NIST Special Publication 1113

Real-Time Monitoring

of Total Inward Leakage

of Respiratory Equipment

Used by Emergency Responders:

Workshop Proceedings

Kathryn M. Butler

Marc R. Nyden

Rodney A. Bryant

NIST Special Publication 1113

Real-Time Monitoring

of Total Inward Leakage

of Respiratory Equipment

Used by Emergency Responders:

Workshop Proceedings

Kathryn M. Butler

Marc R. Nyden

Rodney A. Bryant

Fire Research Division

Building and Fire Research Laboratory

National Institute of Standards and Technology

July 2010

U.S. Department of Commerce

Gary Locke, Secretary

National Institute of Standards and Technology

Patrick D. Gallagher, Director
Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately. Such

identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Special Publication 1113

Natl. Inst. Stand. Technol. Spec. Publ. 1113, 102 pages (July 2010) 

CODEN: NSPUE2
i

ABSTRACT

A workshop on respirator sensors was held at the National Institute of Standards and 

Technology (NIST) on May 1, 2009. The objective of this workshop was to discuss and 

document the need for real-time monitoring of the respiratory intake of emergency 

responders; to identify appropriate sensing technologies; and to identify and discuss a 

scientific strategy (employing both modeling and experimental validation) to determine 

optimal implementation of respirator sensors. This workshop was attended by over 25 

people from the fire service, industry, government agencies, and academia. Collectively, 

they had expertise in the areas of chemical detection, occupational health and safety, 

firefighting technology, fluid flow, and fire protection engineering. Speakers gave eleven 

presentations in three sections that identified the needs of firefighters, reviewed the state 

of sensor technology, and stated the challenges posed by the integration of this 

technology into SCBA respirators. At the conclusion of each of these sections, the 

participants discussed ideas, issues, and concerns with real-time monitoring. This report 

summarizes the knowledge gained from this workshop and our analysis of the scientific 

and engineering challenges involved in the development and implementation of respirator 

sensors.

The report is structured by the three sections of the workshop: First Responder Needs 

and Concerns, Sensor Technologies, and Sensor Integration and Engineering. In each 

section, descriptions of each talk are followed by the points covered in the discussion 

among workshop attendees. The presentations themselves are provided in the 

appendices.
ii

ACKNOWLEDGMENTS

The success of any workshop is dependent on the hard work of the individual speakers, 

facilitators, and participants. These proceedings are an assimilation of the contributions from 

everyone involved in the workshop; copies of the presentations are included in the 

appendices. Thanks to all who made presentations and to those who participated in the 

workshop. 

We appreciate the assistance of Ms. Barbara Huff of NIST, who helped with travel and other 

logistics. Thanks also go to Teresa Vicente, who set up the online workshop registration.

The Science and Technology Directorate of the U.S. Department of Homeland Security 

funded the production of the work presented in this material under Interagency 

Agreement HSHQDC-08-C-00406 with the National Institute of Standards and 

Technology (NIST). The results published herein are solely the responsibility of the 

authors.
iii

ABSTRACT......................................................................................................................... i

ACKNOWLEDGMENTS .................................................................................................. ii

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

2 FIRST RESPONDER NEEDS AND CONCERNS............................................... 4

2.1 NIOSH Fit Test Research – Ziqing Zhuang (NIOSH / NPPTL)........................ 4

2.2 Fit Testing First Responders – Dawn Bolstad-Johnson (Phoenix Fire 

Department)........................................................................................................ 5

2.3 Discussion .......................................................................................................... 7

3 SENSOR TECHNOLOGIES ................................................................................. 9

3.1 Gas Sensing with Porous Silicon Photonic Crystals – Brian King 

(University of California at San Diego) ............................................................. 9

3.2 NIST Chemiresistive Microarray Technology – Kurt Benkstein 

(NIST/CSTL).................................................................................................... 10

3.3 MEMS Sensor Development for End-of-Service-Life Indicators – Nathan 

Lazarus (Carnegie Mellon University)............................................................. 12

3.4 Ultrasound in Respirators: Concepts and Preliminary Results – William 

King (NIOSH/NPPTL)..................................................................................... 14

