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February 2008 Revised TPQ Method for EHS solids in solution

I. Introduction:

EPA is proposing a rule to consider an alternative approach for the threshold planning quantity 

(TPQ) for chemicals on the Extremely Hazardous Substances (EHSs) list that exist as solids in 

solution form. The current TPQ for solids in solution is based on an assumption that the entire 

quantity of the solid chemical in solution at a facility has the potential to become airborne in the 

event of an accidental release. Syngenta petitioned EPA to revise the TPQ for the chemical 

paraquat dichloride, a solid chemical which is used commercially in solution. EPA researched 

data and studies on aerosol air releases of liquids and solutions to investigate how much aerosol 

remains airborne during accidental air releases. Results of this search (USEPA May 2007) show 

that much less than 100% of an amount of solid in solution involved in an accidental release to 

air would remain airborne for distances beyond a facility’s fence line. Therefore, EPA believes 

that the assumption of 100% potential airborne release for solids in solution is too conservative 

and a new approach is warranted. 

II. Background

The TPQs were originally developed as relative measures of concern rather than absolute values. 

The threshold planning quantity is not necessarily applicable to any specific situation with 

respect to predicting the actual quantity or “safe” level. This is best determined by the local 

planning authorities using specific parameters for that facility because there are wide variation of 

site-specific conditions that affect airborne dispersion. 

TPQs were developed using a ranking approach based on each chemical’s physical state, acute 

toxicity and “V”, the potential for the chemical to become airborne. The ranking index was 

defined as the Level of Concern (LOC) divided by V. V was set equal to 1 for gases and solids 

in powder form with particle size less than 100 microns. For solids with particle size of 100 

microns and higher, the TPQ is 10,000 pounds. For chemicals that are molten solids, the amount 

on-site is multiplied by 0.3 to before comparing to the lower TPQ for reporting purposes because 

0.3 is the maximum fraction estimated to potentially volatilize.

For liquid EHS, V is a calculated number based on a volatilization modeling equation and is 

actually the rate of volatilization (mass vaporized per unit time) per mass of liquid spilled for a 

spill, assumed to be 1 cm deep.

III. Proposed Revised Method for Solids in Solution TPQ

Based on review of experimental data on accidental releases of solutions, EPA is proposing that 

the maximum fraction of a release of solid in solution aerosols that could be expected to remain 

airborne and affect a community outside a facility’s fenceline is 0.2. Facilities could use the 
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maximum expected airborne release fraction multiplied by the amount of chemical on site and 

then determine if this amount exceeds the established lower TPQ. This procedure is the same as 

is done now for molten solids, except that molten solids apply a fraction of 0.3 (30%). 

IV. Experimental data on accidental releases of aerosols

EPA found a comprehensive study by US Department of Energy (DOE) that compiled 

experimental data on accidental releases of heavy metal (uranium) salts in solution (USDOE 

1994). The study estimated the fractions of liquid aerosol releases that remained airborne for 

various release scenarios. EPA reviewed and summarized the scenarios with the highest airborne 

fractions (ARF) in USPEPA, May 2007. The accidental release scenario with the highest 

airborne release fraction had an ARF of 0.2 The airborne fractions generally contain aerosol 

sizes less than or equal to 100 microns, the size above which droplets are expected to fall out 

before they reach a community outside a facility. 

EPA also looked at experimental data collected by the Center for Chemical Process Safety 

(CCPS) for aerosol releases of water and cyclohexane, although these liquids did not contain any 

solid material in solution. The reported airborne release fraction for water was 0.03 to 0.54 and 

for cyclohexane was 0.36 to 0.94. CCPS also had experimental release data for CFC-11 and 

chlorine (both gases) and methylamine (a highly volatile chemical), but we did not consider this 

data for use as a good analogy because the releases were probably more in the form of vapor than 

liquid aerosols.

V. Use experimental aerosol release data to revise TPQs for EHS solid in solutions. 

A. Appropriate experimental data 

The original TPQ rationale for solid in solution TPQ was based on using conservative worst case 

scenarios and assuming 100% of the solution would be released as aerosol. Both the DOE and 

CCPS data support the argument that assuming that 100% of the liquid aerosol release remains

airborne is too conservative. 

Cyclohexane is a more volatile chemical than water (95 vs 24 mm Hg vapor pressure), so its 

higher airborne release fraction would not be comparable for a solid chemical in water solution. 

Also, the experimental data for water and cyclohexane used by CCPS had much larger mean 

aerosol diameter sizes, generally over 100 microns. Because aerosols larger than 100 microns 

will likely fall out of the air before they reach a community, the CCPS water and cyclohexane 

aerosol experimental data would not be useful for our purposes of determining TPQs.

The DOE data have results from several release scenarios with the maximum airborne release 

fraction of 0.2. The value is based on the scenarios of aqueous solution or air dried salts under 

gasoline fire on a metal surface. Using the highest airborne release fraction rather than an 
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average result of the scenarios would be more consistent with the intent of the emergency 

planning program to plan for worst case scenario. This data is a good surrogate to use to predict 

maximum potential aerosol release fraction of EHS solids in solution in event of an accidental 

release. Water is probably the most common solvent that would be used with most of the EHS 

solids, whether they are dissolved, suspended or emulsified in water. Many of the EHS solids 

are pesticides and pesticides are commonly applied as water solutions or emulsions. 

