Persistence of Chemical Warfare Agent VX on Building
Material Surfaces
PURPOSE
Chemical warfare agents (CWAs) comprise several chemical classes of which "nerve" agents are
compounds of concern due to their extremely high toxicity. Various nerve agents have recently been
identified as used in Middle East conflicts [I], as well as in attacks against individuals in the United
Kingdom [2] and Malaysia [3] The nerve agent VX (O-ethyl S-(2-[diisopropylamino]ethyl)
methylphosphonothioate) is highly persistent - lasting days to weeks on surfaces under commonly
encountered environmental conditions. VX is one of the most toxic nerve agents and can be introduced
into the body by inhalation, ingestion, skin contact, or eye contact. This brief provides a summary of two
recent U.S. Environmental Protection Agency (EPA) bench-scale studies assessing the persistence of
VX on various building material surfaces [4.5J. This overview provides decision-makers with practical
information on the expected persistency of VX following a chemical incident, which will inform the
remediation strategy prior to reopening contaminated buildings or infrastructure.
INTRODUCTION
EPA's mission is to protect human health and the environment. EPA is also the primary federal agency
responsible for remediation of indoor and outdoor areas at locations in which CWAs might be released.
Therefore, in support of EPA's mission in this area, EPA's Homeland Security Research Program
conducts research to help on-scene coordinators and decision-makers minimize environmental impacts
and human health effects following the release of a chemical agent. Limited data exist on the actual
persistence of nerve agent VX on building materials. EPA conducted bench-scale studies to evaluate the
VX persistence on various building materials, and as a function of temperature and of presence/absence
of air flow. VX amounts on these materials were measured over a period of up to five weeks post-
contamination. One study also evaluated the potential distribution of VX vapor to other materials that
could act as sinks [4|
VX PERSISTENCE RESEARCH
The VX persistency is high [6] because of its low vapor pressure (8.8* 1CT4 mm Hg at 25 °C) and its oily
liquid form. VX evaporates slowly and is expected to last days to weeks on surfaces. The persistence of
the neat agent is expected to be dependent on material type and environmental conditions. Therefore,
persistence should be considered in the overall remediation objectives following a chemical release. The
studies in this brief assessed the influence of temperature on the persistence of VX on ten different
materials, including the porous/permeable materials that are frequently encountered in indoor
environments. The persistence studies measured VX remaining on surfaces through some of the
prevalent processes in an indoor environment (e.g., evaporation, degradation and other physical or
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chemical interactions) over the course of a five-week period. Testing was conducted at three different
temperatures (10 °C, 25 °C, and 35 °C), which represent a range of conditions that might be encountered
in an evacuated abandoned indoor environment following a VX release. Other environmental conditions
such as relative humidity (RH) at 40%, and air flow associated with a specific air exchange rate (none or
one air exchange per hour) were held constant during the testing. Silanized glass served as a nonporous
reference material.
EXPERIMENTAL METHODS
Material coupons (excised samples) of 4.0 cm x 2.5 cm were each spiked with 2 microliters ([j,L) of neat
VX, which is equivalent to an initial average surface concentration of 2.2 g/m2 After a contact period
between VX and a material (ranging from 30 minutes to 35 days, depending on temperature), remaining
VX was extracted from the material coupons and residual mass was quantified via gas chromatography
/mass spectrometry (GC/MS). Table 1 shows the types and function of materials that were included in
the VX persistence studies and their qualitative permeability by water and oil/octanol. Materials that are
generally more permeable can be expected to exhibit longer VX persistency due to penetration and
adherence of agent to the material.
Table 1: Materials Used in VX Persistence Studies
Material
Study
Reference*
Function
Water
Permeability
Oil
Permeability
Sealed concrete
4
Flooring material
None
Low
Galvanized metal
4
HVAC ductwork
None
None
Painted drywall
4
Wall material
Low-Medium
Low
Glazed ceramic tile
4
Wall, floors
None-Low
None-Low
Silanized glass
4,5
Reference Material
None-Low
None
Unsealed concrete
5
Walls, floors
High
Medium-High
Plywood
5
Subfloor material
Medium-High
Medium-High
Rubber
5
Escalator handrail
Low
Low-Medium
HDPE* plastic
5
Water pipes, liner
None-Low
None-Low
Ceiling tile
5
Dropped ceiling
High
Medium-High
Acronyms: HDPE, High-Density Polyethylene; HVAC, heating, ventilation, and air conditioning. *: Full
references are at the end of the brief.
SHORT TERM (<30 MINUTES) INTERACTION OF VX WITH MATERIALS
Experimental results for the shortest contact time of VX with any material (30 minutes) in this study
yielded a noticeable difference in VX amounts recovered. Whereas recovery after immediate (less than 5
minutes) extraction of VX was higher than 70% across all 10 materials (except for unsealed concrete,
17%) [5], Figure 1 shows that of the ten materials included in this study, two materials (unsealed and
sealed concrete) had significantly less VX recovered in comparison to any of the other eight materials,
indicating a high absorption rate into the material, strong adherence to the material, or chemical
degradation by these two materials in comparison to the other materials. The absorption of VX into this
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120
Figure 1. Relative recovered mass of VX after 30 minutes contact time with material
sealant has been observed previously [7], The porous characteristics of unsealed concrete are known to
allow for absorption and likely degradation of VX [8], depending on concrete age and type. Neither the
absorption or degradation process was further investigated as part of this VX persistence study.
Following an actual incident involving VX, sealed and unsealed concrete should therefore be considered
as sinks of VX with an associated difficulty in measuring the remaining amount of VX within these two
materials. Hence, the presence of residual VX associated with these materials as observed by, for
example, wipe sampling may be biased low. The remaining liquid VX on the surface of sealed and
unsealed concrete evaporates in a manner similar to the evaporation of VX from other surfaces.
LONG-TERM (>30 MIN) INTERACTION OF VX WITH MATERIALS
The amount of VX recovered from all surfaces was found to diminish with time. Despite the low
volatility, VX slowly dissipates from all surfaces via evaporation. Further, absorption of VX into some
of the porous/permeable materials (from which it cannot be recovered via solvent extraction) and
possible chemical reactions with the surface result in a loss in the amount of recovered VX. Figure 2
shows the relative VX mass (compared to VX spike controls) recovered from ten building materials as a
function of the contact time of VX on the surface after application at 25 °C (initial VX mass, 2200
|ig/m ate rial) and a condition of one air exchange per hour. Excluding the two concrete materials with a
high affinity to absorb or react with VX on a short timescale, the fastest loss in recovered VX mass was
observed for glazed ceramic tile while the slowest loss in VX mass recovery occurred from the rubber
material. These rates are consistent with glazed ceramic tile as a nonporous material while rubber may
absorb some of the VX, which would slow the overall evaporation rate. It should be noted that all
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materials may degrade over time leading to changes in permeability. For example, weathering of
ceramic tile tends to lead to micro-fractures in the glaze while rubber would become inelastic and brittle.
Contact Time (days)
10	15	20
25
30
TO
0)
03
E
2
M—
"O
<1)
i—
<1)
>
O
O
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Table 2: Fitted half-life and derived 99% dissipation times for VX from surfaces at 10, 25 and 35 °C.