3.5 Chemical Sensors for Aerospace Applications: From Sensor Platforms to 

System Application – Gary Hunter (NASA Glenn Research Center).............. 15

3.6 Discussion ........................................................................................................ 17

4 SENSOR INTEGRATION AND ENGINEERING............................................. 19

4.1 Sensor Development for ESLI & Its Application to Chemical Detection –

Jay Snyder (NIOSH/NPPTL)........................................................................... 19

4.2 Engineering Considerations – Paul Greenberg (NASA Glenn Research 

Center).............................................................................................................. 20

4.3 Measuring and Visualizing Flows Inside Respiratory Masks – Arthur 

Johnson (University of Maryland at College Park).......................................... 21

4.4 Simulating Flows Inside (and Outside) Respiratory Masks – Kathryn Butler 

(NIST/BFRL) ................................................................................................... 23

4.5 Discussion ........................................................................................................ 24

5 CONCLUSIONS.................................................................................................. 26
iv

APPENDIX A Workshop Agenda ............................................................................... 29

APPENDIX B Workshop Registrants and Attendees ................................................. 30

APPENDIX C NIOSH Fit Test Research – Ziqing Zhuang (NIOSH/NPPTL) ........... 31

APPENDIX D Fit Testing First Responders – Dawn Bolstad-Johnson (Phoenix Fire 

Department) ......................................................................................... 38

APPENDIX E Gas Sensing with Porous Silicon Photonic Crystals – Brian King 

(University of California San Diego) ................................................... 46

APPENDIX F NIST Chemiresistive Microarray Technology – Kurt Benkstein 

(NIST) .................................................................................................. 53

APPENDIX G MEMS Sensor Development for End-of-Service-Life Indicators –

Nathan Lazarus (Carnegie Mellon University) .................................... 58

APPENDIX H Ultrasound in Respirators: Concepts and Preliminary Results – 

William King and Jonathan Szalajda (NIOSH/NPPTL) ..................... 63

APPENDIX I Chemical Sensors for Aerospace Applications: From Sensor 

Platforms to System Application – Gary Hunter (NASA Glenn 

Research Center) .................................................................................. 68

APPENDIX J Sensor Development for ESLI & Its Application to Chemical 

Detection – Jay Snyder (NIOSH/NPPTL) ........................................... 76

APPENDIX K Engineering Considerations – Paul Greenberg (NASA) ..................... 81

APPENDIX L Measuring and Visualizing Flows Inside Respirator Masks – Arthur 

Johnson (University of Maryland at College Park) ............................. 84

APPENDIX M Simulating Flows Inside (and Outside) Respirator Masks – Kathryn 

Butler (NIST) ....................................................................................... 96
1

1 INTRODUCTION

The atmosphere within a burning building is deficient in oxygen but rich in carbon 

dioxide, soot, and other aerosol particulates. Dangerous levels of carbon monoxide are 

almost always present in fire effluent, which can also contain hydrogen cyanide, 

hydrochloric acid, and known carcinogens including benzene, formaldehyde, and 

polyaromatic hydrocarbons.

Firefighters and other emergency responders depend on respiratory protective equipment 

to protect them from these hazardous substances while they are working in fire 

environments and under other IDLH (immediately dangerous to life and health) 

conditions. A self-contained breathing apparatus (SCBA) is typically used, consisting of 

a full-facepiece respirator mask covering mouth, nose, and eyes, an air cylinder, and a 

pressure regulator (Figure 1). The SCBA is operated under positive pressure, so that in 

the case of a leak, the flow of gases is designed to be outward. However, momentary 

negative pressure conditions have been recorded in testing, and it is uncertain whether 

there are conditions under which a leak in the seals or exhalation valve may allow 

harmful substances to get inside of the mask.

Figure 1. Firefighter gear including SCBA.