B. Consistency issues

Liquid EHS could also be released accidentally as aerosols via the same catastrophic scenarios 

(overpressure, superheating). It could be argued that perhaps the TPQ method for solids in 

solution could apply to liquids. However, this goes against the intuitive ranking of the extent and 

likelihood of airborne release by physical state as being higher for gases, lesser for liquids and 

even less for solids in solution. Currently the release scenario used for developing the liquid 

TPQs requires only a spill of the liquid, a loss of containment. The liquid then escapes into the 

air by volatilization. An airborne release of solids in solution will require more than a failure of 

containment and will need an energy source such as overpressure or high temperature to drive 

the solution into the air and create aerosol droplets. Not all of the droplets will stay airborne 

(unlike volatilized vapors) and affect the community, whose exposure depends on droplet size 

and distance to the facility fence line. 

Thus, to address any consistency issues, EPA also wants to determine if the current TPQ method 

for liquids gives more conservative (or at least as conservative) TPQs (lower thresholds) than if 

the same proposed TPQ method for solids in solution was applied to pure liquids. A new liquid 

TPQ based on V=1 was calculated. Then this TPQ was multiplied by 5 (1 / 0.2) to determine the 

amount of liquid a facility could have on-site and this result is referred to as the effective TPQ. 

For 117 out of the 163 EHS liquids, current TPQs based on volatilization were still smaller than 

or equal to the new effective TPQs based on aerosolization. Most of the other 46 liquids had 

effective TPQs that were half the current TPQ. 

VI. Alternative approaches considered

A. Use original ranking index method and develop a new TPQs

TPQs were developed using a ranking approach based on each chemical’s physical state, acute 

toxicity and “V”, the potential for the chemical to become airborne. The ranking index was 

defined as the Level of Concern (LOC) divided by V. V was set equal to 1 for gases and solids 

in powder form with particle size less than 100 microns. For solids with particle size of 100 

microns and higher, the TPQ is 10,000 pounds. For chemicals that are molten solids, the amount 

on-site is multiplied by 0.3 to before comparing to the lower TPQ for reporting purposes because 

0.3 is the maximum fraction estimated to potentially volatilize.
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For liquid EHS, V (the potential to become airborne) depends upon the property of volatility 

(evaporation of liquid into gas phase). The same calculation for V for liquids cannot be used for 

solids which are basically non-volatile materials at ambient temperatures. 

One might argue that for solids in solution, V could equal the airborne aerosol fraction released 

based on experimental data. However, this V would not be directly comparable to the V 

calculated for liquids, because it would have no time component. 

B. Use old TPQ multiplied by maximum aerosol airborne fraction.

Developing and listing a third set of TPQs for solids in solution could be potentially confusing to 

the regulated community. For each solid chemical, there already exists two TPQs, one 

developed based on the ranking index methodology of (Index = Level of Concern/V ) and on the 

default TPQ of 10,000 pounds for all solids not meeting criteria for using the lower TPQ. 

VII. Aerosol size 

One issue that EPA agreed to consider in this rulemaking was whether aerosol size should be 

used as a factor in developing new TPQs for solids in solution. Particle size was considered as a 

factor for the TPQ for powdered solids. In the EHS final rule of April 22, 1987, EPA modified 

the TPQs for solid chemicals so that only powdered solids with particle size less than 100 

microns would be subject to the lower TPQ. Otherwise the TPQ defaults to 10,000 pounds. 

This decision was based on data indicating that a particle of a size less than 100 microns are not 

likely to be deposited more than 100 meters from the source. For example, for a typical wind 

speed of 10 mph, particles larger than about 100 microns are like to settle out within 6 to 9 

meters from the point of emission. Particles that are 30 to 100 micron in diameter are like to 

undergo impeded settling and are likely to settle with a few hundred feet (USEPA 1995). The 

100 micron cutoff is also consistent with CERCLA regulations (for reportable quantities) which 

also uses a 100 micron particle size level for powdered materials. For purposes of establishing 

threshold planning quantities, EPA chose a distance of 100 meters (330 feet) to represent the 

distance from a source inside a chemical facility to the point where the community might be 

exposed. 

A. Syngenta’s proposal

The DOE aerosol release data was developed to estimate airborne fractions of liquid aerosol 

releases that were below respirable size. By DOE’s definition, respirable size particles are those

that can be transported through the air and inhaled into the human respiratory system and are

commonly assumed to include particles 10 micron Aerodynamic Equivalent Diameter (AED) or 
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less. A consultant for Syngenta recalculated some of the DOE airborne aerosol fractions using 

the raw experimental data to include only aerosols sizes ≤ 4 micron diameter for some accident 

scenarios. This resulted in smaller airborne fractions. Syngenta claims this approach was

appropriate because the inhalation acute toxicity test for paraquat dichloride is based on 4 micron

size particles. 

