more refined, assessment. The acute exposure scenario assumed worst-case meteorology 

(from one year of local meteorology) and that a person is located downwind at the point of 

maximum impact during this same worst-case 1-hour period, but no nearer to the source than 100 

meters.

Screening for potentially significant acute inhalation exposures also followed the HQ 

approach. We divided the maximum estimated acute exposure by each available short-term 

threshold value to develop an array of HQ values relative to the various acute endpoints and 

thresholds. In general, when none of these HQ values are greater than one, there is low potential 

for acute risk. In those cases where HQ values above one are seen, additional information is 

used to determine if there is a potential for significant acute risks. Refined assessment for 
6

facilities in the Clay Ceramics source category included using aerial imagery of the facilities to 

determine the maximum offsite 1-hour concentrations.

Acid Gases Environmental Risk Screening

In addition to potential health impacts, EPA evaluated the potential for environmental 

impacts when considering whether to exercise discretion under CAA Section 112(d)(4).4 We

applied the environmental risk screen methodology that is used in the RTR Program under 

section 112 of the CAA to evaluate the potential for chronic exposure to HCl and HF emitted by 

Clay Ceramics facilities to cause phytotoxicity and reduced productivity of plants.

Although chlorine may also be emitted from Clay Ceramics facilities, chlorine gas is so 

reactive that it is not expected to remain in the environment very long after it is released. 

Chlorine immediately reacts with both organic and inorganic materials that it comes into contact 

with. Chlorine undergoes direct photolysis in the air and its half-life in the troposphere is on the 

order of several minutes. Therefore, it is not considered in the environmental risk screening for 

the Clay Ceramics source category.

The environmental screen uses air concentrations of HF and HCl from the HEM3 model 

used in the human health exposure and risk analysis. The HEM3 modeling domain extends 50 

km from the center of the facility. The defaults for HEM3 are to model 13 concentric rings at 

various distances (out to 50 km) from the facility with 16 concentration data points (one every 

22.5 degrees) on each ring. Therefore, there are a total of 208 data points. All 16 data points on 

rings that we determine to be onsite are excluded from the 208 total. Therefore, the total number 

of data points included in the analysis generally varies depending on facility size. For the Clay

Ceramics source category, we used aerial imagery of the facilities to determine that 200 m is a 

typical facility radius, and we excluded data points inside this radius. 

We derived an area-weighted average offsite annual ambient air concentration for HF and 

HCl over the 50-km modeling domain for each facility. The area-weighted average 

concentrations were compared directly to the appropriate ecological benchmarks for each

pollutant by dividing the area-weighted average concentration by the appropriate ecological 

benchmark. The result is called a hazard quotient (HQ). An HQ greater than one indicated that 

the area-weighted average concentration exceeded the ecological benchmark.

Dose-Response Information

As discussed above, we assessed the potential for health effects from 1-hour and annual 

exposures, and the potential for environmental effects from annual modeled concentrations. The 

development and/or use of health reference values and ecological values are discussed below.

Chronic Exposures

Dose-response assessment for chronic exposure to acid gases was based on existing 

recommendations for HAPs by the EPA Office of Air Quality Planning and Standards 

(OAQPS).

5

 This information has been obtained from various sources and prioritized according 

 4 See Portland Cement NESHAP Final Rule, 75 FR 54970, 54986/3 (Sept 9, 2010). 5 US EPA, 2012. Table 1. Prioritized Chronic Dose-Response Values (5/21/12). Office of Air Quality Planning and 

Standards. http://www.epa.gov/ttn/atw/toxsource/table1.pdf.
7

to (1) conceptual consistency with EPA risk assessment guidelines and (2) level of peer review 

received. The prioritization process was aimed at incorporating into our assessments the best 

available science with respect to dose-response information. The recommendations are based on 

the following sources, in order of priority: 