The breathing gases are supplied by a cylinder carried by the user. In the US, the 

majority of fire companies use compressed air, which is inexpensive and easy to supply 

using compression equipment at the firehouse or close to the fireground. A disadvantage 

is the limitation of respirator usage time for a single air cylinder to an hour or less, 

depending on bottle size and fill pressure, which can be reduced to as little as 10 minutes 

for exceptionally high workrates. The quality of the compressed air depends on the 

quality of the supply, and care must be taken to place the pump input away from fire 

equipment engine exhaust and other sources of pollutants. Breathing gas cylinders for 

firefighters may alternatively contain air liquified using liquid nitrogen or compressed 
2

oxygen. The latter is the closed-circuit SCBA, which both employs a CO2 scrubber to 

maintain a breathable environment within the respirator and supplements the depleted 

oxygen with addition of O2 from a cylinder. Both liquid air and compressed oxygen 

technologies provide significantly longer respirator usage times, with the disadvantages 

of higher complexity and expense. However, regardless of the type of system, for the 

positive pressure SCBA respirator a leak will increase the rate at which breathing gases 

are used. 

Prior to certification to use a SCBA, each firefighter must pass a fit test to demonstrate 

that the selected facepiece fits sufficiently well. In the past, this fit test asked firefighters 

to report the detection of odors when immersed in an environment containing a test gas, 

such as banana oil. Most fire companies now use the Porta-Count, a device that 

compares the number of particles detected outside the respirator mask with those detected 

within, under conditions that include various head and body movements and talking. The 

degree of protection provided by the respirators is quantified by several measures, 

including the fit factor (FF) measured during the fit test, the assigned protection factor 

(APF) that the respirator is expected to meet, and the workplace protection factor (WPF) 

that is actually experienced in the workplace.1 Ideally, the concentration of each type of 

hazardous gas or particulate within the respirator would be measured in real time and an 

alarm would signal values that exceed the permissible exposure limit. This is not 

currently practical, however, due to the complexity of the hazardous environment and 

limitations in commercial sensor technology. The environment faced by the firefighter is 

not well-characterized. In addition, synergistic effects that may worsen the health effects 

of one component in the presence of another are not well-known.

To maintain their certification to use their SCBA, firefighters must pass fit tests every 

year. This regimen is intended to ensure that the user is not exposed to hazardous 

substances while using their SCBA in fire environments. However, little data exists to 

support this assertion and there are some reasons to question it, as will be discussed later 

in this report. If, however, conditions inside the SCBA mask were monitored in real 

time, the user could be alerted in the event that the equipment fails to provide adequate 

protection due to inward leakage from poor face fit or a mechanical failure, such as a leak 

in the exhalation valve.

Monitoring the respiratory intake of a firefighter using a SCBA is a challenging problem. 

The sensing technology to do this must be adaptable to the constraints imposed by 

facemasks in geometry and in power availability (e.g. battery life requirements) and at 

the same time must be responsive, sensitive, and sufficiently robust to survive and 

perform in a fire environment. Furthermore, respirator sensors must be positioned so that 

the readings accurately reflect respiratory intake. Placement of sensors at the intake 

through the mouth or nose may be representative, but this may not be practical, and 

locating sensors within dead spaces and eddies will adversely affect their response.

A workshop on respirator sensors was held at the National Institute of Standards and 

Technology (NIST) on May 1, 2009. The objective of this workshop was to discuss and 

document the need for real-time monitoring of the respiratory intake of emergency 
3

responders; to identify appropriate sensing technologies; and to identify and discuss the 

development of a scientific strategy (employing both modeling and experimental 

validation) to determine optimal implementation of respirator sensors. This workshop 

was attended by over 25 people from the fire service, industry, government agencies, and 

academia. Collectively, they had expertise in the areas of chemical detection, 

occupational health and safety, firefighting technology, fluid flow, and fire protection 

engineering. Speakers gave a total of eleven presentations in three sections that identified 

the needs of firefighters, reviewed the state of sensor technology, and stated the 

challenges posed by the integration of this technology into SCBA respirators. At the 

conclusion of each of these sections, the participants discussed ideas, issues, and 

concerns with real-time monitoring. What follows is a report of what was learned from 

this workshop and our analysis of the scientific and engineering challenges involved in 

the development and implementation of respirator sensors.