10
°C
25
°C
35
°C
Material
Half life ±
SD
(hours)
99% loss in
recovered
Half life ±
SD
(hours)
99% loss in
recovered
Half life ±
SD
(hours)
99% loss in
recovered

VX ± SD
(days)
VX ± SD
(days)
VX ± SD
(days)
Glazed Ceramic
66 ±4.7
18 ±1.3
13 ± 1.3
3.5 ±0.4
6.1 ±0.47
1.7 ± 0.13
Sealed Concrete1
72 ± 19
20 ±5.4
14 ±2.9
3.9 ±0.8
5.1 ±0.88
1.4 ±0.24
Galvanized Metal
111 ±22
31 ±6.2
23 ±3.3
6.4 ±0.9
7.7 ±0.62
2.1 ±0.17
Glass
181 ±28
50 ±7.8
30 ±2.4
8.4 ±0.7
7.3 ±0.47
2.0 ±0.13
Plywood
153 ±6.9
42 ± 1.9
31 ±2.7
8.5 ±0.8
12 ± 1.4
3.4 ±0.39
Painted Drywall
215 ± 18
60 ±5.0
35 ±3.3
9.8 ±0.9
10 ± 1.5
2.8 ±0.42
Unsealed
Concrete2
150 ±35
41 ±9.7
46 ± 5.2
12.8 ±1.4
18 ± 1.0
5.0 ±0.29
HDPE Plastic
486 ± 98
135 ±27
81 ± 11
22.4 ±3.1
27 ±2.3
7.5 ±0.64
Ceiling Tile
418 ±37
116 ±10
93 ±6.7
25.6 ±1.8
35 ±0.81
9.6 ±0.23
Rubber
79 ± 17
22 ±4.8
118 ±23
32.6 ±6.5
48 ± 5.6
13 ± 1.5
SD: Standard Deviation in in the pseudo-first order fit to the experimental data. HDPE: High density polyethylene.
1: VX permeates through sealant; less than 47% extracted from sealed concrete after 30 min.
2: VX absorbs into concrete; less than 17% extracted from unsealed concrete after 30 min.
As is evident from Figure 3, VX dissipation half-lives from all surfaces were found to be strongly
dependent on (inverse of) temperature with an average reduction in half-life times by a factor eight to
twelve across materials when comparing data collected at 10 and 35 °C, with a relatively minor
1000
CO
X
CO
Q.
W
W
b
X
>
100
10
35
Temperature (°C)
30 25 20 15
10
i i i i i i i


