Standards Development for Gas Phase 

Air Cleaning Equipment in Buildings 

Andrew Persily 

Cynthia Howard-Reed 

Stephanie Watson 

Nicos Martys 

Edward Garboczi 

Heather Davis 

NISTIR 7525

NISTIR 7525

October 2008

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

National Institute of Standards and Technology 

Patrick Gallagher, Acting Director 

Standards Development for Gas Phase 

Air Cleaning Equipment in Buildings 

Andrew Persily 

Cynthia Howard-Reed 

Stephanie Watson 

Nicos Martys 

Edward Garboczi 

Heather Davis

Building and Fire Research Laboratory

Prepared for: 

U.S. Department of Homeland Security 

Science & Technology Directorate 

Washington, DC 

Abstract 

Concerns about building security have resulted in increasing interest in gas phase air cleaning 

(GPAC) and the need for standard methods to determine the effectiveness of these systems. 

Similarly, the ability to predict their installed performance, based on such standard test data, is 

becoming increasingly important. The development and application of these standards and 

prediction tools will provide better protection of building occupants against chemical agents and 

improve the ability of designers and building owners to identify and specify air-cleaning 

equipment with a realistic expectation as to installed performance. The National Institute of 

Standards and Technology (NIST) has conducted an effort to facilitate the development of these 

standards and predictive tools under a project funded by the Department of Homeland Security. 

This report describes the following tasks that were carried out as part of this project: an 

evaluation of gas phase air cleaning technology; an assessment of existing and proposed 

standards, as well as relevant guidance documents; design of a laboratory-scale microreactor to 

evaluate media consistent with current and proposed industry approaches; micro-scale modeling 

to understand the interactions between the media and gaseous contaminants and, building-scale 

modeling to understand the impacts of air cleaning systems in controlling occupant exposure. 

Keywords: air cleaning, building security, filtration, gas phase, standards 
Table of Contents 

INTRODUCTION ......................................................................................... 1 

GPAC TECHNOLOGY ................................................................................. 1 

GPAC STANDARDS .................................................................................... 3 

Existing Standards ......................................................................................... 3 

Standards Under Development .......................................................................... 4 

Other Relevant Standards and Guidelines ............................................................. 5 

MICROREACTOR ....................................................................................... 7 

Design of Benchtop Microreactor ...................................................................... 7 

Design of Experiments ................................................................................. 11 

MICRO-SCALE MODELING ....................................................................... 12 

Transport Model ......................................................................................... 12 

Filter Characterization ................................................................................... 12 

Model Test Case ......................................................................................... 13 

Modeling Contaminant Transport ..................................................................... 15 

Links to Measurement of Concentration over Time ................................................ 17 

Future Research Needs .................................................................................. 18 

BUILDING SCALE MODELING ................................................................... 18 

Simulation Results ....................................................................................... 20 

DISCUSSION ............................................................................................ 24 

ACKNOWLEDGEMENTS .......................................................................... 24 

REFERENCES ......................................................................................... 24 
1

INTRODUCTION 

As concerns about building security increase, the application of particle filtration and gas phase 

air cleaning has been receiving increasing attention (NIOSH 2003). Unlike particle filtration, 

there is currently a lack of standards for determining the effectiveness of gaseous air cleaning 

systems and tools for predicting their installed performance. This lack of standards makes it 

difficult for designers, building owners and operators, and equipment manufacturers to specify 

gaseous air cleaning systems and to incorporate them into system designs. To provide protection 

against gaseous agents to which a building might be exposed, standards and application guidance 

for gas phase air cleaning are needed. 

NIST has initiated a project to advance both the development of the standards and guidance for 

gas phase air cleaning, and to better understand the detailed performance of these systems to 

support standards development and to better predict their installed performance. This report 

presents the findings of several tasks under this project: assessing existing and proposed 

standards, as well as relevant guidance documents; designing a laboratory-scale microreactor to 

evaluate media consistent with current and proposed industry approaches; conducting 

computational fluid flow simulations of the interactions between adsorbent media and gaseous 

contaminants to better understand the mass transport processes occurring in air cleaning systems; 

and, performing building-scale modeling to understand the impacts of air cleaning systems in 

controlling occupant exposure. 

GPAC TECHNOLOGY 

Currently available non-industrial gas phase air cleaning (GPAC) devices use one or more of the 

following removal processes: adsorption, chemisorption, ozone oxidation, air ionization, and 

ultra-violet photocatalytic oxidation. Although several technologies are capable of removing gasphase contaminants, there is no single device available today that can remove all types of 

hazardous gaseous contaminants from an indoor environment. As a result, the different types of 

gas-phase air cleaners can be described by their specific capabilities and limitations. 

Adsorption is the most widely used air cleaning mechanism of gas-phase contaminants in nonindustrial buildings (VanOsdell et al. 1996) and therefore the focus of this project. Physical 

adsorption is an exothermic process where organic vapor molecules are attracted to the surface of 

an adsorbent material and trapped in the material’s pores (< 2 nm diameter). Polarity and pore 

size dictate a material’s affinity for specific contaminants. Adsorbents are typically packed in a 

bed configuration for contaminant removal, but they may also be embedded in fabric filters or 

coated on surfaces of particulate filters. In addition to media characteristics and contaminant 

properties, the effectiveness of sorbent air cleaners also depends on the airflow velocity through 

the media and environmental conditions such as relative humidity and temperature. 

There are many different adsorbent materials available; however, activated carbon is the most 

commonly used adsorbent in heating, ventilation and air-conditioning (HVAC) systems 

(Henschel 1998). Activated carbon consists of a carbonaceous material (e.g., wood, coal, bark, 

coconut shells) that has been partially oxidized to create pore sizes ranging from 0.5 nm to 50 nm 

and surface areas in the range of 1000 m2

/g (NIOSH 2003). Contaminants most likely to be 

removed by adsorption onto activated carbon tend to be non-polar and have low vapor pressures 

(< 1.5 kPa). Activated carbon is less effective for volatile, low-molecular weight gases, such as 

formaldehyde, ammonia, and vinyl chloride. 
2

Other sorbent materials include silica gel, activated alumina, and zeolites. Silica gel and alumina 

have less surface area than activated carbon (150 m2/g to 750 m2/g) but are often used to trap 

polar compounds, such as formaldehyde and sulfur-based contaminants (NIOSH 2003). Their 

affinity for water, however, limits their effectiveness under higher humidity conditions. Zeolites 

are naturally occurring aluminosilicate minerals that are also hydrophilic but tend to have smaller 

pores (0.3 nm to 3.0 nm), making them more suitable for low molecular weight halides, such as 

chlorinated fluorocarbons (NIOSH 2003). It is possible to manufacture synthetic zeolites with 

larger pore sizes and less sensitivity to humidity. 