1) US Environmental Protection Agency (EPA). EPA has developed a doseresponse assessment for chronic exposure to HCl. The dose-response assessment for HCl 

provides a qualitative statement regarding the strength of scientific data and specifies an RfC to 

protect against effects other than cancer. The RfC is defined as an “estimate (with uncertainty 

spanning perhaps an order of magnitude) of a continuous inhalation exposure to the human 

population (including sensitive subgroups) that is likely to be without an appreciable risk of 

deleterious effects during a lifetime.” The EPA disseminates dose-response assessment 

information in several forms, based on the level of review. The Integrated Risk Information 

System (IRIS)6 is an EPA database that contains scientific health assessment information, 

including dose-response information. All IRIS assessments since 1996 have also undergone 

independent external peer review. The current IRIS process includes review by EPA scientists, 

interagency reviewers from other federal agencies, and the public, and peer review by 

independent scientists external to EPA. New IRIS values are developed and old IRIS values are 

updated as new health effects data become available. Refer to the “IRIS Track” website for 

detailed information on status and scheduling of current individual IRIS assessments and updates 

(http://cfpub.epa.gov/ncea/iristrac/index.cfm).

2) US Agency for Toxic Substances and Disease Registry (ATSDR). ATSDR, 

which is part of the US Department of Health and Human Services, develops and publishes 

Minimum Risk Levels (MRLs)7 for inhalation and oral exposure to many toxic substances, 

including chlorine. As stated on the ATSDR web site: “Following discussions with scientists 

within the Department of Health and Human Services (HHS) and the EPA, ATSDR chose to 

adopt a practice similar to that of the EPA's Reference Dose (RfD) and Reference Concentration 

(RfC) for deriving substance specific health guidance levels for non neoplastic endpoints.” The 

MRL is defined as “an estimate of daily human exposure to a substance that is likely to be 

without an appreciable risk of adverse effects (other than cancer) over a specified duration of 

exposure.” ATSDR describes MRLs as substance-specific estimates to be used by health 

assessors to select environmental contaminants for further evaluation. Exposures above an MRL 

do not necessarily represent a threat, and MRLs are therefore not intended for use as predictors 

of adverse health effects or for setting cleanup levels.

3) California Environmental Protection Agency (CalEPA). The CalEPA Office 

of Environmental Health Hazard Assessment has developed a dose-response assessment for HF 

for health effects other than cancer. The process for developing these assessments is similar to 

that used by EPA to develop IRIS values and incorporates significant external scientific peer 

review. As cited in the CalEPA Technical Support Document for developing their chronic 

assessments : “The guidelines for developing chronic inhalation exposure levels incorporate 

 6 US EPA, 2006. Integrated Risk Information System. http://www.epa.gov/iris/index.html. 7 US Agency for Toxic Substances and Disease Registry. 2006. Minimum Risk Levels (MRLs) for Hazardous 

Substances. http://www.atsdr.cdc.gov/mrls/index.html.
8

many recommendations of the U.S. EPA8 and NAS9.” The non-cancer information includes 

available inhalation health risk guidance values expressed as chronic inhalation reference 

exposure levels (RELs)10. CalEPA defines the REL as “the concentration level at or below 

which no health effects are anticipated in the general human population.”

The acid gases, their quantitative inhalation chronic noncancer dose-response values, and the 

source of each value are listed in Table 4.

Table 4. Dose-Response Values for Chronic Inhalation Exposure to Acid Gases

Pollutant CAS Number11

RfC 

(mg/m3) Source

Hydrochloric acid 7647010 0.02 IRIS

Hydrofluoric acid 7664393 0.014 CAL

Chlorine 7782505 0.00015 ATSDR

Acute Exposures

Dose-response assessment for acute exposure to acid gases was also based on the existing 

recommendations of OAQPS for HAPs.12 Depending on availability, the results from screening 

acute assessments are compared to both “no effects” reference levels for the general public, such 

as the California Reference Exposure Levels (RELs), as well as emergency response levels, such 

as Acute Exposure Guideline Levels (AEGLs) and Emergency Response Planning Guidelines 

(ERPGs), with the recognition that the ultimate interpretation of any potential risks associated 

with an estimated exceedance of a particular reference level depends on the definition of that 

level and any limitations expressed therein.