¦ Sealed Concrete

• Glass

^ Galvanized Metal

—T- Painted Drywall

—Glazed Ceramic

—Unsealed Concrete

—~- Plywood

• Rubber

—HDPE Plastic

—Ceiling Tile
i I i I i I i

3.2x10"1
3.3x10"
1-3
3.4x10"3
3.5x10"1
3.6x10-
1-3
1 0/T (1 /Kelvin)
Figure 3. VX dissipation half-life as function of temperature
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additional material dependence. One noticeable exception was for the rubber material for which the
fitted VX dissipation half-life at 10 °C was found to be shorter than at 25 °C. This difference is
attributed to an inexplicable anomaly in the recovered amounts of VX for the rubber material after 7 and
14 days [5] at 10 °C, resulting in a poor quality of the pseudo-first order fit to the experimental data sets
at these two temperatures. Rates (except for rubber) followed the Arrhenius equation for the temperature
dependence in (dissipation) rate constants,
-Eg
k = A x e rt	[1]
where k is the (dissipation) rate constant, T is the absolute temperature, A is a constant, Ea is the
activation energy for the dissipation of VX and R is the universal gas constant. The near equal slopes of
the curves in Figure 3 shows that the activation energy, which is associated with the heat of evaporation,
is only somewhat dependent on the material, suggesting that the VX dissipation is independent of the
material and dominated by the evaporation from the initial VX liquid droplet on the material.
IMPACT OF A LACK OF AIR EXCHANGE ON DISSIPATION OF VX AT 25 °C
VX persistence data were collected in a subset of five building materials at 25 °C without an air
exchange present [4], In this study, using an approximately 200 L volume chamber, half-life times for
the persistence of VX on these materials were found not to be statistically significantly different from
the persistence half-lives for VX in the presence of one air exchange on the same materials.
MIGRATION OF VX TO OTHER SURFACES
The potential distribution of VX via movement of its vapor from contaminated materials to other
initially noncontaminated materials (leather upholstery, high density polyethylene, painted metal, desk
laminate, and cubicle divider cloth) was investigated through extraction of these materials that were
placed near the contaminated material (glass). After a seven-day contact period of VX with the glass
materials, the spiked glass coupons had a mean VX recovery of approximately 2% of the initially spiked
VX amount, which is similar to the reported mass recovered after seven days in other VX persistence
experiments. After seven days, VX was also recovered from leather upholstery, high density
polyethylene, and cubicle divider cloth (mean VX recoveries were <0.2% of initial amount). VX was not
recovered from painted metal or desk laminate. The highest VX amount recovered from an unspiked
coupon was 0.25% of the amount of VX spiked for a cubicle divider cloth material. This observed
migration of VX vapor may be biased low. Measurement of the material-specific extraction efficiencies
for VX were beyond the scope of this study to characterize the precision and accuracy of the analysis of
the various coupon materials and may, therefore, underestimate the VX amount measured on these
surfaces due to the volatilization of VX from the contaminated material. This limited study provides
strong evidence that some distribution of VX to initially uncontaminated materials occurs, and these
materials may become sinks and would also require decontamination.
IMPACT OF VX ATTENUATION ON REMEDIATION ACTIVITIES
Starting with a 2.2 g/m2 surface concentration of VX, the 99% dissipation time implies that 22 mg/m2 or
2.2 |ig/cm2 would remain on a surface after 4 to 33 days across all materials at 25 °C, with shorter times
at higher temperatures and longer times at lower temperatures. This residual surface concentration is
expected to be significantly higher than a cleanup goal surface concentration value. As a comparison,
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the preliminary remediation goal (PRG), risk-based goal for surfaces contaminated with VX as