In general, the addition of sorption media to an existing HVAC system is relatively 

straightforward. However, sorption-based air cleaning systems are associated with a significant 

pressure drop, which can increase fan energy requirements in HVAC systems and therefore both 

first and operating costs. Most sorbent media can be regenerated with high temperatures or 

solvent extraction, but this process can be typically difficult to perform in situ. However, if there 

are no space or structural limitations, pressure swing adsorption could be used to regenerate 

installed media. Pressure swing adsorption uses high temperature air at a high pressure to purge 

contaminants from activated carbon and exhaust them to the outdoors or to a separator. Sorption 

has another advantage of producing no harmful byproducts. However, there is potential for 

media to desorb a portion of the originally sorbed species when the air cleaner becomes saturated 

and/or the upstream contaminant concentration goes to zero. As a result, gas-phase air cleaner 

performance should be based on both the initial contaminant removal and breakthrough time 

(Chen et al. 2005). Another concern is the disposal of used media. 

Chemisorption can extend the capabilities of adsorption through use of a reactive chemical 

coating on the sorbent surface that causes certain contaminants to bond to the media. For 

example, a number of high-vapor pressure chemicals (e.g., hydrogen cyanide and formaldehyde) 

are not retained on activated carbon by physical adsorption due to their high volatility. 

Impregnating the carbon with a reactant will bond the contaminant to the substrate and convert it 

to more benign chemicals. For example, potassium permanganate is used to oxidize 

formaldehyde into water and CO2. In addition to potassium permanganate, chemisorption 

compounds include sodium sulfide, bromine, phosphoric acid, sodium carbonate, and metal 

oxides. The US military has developed an impregnated carbon, ASZM-TEDA, to specifically 

remove a wide range of chemical warfare agents. ASZM-TEDA is a coal-based activated carbon 

impregnated with copper, zinc, silver, molybdenum and triethylenediamine (NIOSH 2003). 

While chemisorption has the advantage of removing a wider range of gas-phase contaminants 

than physical adsorption alone, it is a slower process and is not reversible. Thus, the 

chemisorption media is not reusable and must be replaced, emphasizing the importance of filter 

maintenance schedules and proper disposal of spent media. Another drawback to oxidative 

chemisorption is the potential production of harmful byproducts resulting from chemical 

reactions. For example, hydrochloric acid may be produced when chlorinated hydrocarbons are 

oxidized (Godish, 1989). 

While the most common, physical adsorption and chemisorption are not the only technologies 

available for removing gas-phase air contaminants from indoor air. For example, ozone 

generators and air ionizers have been shown to remove aliphatic and aromatic alkenes (Boeniger 

1995; Daniels 2002), but have potential drawbacks related to ozone exposure and potentially 

harmful byproducts (Weschler 2000; Boeniger 1995). Ultraviolet-photocatalytic oxidation (UVPCO) is a more recently developed technology that has been shown to remove several classes of 

volatile organic compounds (VOCs) and can be characterized by lower maintenance 
3

requirements and longer service life (Jacoby et al. 1996; Alberici and Jardim 1997; Blake 1994). 

However, questions remain regarding catalyst poisoning, potentially harmful byproducts and 

building conditions that impact performance. 

So far, questions about many of the non-adsorption-based air cleaners have hindered wide 

acceptance in the marketplace. As a result, discussions of gas-phase air cleaning equipment 

standards, test methods and guidance documents have focused on physical adsorption and 

chemisorption air cleaner technologies. 

GPAC STANDARDS 

A primary goal of this project is to support the development of the standards needed to 

effectively implement gas phase air cleaning as a means of improving building safety and 

security. A first step in achieving this goal is to assess current standards that are relevant to this 

technology and which may serve as a vehicle to meet the project goals. At this time, there is a 

notable lack of standards for determining the effectiveness of gaseous air cleaning systems and 

their installed performance. In addition to standards, there is a need for better guidance on 

installation, operation and maintenance of these systems. Note that this need for guidance also 

applies to particulate filtration. 

Existing Standards 

This section reviews a number of existing standards that apply to air cleaning systems and 

discusses their relevance to this project and the application of GPAC to building security. 

ASME N510-1989, Testing of Nuclear Air Treatment Systems: This standard provides a basis 

for the development of test programs and detailed acceptance and surveillance test procedures 

for high efficiency air-cleaning systems for nuclear power plants. It also specifies minimum 

requirements for the reporting of test results. The standard covers requirements for the postdelivery field-testing of high efficiency air-cleaning systems for both gas phase and particle 

matter for nuclear power plants and other nuclear applications. The tests covered by this standard 

are of two types: (a) acceptance tests, which verify that the systems have been correctly installed 

and meet the requirements of project specifications; (b) surveillance tests, which monitor the 

condition of the systems. The actual tests described by the standard include the following: visual 

inspection, duct and housing leakage, structural capability, mounting frame pressure leakage, 

airflow capacity and distribution, air-aerosol mixing uniformity, high efficiency particulate air 

(HEPA) filter in-place leakage, adsorber bank in-place leakage, duct damper bypass, system 

bypass, air heater performance, and laboratory testing of adsorbent. Only the last test relates 

directly to the removal efficiency of the filter, in this case only for radioiodine removal using 

ASTM standard D 3803. 