California Acute Reference Exposure Levels (RELs). The California Environmental 

Protection Agency (CalEPA) has developed acute dose-response reference values for many 

substances, expressing the results as acute inhalation Reference Exposure Levels (RELs). The 

acute REL (http://www.oehha.ca.gov/air/pdf/acuterel.pdf) is defined by CalEPA as “the 

concentration level at or below which no adverse health effects are anticipated for a specified 

exposure duration.13 RELs are based on the most sensitive, relevant, adverse health effect 

reported in the medical and toxicological literature. RELs are designed to protect the most 

sensitive individuals in the population by the inclusion of margins of safety. Since margins of 

safety are incorporated to address data gaps and uncertainties, exceeding the REL does not 

automatically indicate an adverse health impact.” Acute RELs are developed for 1-hour (and 8-

 8 US EPA, 1994. US Environmental Protection Agency. Methods for Derivation of Inhalation Reference 

Concentrations and Application of Inhalation Dosimetry. EPA/600/8-90/066F. Office of Research and 

Development. Washington, DC: U.S.EPA.

9 NRC, 1994. National Research Council. Science and Judgment in Risk Assessment. Washington, DC: National 

Academy Press.

10 CA Office of Environmental Health Hazard Assessment, 2005. Chronic Reference Exposure Levels Adopted by 

OEHHA as of December 2008. http://www.oehha.ca.gov/air/chronic_rels/AllChrels.html. 11 Chemical Abstract Services identification number. For groups of compounds that lack a CAS number we have 

used a surrogate 3-digit identifier corresponding to the group’s position on the CAA list of HAPs. 12 US EPA, 2011. Table 2. Acute Dose-Response Values for Screening Risk Assessments (12/19/2011). Office of 

Air Quality Planning and Standards. http://www.epa.gov/ttn/atw/toxsource/table2.pdf. 13 CA Office of Environmental Health Hazard Assessment, 2000. All Acute Reference Exposure Levels developed 

by OEHHA as of May 2000. http://www.oehha.ca.gov/air/acute_rels/allAcRELs.html.
9

hour) exposures. The values incorporate uncertainty factors similar to those used in deriving 

EPA’s Inhalation Reference Concentrations (RfCs) for chronic exposures (and, in fact, 

California also has developed chronic RELs).

Acute Exposure Guideline Levels (AEGLs). AEGLs are developed by the National 

Advisory Committee (NAC) on Acute Exposure Guideline Levels (NAC/AEGL) for Hazardous 

Substances, and then reviewed and published by the National Research Council. As described in 

the Committee’s “Standing Operating Procedures (SOP)”,14 AEGLs “represent threshold 

exposure limits for the general public and are applicable to emergency exposures ranging from 

10 min to 8 h.” Their intended application is “for conducting risk assessments to aid in the 

development of emergency preparedness and prevention plans, as well as real time emergency 

response actions, for accidental chemical releases at fixed facilities and from transport carriers.” 

The document states that “the primary purpose of the AEGL program and the NAC/AEGL 

Committee is to develop guideline levels for once-in-a-lifetime, short-term exposures to airborne 

concentrations of acutely toxic, high-priority chemicals.” In detailing the intended application of 

AEGL values, the document states that, “It is anticipated that the AEGL values will be used for 

regulatory and nonregulatory purposes by U.S. Federal and State agencies, and possibly the 

international community in conjunction with chemical emergency response, planning, and 

prevention programs. More specifically, the AEGL values will be used for conducting various 

risk assessments to aid in the development of emergency preparedness and prevention plans, as 

well as real-time emergency response actions, for accidental chemical releases at fixed facilities 

and from transport carriers.” 