calculated via EPA's Risk Assessment Guide for Superfund (RAGS) methodologies [9] is
approximately 10,000 times lower. Although these PRGs may not be the ultimate clearance goal for VX
remediation, the significant difference between the residual amount of VX observed at the end (4-5
weeks) of these dissipation tests and the PRG indicates that the attenuation of VX by itself is not an
effective approach for remediating a contaminated site. Decontamination approaches may likely be
needed to remediate a VX contaminated area. Waste generated from remediation activities is likely to
contain residual VX. Further, volatilization of VX will lead to elevated air concentrations and to the
spread of the contaminant.
DISCLAIMER
The U.S. Environmental Protection Agency through its Office of Research and Development funded and
managed the research described herein under Contract Number EP-C-11-038, Task Order 23 with
Battelle. This summary has been subjected to the Agency's review and has been approved for
publication. Note that approval does not signify that the contents reflect the views of the Agency.
Mention of trade names, products, or services does not convey EPA approval, endorsement, or
recommendation.
REFERENCES
1.	Organisation for the Prohibition of Chemical Weapons (OPCW). "Syria and the OPCW." See
https://www.opcw.org/media-centre/news/2019/03/opcw-issues-fact-finding-mission-report-
chemical-weapons-use-allegation. Last accessed May 28, 2019.
2.	Organisation for the Prohibition of Chemical Weapons (OPCW). "Incident in Salisbury." See
https://www.opcw.org/media-centre/featured-topics/incident-salisbury Last accessed May 28, 2019.
3.	Organisation for the Prohibition of Chemical Weapons (OPCW). "Malaysia. Statement by the
Delegation of Malaysia at the eighty-sixth Session of the Executive Council." See
https://www.opcw.org/fileadmin/OPCW/EC/86/en/ec86natl2 e .pdf. Last accessed May 28, 2019.
4.	U.S. EPA. 2016. "Natural Attenuation of Persistent Chemical Warfare Agent VX on Selected
Interior Building Surfaces." EPA/600/R-16/110, Research Triangle Park, NC: U.S. Environmental
Protection Agency.
5.	U.S. EPA. 2017. "Natural Attenuation of the Persistent Chemical Warfare Agent VX on Porous and
Permeable Surfaces." EPA/600/R-17/186, Research Triangle Park, NC: U.S. Environmental
Protection Agency.
6.	Centers for Disease Control and Prevention. "Facts about VX." See
https://emergencv.cdc.gov/agent/vx/basics/facts.asp. Last accessed May 28, 2019.
7.	U.S. EPA. 2016. "Fate and Transport of Chemical Warfare Agents VX and HD across a Permeable
Layer of Paint or Sealant into Porous Subsurfaces." EPA/600/R-16/173, Research Triangle Park,
NC: U.S. Environmental Protection Agency.
8.	Williams, J.M. et al., Degradation kinetics of VX on concrete by secondary ion mass spectrometry,
Langmuir 21(6):2396-2390 (2005).
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9. U.S. EPA. "Superfund Risk Assessment: Human Health Topics." See
https://www.epa.gov/risk/superfund-risk-assessment-human-health-topics. Last accessed May 28,
2019.
CONTACT INFORMATION
For more information, visit the EPA Web site at http://www2.epa.gov/homeland-security-research.
Technical Contact: Lukas Oudejans (Oudejans.lukas@epa.gov)
General Feedback/Questions: Amelia McCall (mccall.amelia@epa.gov)
U.S. EPA's Homeland Security Research Program (HSRP) develops products based on
scientific research and technology evaluations. Our products and expertise are widely used in
preventing, preparing for and recovering from public health and environmental emergencies
that arise from terrorist attacks or natural disasters. Our research and products address
biological, radiological, or chemical contaminants that could affect indoor areas, outdoor areas,
or water infrastructure. HSRP provides these products, technical assistance, and expertise to
support EPA's roles and responsibilities under the National Response Framework, statutory
requirements, and Homeland Security Presidential Directives.
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