ASME N509-2002, Nuclear Power Plant Air-Cleaning Units and Components: This standard 

identifies and establishes requirements for filters, adsorbers, moisture separators, air heaters, 

filter housings, dampers, valves, fans, ducts, and other components of nuclear air-treatment 

systems for a specific application in a nuclear power plant. Requirements for operability, 

maintainability and testability of systems necessary for the maintenance of system reliability for 

the design conditions are also included. The standard specifies acceptance testing, including 

minimum acceptance requirements, in accordance with Standard ASME N510. 

The standard covers requirements for the pre-delivery design, construction, and qualification and 

acceptance testing of the air-cleaning units and components that make up high efficiency air and 
4

gas cleaning systems used in nuclear power plants. Qualification and acceptance testing 

provisions are specified to verify the adequacy of the air-cleaning unit and component design, to 

verify that components have been properly fabricated and installed, and that the system will 

perform in accordance with specification requirements. The categories covered by the standard 

are as follows: functional design, components, packaging, shipping, receiving, storage, and 

handling of components, installation and erection, quality assurance, and acceptance testing. This 

standard does not address determination of removal efficiencies. 

ASHRAE Standard 62.1-2007, Ventilation and Acceptable Indoor Air Quality: This standard 

contains various requirements related to both ventilation and indoor air quality, primarily 

minimum ventilation requirements and various system requirements related to moisture control 

and other issues. It does not contain requirements for gaseous air cleaning (except in the case of 

elevated outdoor ozone levels), but its relevancy to this discussion is based on the inclusion of a 

performance path for compliance referred to as the Indoor Air Quality (IAQ) Procedure. The 

IAQ Procedure is an alternative to the prescriptive Ventilation Rate Procedure, which specifies 

outdoor air requirements as a function of space type. The alternate procedure is based on the 

identification of contaminants likely to be present in the indoor environment and the 

determination of the levels of ventilation and air treatment (including gaseous air cleaning) 

required to maintain their concentrations at acceptable levels. Therefore, gaseous air cleaning is 

important in that it provides a means to comply with this performance procedure, and the 

development of test methods for evaluating gaseous air cleaning is key to its application. 

Standards Under Development 

This section discusses the activities of ASHRAE committee SPC 145P, which is working on 

three test methods for gas phase air cleaning equipment. The first, referred to as 145.1, is a 

small-scale laboratory method for evaluating media. The second is a laboratory method for 

evaluating full-scale devices in ductwork and is referred to as 145.2. The last method, 145.3, is a 

field test for installed performance. 

While not yet approved, ASHRAE Standard 145.1P, Laboratory Test Method for Assessing the 

Performance of Gas-Phase Air Cleaning Systems: Loose Granular Media, is expected to define a 

small-scale laboratory test method for measuring the contaminant removal efficiency of loose 

granular sorptive media. Tests are to be performed at elevated gas challenge concentrations 

under steady-state conditions for comparing the different media, rather than for predicting their 

installed performance. The standard will also define methods of calculating and reporting results. 

The approved scope describes the media types that the standard does not apply to as follows: 

bonded carbon panels, beaded activated carbon, carbon cloths, absorbant loaded nonwovens, dry 

process carbon composites, and particulate removal equipment. 

The testing apparatus and material will only be applicable to granular (pelletized) media with 

mean particle diameters of less than 5 mm. Two groups of chemical contaminants are expected 

to be prescribed for use in the testing, outdoor air acid gases and indoor air volatile organic 

compounds (VOCs), and the user may select one or both of the groups for testing. Although 

other contaminant groups may be tested, they may not necessarily be used in rating comparisons. 

There will be requirements for column length and inside diameter for the test canister, which will 

be included to average out anomalies due to filter media variations and to avoid significant 

edge/wall effects. The tests are run to a 50 % breakthrough of the initial challenge concentration, 

where breakthrough is defined as the point at which the downstream contaminant concentration 

is measurable and begins to rise rapidly. 
5

The second standard being developed by the ASHRAE committee is referred to as 145.2P, with 

the tentative title Method of Testing Gaseous Contaminant Air Cleaning Devices for Removal 

Efficiency. This test method will describe how to perform an in-duct, large-scale, sorptive media 

laboratory test for measuring the performance of gaseous contaminant air-cleaning devices with 

operating ranges from 0.24 m3

/s to 0.94 m3/s (500 cfm to 2000 cfm). The standard will include a 

large-scale test procedure with quality control constraints to measure percent removal efficiency 

and removal capacity of sorption-based gas phase air cleaning devices, when they are challenged 

under steady-state conditions by specified gaseous contaminants. The test is intended to simulate 

the capture performance of commercially available HVAC media under controlled, but 

representative conditions. 

This procedure will incorporate a number of steps that are designed to reduce personal inhalation 

exposures during testing to levels well below the respective Permissible Exposure Limits (PELs) 

and should be followed carefully. A primary focus is careful sealing of apparatus leaks, and 

applications of tests to demonstrate that leaks into the operator work area do not constitute a 

significant health hazard. The laboratory test apparatus, equipment, test protocol, quality control 

guidelines, and equipment calibration recommendations are designed to ensure repeatability 

within ± 10 % of the measured value. 

Other Relevant Standards and Guidelines 

This section describes other standards and guidelines that are relevant to the goals of this project. 

ASHRAE Standard 52.2-2007, Method of Testing General Ventilation Air-Cleaning Devices for 

Removal Efficiency by Particle Size: This standard establishes a test procedure for evaluating the 

performance of air-cleaning devices as a function of particle size and is a good example of the 

type of method of test needed for the gaseous air-cleaning systems. The standard addresses two 

air cleaner performance characteristics of importance to users: the ability of the device to remove 

particles from the airstream and its resistance to airflow. When air cleaners are tested and 

reported for efficiency in accordance with this standard, there is a basis for comparison and 

selection for specific applications. 