The NAC/AEGL defines AEGL-1 and AEGL-2 as:

“AEGL-1 is the airborne concentration (expressed as ppm or mg/m3) of a substance 

above which it is predicted that the general population, including susceptible individuals, could 

experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, 

the effects are not disabling and are transient and reversible upon cessation of exposure.”

“AEGL-2 is the airborne concentration (expressed as ppm or mg/m3) of a substance 

above which it is predicted that the general population, including susceptible individuals, could 

experience irreversible or other serious, long-lasting adverse health effects or an impaired ability 

to escape.”

“Airborne concentrations below AEGL-1 represent exposure levels that can produce 

mild and progressively increasing but transient and nondisabling odor, taste, and sensory 

irritation or certain asymptomatic, nonsensory effects. With increasing airborne concentrations 

above each AEGL, there is a progressive increase in the likelihood of occurrence and the severity 

of effects described for each corresponding AEGL. Although the AEGL values represent 

threshold levels for the general public, including susceptible subpopulations, such as infants, 

children, the elderly, persons with asthma, and those with other illnesses, it is recognized that 

individuals, subject to unique or idiosyncratic responses, could experience the effects described 

at concentrations below the corresponding AEGL.”

 14 http://www.epa.gov/opptintr/aegl/pubs/sop.pdf.
10

Emergency Response Planning Guidelines (ERPGs). The American Industrial 

Hygiene Association (AIHA) has developed Emergency Response Planning Guidelines 

(ERPGs)15 for acute exposures at three different levels of severity. These guidelines represent 

concentrations for exposure of the general population (but not particularly sensitive persons) for 

up to 1 hour associated with effects expected to be mild or transient (ERPG-1), irreversible or 

serious (ERPG-2), and potentially life-threatening (ERPG-3). 

ERPG values16 are described in their supporting documentation as follows: “Emergency 

Response Planning Guidelines (ERPGs) were developed for emergency planning and are 

intended as health based guideline concentrations for single exposures to chemicals. These 

guidelines (i.e., the ERPG Documents and ERPG values) are intended for use as planning tools 

for assessing the adequacy of accident prevention and emergency response plans, including 

transportation emergency planning and for developing community emergency response plans. 

The emphasis is on ERPGs as planning values: When an actual chemical emergency occurs 

there is seldom time to measure airborne concentrations and then to take action.” 

ERPG-1 and ERPG-2 values are defined by AIHA as follows:

“ERPG-1 is the maximum airborne concentration below which it is believed that nearly 

all individuals could be exposed for up to 1 hour without experiencing other than mild transient 

adverse health effects or without perceiving a clearly defined, objectionable odor.” 

“ERPG-2 is the maximum airborne concentration below which it is believed that nearly 

all individuals could be exposed for up to 1 hour without experiencing or developing irreversible 

or other serious health effects or symptoms which could impair an individual's ability to take 

protective action.”

The acid gases, their quantitative inhalation acute dose-response values, and the source of each 

value are listed in Table 5.

Table 5. Dose-Response Values for Acute Inhalation Exposure to Acid Gases

Pollutant CAS Number

AEGL-1

(1-hr)

(mg/m3)

AEGL-2

(1-hr)

(mg/m3)

ERPG-1

(mg/m3)

ERPG-2

(mg/m3) REL 

Hydrochloric acid 7647010 2.7 33 4.5 30 2.1

Hydrofluoric acid 7664393 0.82 20 1.6 16 0.24

Chlorine 7782505 1.5 5.8 2.9 8.7 0.21

Ecological Risk Benchmarks for Acid Gases

The environmental screening analysis evaluated potential damage and reduced 

productivity of plants due to chronic direct exposure to HCl and HF in the air. The chronic 90-

day benchmarks used in the environmental risk screen for the acid gases are shown in Table 6 

and discussed in the following section. 