1. Small size aerosols used in toxicity tests

Aerosols may be defined as a suspension of solid or liquid particles in air. Inhalation acute 

toxicity tests are purposely designed with very small diameter particles in order to ensure that 

particles are small enough to enter rodent lungs and test the toxicity in the lungs. Larger 

particles may not enter deep areas of the lungs and thus test results may be misinterpreted if little 

inhalation toxicity is shown. EPA is not attempting to use the airborne aerosol fraction for 

purposes of risk assessment, but only as a tool to set screening levels for the amount of chemicals 

on-site which may potentially cause harm if accidentally released. One cannot assume that the 

size aerosols used in an animal laboratory test would be the same as those in aerosols people may 

be exposed during an accidental release of a solution of solid chemical.

2. Particle size and exposure

According to EPA, inhalable particles enter the respiratory tract, including the head airways and 

are generally ≤ 100 microns. Thoracic particles (≤ 10 microns) travel past the larynx and reach 

the lung airways and the gas-exchange regions of the lung. Respirable particles (≤ 4 microns) 

are a subset of thoracic particles that are more like to reach the gas-exchange region of the lung. 

DOE uses a different size cut (< 10 micron) because radioactive materials they are concerned 

with can be removed from the skin but not following inhalation or ingestion. 

Most particles that are deposited in upper airways are trapped in mucous that moves to the throat 

and is swallowed in a few hours. Thus instead of inhalation exposure deep in the lungs, 

exposure to larger particles of chemicals may occur through dermal exposure to mucous 

membranes or oral exposure through swallowing. Emergency planning for EHS chemicals is not 

limited to inhalation exposure only although many of the EHS chemicals listings are based on 

inhalation toxicity. Although airborne exposure is the mostly likely route of exposure, it is not 

the only route of exposure. EPA is concerned about all routes of exposure (inhalation, dermal 

and oral) to the community. Thus exposure to larger size aerosols (e.g. those above 4 or 10 

microns) by any route should not be ignored when setting TPQs. 

3. Solvent evaporation from aerosols

Even after liquid aerosol droplets are released, some of the solvent may evaporate in air. This 

would result in even smaller size of aerosols or solid EHS particulates in the air to which a 

community would be exposed. One concern is that droplet of size greater than 100 micron could 
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settle quickly, dry into a smaller particle size and then become airborne again (resuspension). 

In event of an accidental release, the responsible party should cleanup chemicals deposited on 

facility grounds before additional exposure to the community would take place. 

Data from the DOE Handbook for resuspension of liquids and powders from soils and other 

surfaces can be used to estimate the fraction of material resuspended to be from 4E-3 to 1E-2.

If the worst case airborne fraction that leaves the facility’s site is assumed to be 2E-1, then the 

fraction of the release that would fall out and be deposited on the facility’s site would be 8E-1. 

Assuming that a worst case fraction 1E-2 (whether as dried solids or liquid droplets) of this 

deposit could be resuspended, the additional fraction of the original release to become airborne 

would be 8E-3 (8E-1 times 1E-2). Adding the resuspended fraction of 8E-3 to the original 

airborne release fraction of 2E-1 would yield a total fraction of 0.208. This does not much add 

much to the original amount and rounding appropriately to 1 significant digit would still result in 

an overall ARF of 2E-1.

B. Feasibility of applying aerosol size cutoff

The particle size of the accidental aerosol release is the relevant consideration and not the 

solution form stored on-site. Aerosol size of an accidental aerosol release is not likely to be 

known or measured by facilities. EPA was able to apply a particle size cutoff for powdered 

solids because the form of the chemical released is the same as that stored on-site. Therefore, a 

particle size cutoff that facilities could apply to solids in solution similar to the size cutoff for 

powdered solids would not be feasible. 

C. Size of experimental aerosols

Most of the DOE experimental aerosol release data had aerosol sizes of less than 100 microns. 

The water and cyclohexane aerosol release data compiled by CCPS had much larger mean 

aerosol diameter sizes, generally over 100 microns. Because it is likely that aerosol releases with 

diameters larger than100 microns will fall out of the air before they reach a community, the 

water and cylcohexane aerosol experimental data are not that useful for our purposes of 

determining TPQs for solids in solution. 

REFERENCES

USDOE. 1994. DOE Handbook, Airborne Release Fractions/Rates and Respirable Fractions for 

Nonreactor Nuclear Facilities. December 1994. US Department of Energy, Washington, DC 

20585 DOE-HDBK-3010-94. Volume 1- Analysis of Experimental Data and Volume IIAppendices. 

USEPA. May 2007. Literature Survey for Liquid Aerosol Release Data to Support TPQ 
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Methodology for EHS Solids in Solution, May 11, 2007. Office of Solid Waste and Emergency 

Response, Office of Emergency Management. 

USEPA 1995. AP-42 Emission Factors, Fifth Edition, Volume I. Chapter 13 Miscellaneous 

Dust Source. 13.2 Introduction to Fugitive Dust Sources. USEPA, Office of Air Quality 

Planning and Standards.