This standard outlines a method of loading the air cleaner with synthetic dust to simulate field 

conditions. The standard defines procedures for generating the aerosols required for conducting 

the test and provides a method for counting airborne particles of particular sizes in order to 

calculate removal efficiency by particle size. This standard also establishes performance 

specifications for the equipment required to conduct the tests, defines methods of calculating and 

reporting the results obtained from the test data, and provides a minimum efficiency reporting 

system that can be applied to air-cleaning devices covered by this standard. 
6

IEST: The Institute of Environmental Sciences and Technology (IEST) has prepared a Handbook 

of Recommended Practices (RPs) Relating to Air Filtration in Cleanrooms and Other Controlled 

Environments. These documents address filter and filter media testing, filter construction, and 

various applications of filters in cleanrooms for the removal of airborne particulate 

contamination. The handbook includes: 

IEST-RP-CC001 (1993): HEPA and ULPA* Filters 

IEST-RP-CC006 (2004): Testing Cleanrooms 

IEST-RP-CC007 (1992): Testing ULPA Filters 

IEST-RP-CC0021 (2005): Testing HEPA and ULPA Filter Media 

IEST-RD-CC011 (1995): A Glossary of Terms and Definitions Relating to Contamination 

Control 

IEST-RP-CC034.1 (2005): HEPA and ULPA Filter Leak Tests 

The recommendations outlined in these documents for particulate removal are models of the type 

of guidance documents for establishing standards and recommended practices for filter testing 

and media characterization needed for gaseous air-cleaning devices. 

NAFA (National Air Filtration Association), Installation, Operation and Maintenance of Air 

Filtration Systems (1997): This manual was created to provide guidance on the installation, 

operation and maintenance of air filtration systems and is based on situations encountered during 

actual field experience and applications. Its prime concern is to help ensure that filters are 

properly designed, installed, operated, and maintained for maximum efficiency and safety. The 

manual instructs the user on the following subjects: air filtration framing systems, filter 

installation, and filter replacement for different type of particle filters and gas phase air cleaning 

systems. 

NAFA Guide to Air Filtration (2001): This document is an information manual intended for 

those interested in the fundamentals of air filtration, and is required reading to become NAFA 

Certified Air Filtration Specialists. The manual covers the following topics: the importance of air 

filtration, principles of air filtration, types of filters, electronic air cleaners, HVAC filter testing, 

HEPA and ULPA filter testing, controlled environments, airborne microorganisms, gaseous 

contaminants, indoor air quality, owning and operating costs, and brief descriptions of the 

ASHRAE Standards 52.1 and 52.2. 

 

*

 ULPA refers to ultra low particulate air. 
7

MICROREACTOR 

As part of this project, NIST has defined experimental and simulation efforts to better understand 

gas phase air cleaner performance. One aspect of the experimental program involved the design 

of a laboratory-scale microreactor for measuring media performance. While the microreactor was 

not constructed as part of this project, the design work is valuable for use in future projects. This 

section describes the microreator design and its potential application. 

The microreactor was designed for small-scale laboratory measurements of contaminant removal 

efficiency of loose granular sorptive media. The testing envisioned would examine the sorptive 

media used in gas-phase air cleaning equipment as installed in an airstream and subsequently 

challenged with test gases at concentrations that are elevated relative to typical applications. The 

apparatus design is based on the proposed ASHRAE Standard 145.1, and was being considered 

for potential use in round-robin tests of ASHRAE 145.1 after it is published. The apparatus was 

also designed to measure permeability of the sorptive media by measuring the change in 

challenge gas concentrations/volumes after sorptive media introduction. 

As noted above, ASHRAE 145.1 is expected to outline a test procedure with quality control 

restraints to develop a composite rating of small media samples, in granular or pellet form, when 

it is challenged under steady-state conditions by a number of gaseous contaminants. Such media 

include untreated or chemically impregnated activated carbon, activated alumina, and other 

adsorbent materials or catalysts. The test is to be conducted at elevated gas challenge 

concentrations, relative to ventilation applications, and at a single temperature and relative 

humidity (RH), to compare the media rather than directly predict performance in any particular 

application. Gas analyzers are used to measure the changes in challenge gas concentrations over 

the course of the experiment. The composite rating for small media is based on percent removal 

efficiency and removal capacity. Thus, the results of these tests can provide information for the 

design and selection of air cleaning equipment and the design of air cleaning systems for 

controlling indoor concentrations of gaseous air contaminants. 

Design of Benchtop Microreactor System 

The test apparatus was designed with certain modifications of the system in the proposed 

ASHRAE 145.1P. In particular, these modifications include changes to the media column or 

reactor, the range in challenge gas concentration, and the experimental range of temperature and 

RH. In addition, the method of gas analysis was specified as gas chromatography-mass 

spectrometry (GC-MS) to enable exact identification of the challenge gas and possible secondary 

reaction products, and a mode to re-circulate the challenge gas over the media bed was added to 

more closely mimic air recirculation in building ventilation systems. These modifications were 

selected to enable the benchtop microreactor system to more accurately characterize the air 

media interaction with various challenge gases under more realistic variables, i.e. temperature, 

RH, and re-circulation mode. Additionally, the air media itself would be easily analyzed 

spectroscopically for chemical changes after exposure to the challenge gas. 

A schematic of the benchtop microreactor system is shown in Figure 1. Two separate gas 

cylinders contain the challenge gas/air mixture and an air purge. Gas flow is controlled by mass 

flow controllers. The RH generator, mass flow controllers, media column, and re-circulating 

pump are housed in a temperature controlled chamber. The chamber is commercially available 

and would be modified in-house to maintain constant temperature conditions. The control of RH 

for the microreactor system is performed using a humidity generator designed for the Simulated 

Photodegradation via High Energy Radiant Exposure (SPHERE) in the Polymeric Materials 
8

Group of NIST as shown in Figure 2. In this humidity generator, RH is controlled by the 

controlled mixing of moisture saturated and dry air streams. Air is passed through an air drier 

and split into two streams. One stream is moistened by routing it through a saturated cotton filter 

wick in a commercial stainless steel water tank (Figure 3) partly filled with distilled, de-ionized 

water. The water level in the stainless steel tank is maintained by an automated feedback 

controlled filling system. The flow rates and proportions of the two streams are controlled by a 

proportional integral differential (PID) control loop, which is driven by voltages generated by a 

microprocessor board in response to the difference between the set-point and the measured RH. 

On leaving the water tank, the saturated air stream is immediately mixed with the dry air stream. 

The combined stream is then passed to a rotary valve and mass flow controller before passing 

through the media column. 