 15 American Industrial Hygiene Association, 2011. Current AIHA ERPG Values. 

http://www.aiha.org/insideaiha/GuidelineDevelopment/ERPG/Documents/2011erpgweelhandbook_table-only.pdf. 16 http://www.aiha.org/insideaiha/GuidelineDevelopment/ERPG/Pages/default.aspx.
11

Table 6. Acid Gas Benchmarks Included in the Environmental Risk Screen

a- Note that the human health RfC is 20 μg/m3, which is lower than the ecological benchmark.

Hydrogen Chloride. For HCl, EPA identified chronic benchmark concentrations as 

described in a 2009 EPA document on RTR risk assessment methodologies.17 The chronic 

benchmark for HCl was based on a LOEL from a short-term exposure (20 minutes) that related 

HCl concentration to “changes” in the leaves of 7 out of 8 plant species as reported by Lerman et 

al.18 It was the lowest exposure concentration at which effects of any type were seen (visible 

injury to some proportion of leaves). Haber’s law was used to extrapolate the 1.5 mg/m3 LOEL 

concentration (20-minute exposure) to a 0.5 mg/m3 concentration expected to produce the same 

effect after one hour. The 1-hr estimated LOEL was extrapolated to a chronic benchmark by 

dividing by a factor of ten to yield 0.050 mg/m3, or 50 μg/m3.

Hydrogen Fluoride. For HF, EPA used two chronic benchmark concentrations for 

plants in the environmental screening analysis (Table 6). The value of 0.5 μg HF/m3 is based on 

the Washington State criterion for gaseous HF, and represents a LOEL. The value of 0.4 μg 

HF/m3 is based on the Environment Canada criteria and also represents a LOEL.

To protect vegetation from adverse effects resulting from HF exposure, the Canadian 

Council of Ministers of the Environment19 recommends that HF concentrations not exceed 0.4 

μg/m3 over a 30- to 90-day period; HF concentrations can be higher for shorter exposures). 

Environment Canada20 defined the effect represented by the level of 0.4 μg HF/m3 as:

"The level above which there are demonstrated effects on human health and/or the 

environment. It is scientifically based and defines the boundary between the LOAEL and 

the NOAEL. It is considered to be the level of exposure just below that most likely to 

result in a defined and identifiable but minimal effect. The reference levels have no safety 

factors applied to them, as they are related directly to the LOAEL, and are the most 

conservative estimates of the effect level."

 17 US EPA, 2009. Risk and Technology Review (RTR) Risk Assessment Methodologies: For Review by the EPA’s 

Science Advisory Board with Case Studies – MACT I Petroleum Refining Sources and Portland Cement 

Manufacturing. EPA-452/R-09-006. http://www.epa.gov/ttn/atw/rrisk/rtrpg.html. 18 Lerman, S., O.C. Taylor, and E.F. Darley, 1976. Phytotoxicity of Hydrogen Chloride Gas with a

Short-Term Exposure. Atmospheric Environment, Vol.10, pp. 873-878.

19 CCME. 1999b. Canadian National Ambient Air Quality Objectives: Process and Status. In: Canadian 

Environmental Quality Guidelines, 1999, Canadian Council of Ministers of the Environment, Winnipeg. Publication 

No. 1299, ISBN 1-896997-34-1. Available at http://ceqg-rcqe.ccme.ca/download/en/133/. 20 EC. (Environment Canada). 1996. National Ambient Air Quality Objectives for Hydrogen Fluoride (HF). Science 

Assessment Document. A Report by the CEPA/FPAC Working Group on Air Quality Objectives and Guidelines. 

July. ISBN 0-662-25641-7, Catalogue En42-17/6-1997. Available online at: 

http://www.bape.gouv.qc.ca/sections/mandats/ap50_rio_tinto_alcan/documents/DQ3.1.1.pdf.

Acid Gas Chronic 90-day Benchmark in μg/m3

Hydrochloric acid - NEL 50a

Hydrofluoric acid – Plant Community LOEL 0.5

Hydrofluoric acid – Plant Community LOEL 0.4
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High concentrations of HF in the air have also been linked to fluorosis in livestock. 