Figure 1. Schematic of the benchtop microreactor system. Green circles with RH/T represent 

relative humidity/temperature sensors. Green circles with P represent gas pressure sensors. 
9

Figure 2. Schematic of the controller for the heating and humidity generation system. 

Figure 3. Diagram of a stainless steel water tank used in the humidity generation system. The 

units for the values of the dimensions are millimeters. 

Mass Flow

Controllers

Deionized

Water

Auto fill

Check

Valves

Specimen 

Holder Heated

Humidity

Generator Cotton

Wick

Vent

+

Contact

Heaters

RH Sensor

Dry Air

Regulator

Temperature

Sensor

Temperature

Sensor

Temperature

Sensor

Mass Flow

Controllers

Deionized

Water

Auto fill

Check

Valves

Specimen 

Holder Heated

Humidity

Generator Cotton

Wick

Vent

+

Contact

Heaters

RH Sensor

Dry Air

Regulator

Temperature

Sensor

Temperature

Sensor

Temperature

Sensor

Mass Flow

Controllers

Deionized

Water

Auto fill

Check

Valves

Specimen 

Holder Heated

Humidity

Generator Cotton

Wick

Vent

+

Contact

Heaters

RH Sensor

Dry Air

Regulator

Temperature

Sensor

Temperature

Sensor

Temperature

Sensor

Mass Flow

Controllers

Deionized

Water

Auto fill

Check

Valves

Specimen 

Holder Heated

Humidity

Generator Cotton

Wick

Vent

+

Contact

Heaters

RH Sensor

Dry Air

Regulator

Temperature

Sensor

Temperature

Sensor

Temperature

Sensor
10

The benchtop microreactor system will accommodate a re-circulating mode using a varistaltic 

type pump. A means of maintaining RH for the media column system under re-circulating 

conditions has yet to be determined. However, for most air media samples the addition of water 

vapor would be needed to maintain a specified level of RH. Moisture may be added by mixing in 

moist air and/or moist challenge gas/air mixture into the re-circulation loop. In either case, the 

challenge gas concentration in the re-circulating loop would change. Furthermore, monitoring 

the RH within the re-circulation loop would require additional RH/temperature sensors to be 

placed within the re-circulation loop. Initial challenge gas experiments would be conducted 

without adjusting RH during re-circulation to determine the operating conditions for each air 

media sample and the amount of additional RH needed to maintain constant RH. RH adjustments 

would be added at a later phase in the testing. 

The media column is modified to insure proper media packing, media bed placement, and 

adequate seals to maintain pressure and humidity in the system. This media column is 

constructed mostly of glass, a highly non-reactive material, to reduce air media contamination 

errors. Glass frits are used as support disks and an inner glass ring, available in adjustable sizes, 

is used to properly pack the media bed to prevent bed fluidization. The design is outlined in 

Figure 4. 

The analysis method for the contaminant gas and secondary products for this benchtop 

microreactor is gas chromatography-mass spectrometry with a sample pre-concentrator, for 

detection of volatile chemicals at low initial challenge gas concentrations on the order of 1 μg/L 

or less. A commercial rotary valve is used to collect samples of the test gas stream for GC/MS 

analysis. The mass spectrometer detector allows the direct identification of the challenge gas and 

secondary products in addition to their concentrations. 

Figure 4. Schematic of the media column. The units for the values of the dimensions are 

millimeters. 
11

Design of Experiments 

A variety of experimental conditions to be used with the benchtop microreactor system are 

outlined in Table 1. These experimental values were chosen to cover a range of conditions found 

in buildings and suggested in ASHRAE 145.1P. The allowable variance in the target temperature 

and target RH, based on ASHRAE 145.1P, is ± 2 °C and ± 5 % RH, respectively. The test 

airflow rate to be used was that necessary to achieve a 0.1 s residence time, which is theoretical 

time that an increment of air is within the confines of a media bed, as determined in ASHRAE 

145.1P. Residence times and face velocities are calculated using the following relationships: 

 tr = d/v (1) 

v = Q/A (2) 

where tr is residence time in s, d is the media depth in mm and v is the air/gas velocity in mm/s, 

Q is the air/gas volumetric flow rate in mm3

/s and A is sample face area, equal to πd2, in mm2. 

Given a typical media depth of 25 mm, Q is equal to 0.00049 m3

/s. The variances in the 

conditions within the benchtop microreactor system are listed in Table 2. 

Table 1. Sample experimental conditions for microreactor system challenge gas experiments. 

Experiment Temperature (°C) Relative Humidity (%) 

1 23 50 

2 43 50 

3 0 50 

4 32 90 

5 27 65 

Table 2. Experimental condition variances allowable within the benchtop microreactor system. 

Experimental Variable Variance 

Temperature ± 0.2 °C 

RH ± 2 % of value 

Airflow ± 1 % 

Challenge Gas Concentration ± 0.2 % 

The challenge gases to be examined are based on the ASHRAE Standard 145.1P and are 

categorized into three groups: Acid Challenge Gases, Volatile Organic Carbon (VOC) Gases, 

and Other Common Challenge Gases. Table 3 outlines the three challenge gas groups and the 

concentrations that could be used in the experiments. The initial experiments for such studies 

could focus on VOC challenge gases and examine the top three VOC challenge gases, 

individually, to obtain preliminary results. 
12

Table 3. Challenge Gases and Experimental Concentrations 

Acid Challenge Gases VOC Challenge Gases Other Common Gases 

Compound Conc. 

(mg/m3

) 

Compound Conc. 

(mg/m3) 

Compound Conc. 

(mg/m3) 

sulfur 

dioxide 

260 ± 26 toluene 377 ± 38 formaldehyde 1.23 ± 12 

nitrogen 

dioxide 

190 ± 19 acetaldehyde 180 ± 18 ozone 1.96 ± 2 

nitric oxide 123 ± 12 hexane 352 ± 35 ammonia 70 ± 7 

hydrogen 

sulfide 

140 ± 14 2-butanone 295 ± 30 

chlorine 290 ± 29 isobutanol 303 ± 30 

 dichloromethane 347 ± 35 

 tetrachloroethylene 537 ± 54 

MICRO-SCALE MODELING 

Micro-scale modeling of the interaction of contaminants and sorbent media was carried out to 

better understand the processes and parameters involved, and ultimately to support macro-scale 

measurements and building modeling efforts. The main thrust of this work was to develop 

models that account for all the important processes thought to take place in an air cleaning 

device. In support of this, an attempt was made to characterize the pore system of the filter at the 

micrometer scale to supply a physical basis for permeability estimates of intra-bead permeability 

and diffusivity. 

Transport Model 

A numerical algorithm based on the lattice Boltzmann method was developed for modeling 

Navier-Stokes fluid flow in a bead pack geometry similar to that used in filter systems (Martys 

and Chen 1996; Nie and Martys 2007). The solution of flow fields obtained from the simulation 

is consistent with the Brinkman equation. That is, this model accounts for flow between beads as 

well as allowing for a non-zero bead permeability to account for possible flow through the beads. 

Once the local flow fields are determined, they serve as input into a code that solves for the 

advection, diffusion absorption and desorption of contaminants. The influence of flow through 

the bead on the absorption of contaminants in the bead pack can then be determined. 

Filter Characterization 

There are three levels of pore structure in a gaseous air-cleaning filter of the type considered in 

this modeling effort. The first is the pores between beads in the packing. This can be easily 

modeled, as will be seen below. The second is the pores in the beads that allow gas penetration 

into the beads. This pore scale was characterized via scanning electron microscopy (SEM) of cut 

beads and X-ray microcomputed tomography of whole beads, at the micrometer scale. The final 

level is the very small nanopores inside the beads that allow for sorption of contaminants. This 

level could potentially be characterized with transmission electron microscopy-based 

tomography and with focused ion beam tomography, but these techniques were not applied as 

part of the current project. 
13

Figure 5 shows optical and SEM images of a sliced activated carbon-based bead. The optical 

images show the gross overall morphology of the beads, as well as the cross-section. SEM 

images show finer detail of the structure inside of a bead. Figure 6 shows one slice out of many 

slices making up an X-ray tomographic image of a bead. While the SEM pictures provide 

qualitative insight into the intra-bead pore system, a 3-D pore system model could not be derived 

from the limited micrometer-scale tomography due to low resolution of the micropores in the 

bead. X-ray tomography may be able to provide a more complete pore model in the future. 

Figure 5: Optical and SEM images of beads. Top left – several beads, scale divisions are mm; 

top right – cross section of single bead; bottom left – SEM of cross-section, scale bar is 1 mm; 

bottom right – higher magnification image of cross-section, scale bar is 10 μm. 

Model Test Case 

Once the transport model was developed, the flow through a random packing of monosize beads 

was studied to determine the permeability of the total bead pack as a function of the permeability 

of the individual beads, since an experimentally-measured 3-D pore system model was not 

available. A monosize bead pack was used as an approximation, since the diameter of the actual 

beads did not vary by more than a factor of two. The beads were treated as continuum objects 

with a given permeability. The volume fraction of beads was about 0.50, which is typical for 

random deposition of mono size spheres. When modeling the dispersion, absorption and 

desorption of contaminant, it was assumed that the contaminant did not affect the actual flow 

field but moved passively. For test purposes, a simple linear absorption model was assumed. 
14

Figure 6: Cross-section slice from X-ray microcomputed tomography. Dark spots indicate pores. 

Scale is about 2.5 μm per pixel, and sample was about 4 mm across in this slice. 

Figure 7: Bead pack, arrow indicates flow direction 

Figure 7 illustrates a packing of spheres that was used in the simulation. The spheres may be 

inert solid or assigned a porosity, permeability, absorption/desorption coefficients and a relative 

diffusivity. The relative diffusivity is the diffusivity of a molecule relative to that in free space 

(air). That is, if a molecule has a certain diffusivity in air, its diffusivity is lower in the porous 

medium due to its interaction with the microstructure of the medium. These properties may be 

assigned individually, for each sphere, to allow for a distribution of properties as needed. A 
15

pressure drop is applied across the medium from left to right. Because periodic boundary 

conditions are maintained for the flow in the simulation, the fluid cycles through the medium. 

The system can be set up so that, initially, a concentration of contaminants is uniformly 

distributed. Alternately, a source or a sink term, allowing for addition or removal of contaminant 

at the inlet or outlet of the system, can be added. 

Figure 8 shows the dependence of the total permeability of the filter on the permeability of the 

beads. For small values of the bead permeability, the vast majority of the flow goes between the 

beads and thus there is little effect on the overall permeability. As the bead permeability becomes 

1/30th of the overall permeability, a sharp rise in the overall permeability begins. At very low 

bead permeability, the filter behaves like a packed hard sphere system. As the permeability of the 

beads is increased, two factors control the overall permeability of the system. Obviously the first 

cause of increase is the fact that there is additional flow though the beads. A second cause of 

increase is the fact that there is now an effective slip velocity at the bead surface corresponding 

to the original hard sphere surface. The slip velocity can significantly enhance total flow. 

Figure 8: Bulk permeability of bead pack as a function of the individual bead permeability. The 

arrow indicates the bulk permeability in the limit of zero bead permeability. 

Modeling contaminant transport 

The transport of contaminants in the fluid phase is described by the advection-diffusion equation 

CV D C

dt

dC t 2 ( ) = −∇ ⋅ + ∇ r r where D is the molecular diffusion coefficient of the contaminant in 

the fluid. A dimensionless number, Peclet number, Pe = Vl/D, where l, is a typical length scale, 

taken to be the bead diameter, that characterizes the relative importance of flow vs. diffusion in 
16

transport phenomena. Here cases where Pe = 1 and 10 are considered for the flow fields 

determined in the simulation. These values of Peclet number are representative of flow rates 

through filters corresponding to the lower flow rate regime. As the contaminant is transported, it 

may be adsorbed into or desorbed from the beads. This phenomena is modeled by including a 

sink/source term to the transport equation 

dCs(t)

dt = αC(t) − βCs(t). This is a simple linear 

model of the process, used for convenience, where Cs corresponds to the concentration in the 

porous medium, and α and β are adsorption and desorption coefficients. Figures 9 and 10 

illustrate how the performance of a filter system may be studied with this numerical approach. 