However, the HF concentrations at which fluorosis in livestock occur are higher than those at 

which plant damage begins. Therefore, the benchmarks for plants are protective of both plants 

and livestock.

Results and Risk Characterization

Because the 600 tpy level of HCl-equivalent emissions was determined such that no 

facility would exceed a TOSHI of one, no facility in the individual acid gas model runs exceeded

this level. Therefore, adverse chronic health effects are not likely to occur. However, because 

the emissions equivalency was based on chronic dose-response values, the 600 tpy level does not 

necessarily ensure that acute reference levels will not be exceeded. For the HCl and Cl2 model 

runs, there were no facilities with acute screening HQ values exceeding one. For the HF model 

run (420 tpy), there were multiple facilities with screening HQ values (based on the REL) 

exceeding one, with the highest being an HQ of two. There were no facilities with an acute 

screening HQ value based on the AEGL-1 that exceeded one. We reviewed the cases where the 

HQ (REL) exceeded one by examining aerial imagery of the facilities to determine the maximum 

offsite HQ values. Table 8 gives the facilities with screening HQ (REL) values exceeding one, 

along with their screening HQ values and maximum offsite HQ values. 

Table 8. Maximum Acute Screening and Offsite HQ (REL) Values for Clay Ceramics

Facilities (based on 420 tons per year of HF).

Facility

Maximum Acute Screening

HQ (REL) Value

Maximum Offsite Acute HQ 

(REL) Value

01077NEI8517 2 2

40101NEIOK1015305 2 <1

45083NEI41474 2 2

47125NEITNRCC123 2 1

48049NEI13131 2 1

48113NEI12943 2 <1

48141NEI41683 <1 <1

55117NEIWI1179993 2 1

Although the maximum offsite acute HQ (REL) values for several facilities exceed one, 

we conclude that there is low potential for acute health effects for several reasons. First, the 

maximum offsite HQ (REL) value is only two, and the maximum offsite HQ based on the 

AEGL-1 is less than one. Also, the acute scenario is conservative in that it assumes there is a 

person present at the location and time where the maximum HQ value occurs. Finally, it is 

unlikely that a facility would emit only HF, and emissions of the other acid gases did not result 

in HQ values exceeding one.

For HCl, the environmental risk screen indicated that the area-weighted average modeled 

concentration of HCl around each facility (i.e., the area-weighted average concentration of all 

offsite data points in the modeling domain) did not exceed the ecological benchmark. In 

addition, the ecological benchmark was not exceeded at any offsite receptor location for any 

facility. For HF, the environmental risk screen indicated that the area-weighted average modeled 

concentration of HF around each facility did not exceed the ecological benchmark. There were 
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multiple facilities with modeled concentrations of HF at offsite receptor locations that exceeded 

the ecological benchmark, but the area over which the value was exceeded was no greater than

one percent of the offsite modeling domain for each facility, indicating that there would not be 

any significant or widespread environmental effects.

In summary, we conclude that 600 tpy of HCl-equivalent emissions per Clay Ceramics

facility is unlikely to result in adverse chronic or acute human health effects at any facility in the 

source category. The emissions level is such that the chronic health reference values are not 

exceeded at any facility. Although the acute screening assessment indicates there are small 

exceedances of the acute REL values at some facilities, we conclude that there is low potential 

for acute health effects because the maximum offsite HQ (REL) value is only two and the acute 

exposure scenario is conservative in that it assumes there is a person present at the location and 

time where the maximum HQ value occurs. We also conclude that there is low potential for 

significant and widespread environmental effects. For HCl and HF, the environmental risk 

screen indicated that the area-weighted average modeled concentrations around each facility (i.e., 

the area-weighted average concentration of all offsite data points in the modeling domain) did 

not exceed the ecological benchmarks. In addition, the HCl benchmark was not exceeded 

anywhere offsite for any facility, and the area over which the HF benchmark was exceeded was 

no greater than one percent of the offsite modeling domain for each facility in the category.