Figure 10 is the same case as that in Fig. 9, but with the diffusivity of the contaminant in a bead 

reduced by a factor of ten. All other factors, bead packing, initial concentrations, flow rates, and 

adsorption/desorption coefficients were kept the same. In this flow regime Pe = 10, so that 

hydrodynamic forces are more important than the diffusive motion of the contaminant. The 

concentrations of contaminant in the macro-pore space (blue), in the bead micropores (red), and 

in absorbed contaminants (green) were tracked in time. Clearly the efficiency of the filter in 

removing contaminants diminishes as the diffusivity of the contaminant in the bead decreases. 

Figure 9: Relative concentration vs. time in filter. The blue line is the concentration of 

contaminant in the macro-pores. The red line is the contaminant in the bead micropore space and 

the green is that adsorbed into the bead material. Note, the units of concentration here is 

dimensionless as we normalized the concentration of contaminant found in either the micropores 

or that absorbed by the beads by the total contaminant initially found in the macro-pores space. 

This way we know the fraction of contaminant residing in each part of the microstructure. 
17

Figure 10: Relative concentration vs. time in filter. Same color scheme as Fig. 9. In this case, the 

diffusivity of the contaminant was reduced by a factor of 10. As a result, the capability of rapid 

absorption is significantly reduced. 

Link to measurements of concentration over time 

It is important to note that the model used in this study is phenomenological and needs to be 

validated by experiment or replaced by a model based on experimental data for the medium 

concerned. This section describes one way in which this goal may be attained. In the case of the 

linear model used in this study, the values of absorption/desorption coefficients may be obtained 

from study of the concentration vs. time profiles associated with a dispersed contaminant in 

contact with a material (e.g. media used in filter) of interest. In general, depending on the form of 

the concentration vs. time profile, suitable reaction rate equations can be identified whose 

coefficients can be fit to the data. For example, let us assume that the time value concentration 

profile is well described by an exponential decay to some equilibrium. That is, 

C(t) = (C0 – C∞) exp(-kt) + C∞ , where C(t) is the concentration, in units of mass/volume, as a 

function of time, t, C0 is the initial concentration, C∞ is the final or equilibrium concentration and 

the decay rate, k, is determined by fitting to the data. Such profiles can be related to a linear 

reaction rate model describing the rate of adsorption of contaminant on a surface 

( ) ( ) ( ) C t C t

dt

dC t s

s =α − β , where C (t) s is the amount of contaminant (mass) per surface area, 

and α and β are rate coefficients. For this model, it can be shown that (1 ) C0

C

S

Vk ∞ α = − and 
18

C0

Ck ∞ β = , where V is the contaminated fluid volume in the experimental apparatus and S is the 

bead surface area. The above results assume that the concentration of contaminant corresponds to 

a dilute regime, the contaminant is uniformly mixed in the experimental cell and the adsorption 

process is reversible. Including effects such as irreversible reactions may be possible to model by 

including a sink term in the surface interaction term. Such analysis may also be extended to other 

cases such as when there is an upper bound to the amount adsorbed by a surface 

Future Research Needs 

This section described a model of the flow between and in microporous materials. It was 

demonstrated that flow in small micropores is highly restricted once the bead permeability was 

below some value. It was also found that the absorption is greatly diminished in this case. The 

effects of changing bead diffusivity were also modeled and the effect on the concentration of 

contaminant in the macro-pore space between the beads, in the bead micropores, and absorbed 

contaminants inside the beads were demonstrated. 

Future research needs include a validation program to compare simulation results with 

experiments. While accurate flow and contaminant transport simulations are possible, to predict 

the degree of contamination of a bead filter system, a boundary condition is needed that 

describes the rate at which the contaminant is adsorbed or desorbed from a surface. This may be 

obtained by study of the concentration vs. time profiles in experiments similar to that used for 

determining isotherms. As there are a plethora of phenomenological equations describing 

reaction rates, it is important to link parameters of models with experimental data when possible. 

Indeed, for a computational model to be a successful predictor, such accurate materials 

characterization is crucial. Increased microstructure characterization efforts, including focused 

ion beam and transmission electron microscopy-based tomography should give interesting and 

useful results and provide a more physical basis for bead properties. 

BUILDING SCALE MODELING 

In order to assess the potential impacts of gas-phase air cleaning on building occupant exposure, 

whole building simulations were performed using network modeling. These simulations are 

based on a previous study of retrofits for improved building protection (Persily et al. 2007), 

however the simulations reported here are more detailed. The building simulations employed the 

multizone airflow and contaminant dispersal model CONTAM (Walton and Dols 2005), which 

was applied to three generic buildings intended to represent U.S. commercial buildings. 

The three buildings include a single-zone one-story building, a two-story building with more 

realistic zoning, and a high-rise office building with central air handlers. Details on the buildings 

are provided in Persily et al. (2007), including building layout, occupancy levels, and ventilation 

systems. Table 4 describes the size and system types of the three buildings. The systems are 

assumed to have constant airflow rates, with the outdoor air intake rate equal to 10 % to 20 % 

(depending on the building) of an assumed supply airflow rate per unit floor area of 5 L/s•m2

(roughly 1 cfm/ft2). The envelope infiltration rate in the single-zone building is assumed to be 

constant at an air change rate of 0.2 h-1; in the other two buildings the infiltration rate is 

calculated with CONTAM based on airtightness values expressed as effective leakage areas at 

4 Pa of 5 cm2/m2 of wall area (0.07 in2/ft2) in the two-story office and 8.7 cm2/m2 (0.13 in2/ft2) in 

the high-rise. The baseline building models have no gas phase air cleaning. 
19

 # of stories Floor area, m2

 (ft2) Ventilation System Type 

1, Single-zone 1 1000 (10 800) Simple air handler 

2, Office 2 2600 (28 000) Single air handling system 

3, Office 14 11,900 (128 000) Central air handling systems 

Table 4 Simulated Buildings 

The simulations employed a nonreactive gaseous contaminant and the following four release 

locations: 

Exterior release distant from building 

Exterior release at outdoor air intake(s) 

Interior release in lobby 

Interior release into ventilation system return 

The outdoor, distant release is represented by a constant, elevated outdoor concentration of 

1 mg/m3

 for a period of 60 s. The release at the intake is modeled as a localized increase in the 

outdoor concentration at that specific location, again lasting 60 s. The indoor releases are 

represented by contaminant release rates of 10 g/s for 60 s in the designated location. Only the 

outdoor release was considered for the single-zone building. 

The calculated concentrations and the assumed release rates have no significance in relation to 

any particular chemical agent but were chosen to yield indoor concentrations in a reasonable 

range of interest. Given the generic nature of these contaminants and releases, and in keeping 

with the purpose of the project, the calculated concentrations cannot be used to estimate health 

impacts. As a result, the simulation results in terms of the relative concentration or exposure 

between the various simulated cases are of far more relevance than the absolute concentration 

and exposure values themselves. 

The air cleaning options considered in the simulations include removal efficiencies of 95 %, 

97.5 %, 99 %, 99.9 % and 99.99 %, located in either the outdoor air intake or the mixed 

airstream (downstream of where the outdoor air intake and recirculation air mix). In addition, a 

tighter envelope, corresponding to an air change rate of 0.01 h-1 in the single zone building and 

effective leakage areas of 0.7 cm2

/m2 (0.01 in2/ft2) in the two-story and high-rise buildings, was 

combined with the gaseous air cleaner in the outdoor air intake. Also in these two buildings, the 

increased envelope airtightness and enhanced outdoor air filtration were combined with a 

doubling of the outdoor air intake rate in an effort to pressurize the building interior. 

These simulations were run for 12 h with a 5 s time step with a wind speed of 5 m/s (11 mi/h) 

and an indoor-outdoor temperature difference of 20 °C (36 °F). The simulations yield 

contaminant concentrations as a function of time in each building zone, which were subsequently 

used to determine occupant exposures. The exposure of an individual building occupant is the 

average contaminant concentration to which he or she is exposed over the simulation period in 

units of mg•min/m3

. The average exposure for all occupants of the building is then determined 

from the individual occupant exposures. These exposures are calculated over 6 h, starting 1 h 

before the release and continuing 5 h after. For each simulation, the average building exposure is 

compared with the baseline exposure, i.e., without any gaseous air cleaning, to determine the 

exposure as a percentage of the baseline exposure. Therefore the baseline case corresponds to 

100 %, while a reduction in exposure results in a value below 100 %. 
20

Simulation Results 

Figure 11 is a plot of the simulated concentrations for the single zone baseline and enhanced 

filtration cases with a 95 % removal efficiency, subject to the outdoor release. The baseline case 

is the solid line with a peak concentration just under 0.025 mg/m3. The other lines show the 

impacts of air cleaner location (outdoor or mixed airstream) and of combining reduced 

infiltration (airtightening) with the air cleaner located in the outdoor air intake. Locating the air 

cleaner in the outdoor air intake reduces the indoor concentrations significantly, but the mixed 

air location is more effective given the high recirculation airflow rates. Tightening the building 

results in significantly less contaminant entering the building due to infiltration, corresponding to 

the lowest concentrations in Figure 11. 

Figure 11 Simulation results for single-zone model with 95 % efficient air cleaner 

Figure 12 shows the simulation results for the outdoor air intake release over the five values of 

the removal efficiency. The incremental concentration reductions associated with the increased 

efficiencies are evident, but the existence of a significant infiltration rate results in diminishing 

returns given that this infiltrating air is not cleaned. 
21

Figure 12 Simulation results for single-zone model with varying removal efficiencies 

Figure 13 summarizes all the simulation results for the single-zone model in terms of the 

exposure relative to the baseline case with no gas phase air cleaning. This figure shows the 

higher impact of locating the air cleaner in the mixed air relative to the outdoor air, as well as the 

exposure reductions associated with airtightening. However, the residual infiltration for each air 

cleaning configuration is seen to limit the exposure reductions that can be achieved with higher 

removal efficiencies. 
22

Figure 13 Summary of the simulation results for single-zone model 

Figures 14 and 15 summarize the simulation results for the two-story and high-rise office 

buildings. In these cases, four release scenarios were considered: a general increase in the 

outdoor concentration, a local release at the intake, and releases in the building lobby and a 

return vent. The two indoor releases were only considered for the mixed-air cases. As was the 

case for the single-zone simulations, the mixed air location is seen to be more effective than 

cleaning only the outdoor air. However, for both locations, the residual infiltration limits the 

overall reduction that is achievable through increased removal efficiencies. Reducing infiltration 

by tightening the building or increasing the outdoor air intake in an attempt to pressurize the 

building results in very low relative exposures at the higher efficiencies. 
23

Figure 14 Summary of the simulation results for two-story office building 

Figure 15 Summary of the simulation results for two-story office building 
24

The whole building simulations show the significant reductions in exposure that can be achieved 

through increasing levels of gas-phase air cleaning. However, they also show the importance of 

system location and the decrease in overall effectiveness that will occur when outdoor 

contaminants enter a building through infiltration, which does not pass through the air cleaning 

system. Reducing such infiltration through building tightening and increased levels of outdoor 

air intake will increase the overall air cleaning effectiveness. 

DISCUSSION 

Gas phase air cleaning is playing a key role in building protection strategies, which is likely to 

increase as new technologies are developed and new design and performance criteria are 

developed. In order to meet these new demands, standardized methods of test for such systems 

need to be developed to support design decisions along with standards and other guidance on 

installation, operation and maintenance activities. While some such standards have been 

progressing, such as ASHRAE Standard 145.1P, these needs still exist. Additional research is 

needed to define more reliable and perhaps cheaper and faster laboratory test methods and to 

relate the results of these tests to installed performance in the field. The efforts described in this 

report represent some initial steps in meeting these needs, but additional work needs to be 

pursued to support the standards that are required into the future. 

ACKNOWLEDGEMENTS 

This work was supported by the Directorate of Science and Technology of the U.S. Department 

of Homeland Security, 

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