The report is structured by the three sections of the workshop: First Responder Needs 

and Concerns, Sensor Technologies, and Sensor Integration and Engineering. In each 

section, descriptions of each talk are followed by the points covered in the discussion 

among workshop attendees. The presentations themselves are provided in the 

appendices.
4

2 FIRST RESPONDER NEEDS AND CONCERNS

Presentations on issues associated with respirator fitting and the use of SCBAs by 

firefighters were given by Ziqing Zhuang, a research engineer at the National Personal 

Protective Technology Laboratory (NPPTL) of the National Institute for Occupational 

Safety and Health (NIOSH), and Dawn Bolstad-Johnson, an industrial hygienist with the 

Phoenix Fire Department. 

2.1 NIOSH Fit Test Research – Ziqing Zhuang (NIOSH / NPPTL)

To ensure safe use of any respirator with a tight-fitting facepiece, first responders must 

pass a fit test. For SCBAs, this must be a Quantitative Fit Test (QNFT), in which the 

concentrations of a surrogate substance are measured in the surrounding atmosphere and 

within the respirator mask. The widely-used Portacount instrument, for example, counts 

the number of particles inside and outside the mask. The fit test is typically mandated 

prior to the initial use of the respirator, at least annually thereafter, and also whenever 

there is a change in the respirator facepiece or in the physical condition of the wearer. 

The ratio of the concentration outside to the concentration inside the facepiece, referred 

to as the fit factor (FF), is a measure of how effective the mask is in preventing inward 

leakage of gases and particulates. For the purposes of the fit test, the SCBA air tank is 

disconnected and HEPA filters are attached to the facepiece so that the respirator is 

functioning in negative pressure mode. To pass, the overall FF must exceed 500. This 

corresponds to an assigned protection factor (APF), the level that must be met in the 

workplace by the respirator, of about 10,000 under normal operating conditions in which 

the pressure is positive.

The fit test is an art as well as a science. In the attempt to predict workplace protection 

from a test performed in a relatively clean room under controlled circumstances, the test 

must incorporate a large margin of safety. The overall FF is calculated over several 

exercises, including bending, turning side to side, turning head up and down, and talking. 

During the exercises, a leak in the chin region is sometimes observed. Talking may result 

in additional particles in the mask emitted by the lungs, confounding Portacount 

measurements that compare the number of particles inside the mask to outside. Two 

types of error are identified for fit factors: the alpha error, which describes the error that 

the wearer fails the fit test while the respirator adequately fits the wearer, and the beta 

error, which describes the error that allows a respirator to pass the fit test even though it 

doesn't fit very well.

Given these issues, validating fit test methods is an important focus of recent 

NIOSH/NPPTL research efforts. A fit testing milestone is shown in Figure 2, in which a 

correlation was demonstrated between FF and actual exposure (measured by analyzing 

end-exhaled air for Freon-113 concentration) experienced by the user of half-mask 

respirators.2

 Recent research efforts also found correlation between quantitative fit test 

results with actual workplace protection for half-facepiece respirators.3
5

Figure 2. Correlation between fit factor and actual exposure 

for Continuous High Flow Deep Probe fit test.

Current respirator fit research at NIOSH includes the characterization of worker faces 

through anthropometry, the investigation of the correlation of facial dimensions with fit, 

and the definition of new fit test procedures, such as multiple donnings and variations in 

exercise type and duration.4,5 The goal is to reduce both alpha and beta errors. One 

approach is to determine which fit test exercises (talking? nodding?) contribute most to 

the variability of results. The measurement of FF in the workplace continues to be a 

challenge. Possible sensor measurements to monitor FF include pressure drop, ambient 

particles, and CO2 (exhalation vs. inhalation). In addition, there is the question of 

whether the Portacount instrument could be made transportable. A potential remote 

measurement is surface temperature measured with an infrared (IR) camera, which is 

currently being used to look for flu victims.

2.2 Fit Testing First Responders – Dawn Bolstad-Johnson (Phoenix Fire 

Department)

The Fire Department in Phoenix, Arizona dispatches approximately 145,000 emergency 

calls each year. Of these, about 10 % (14,500) are fires, and about 1000 are large fires. 

This represents a significant base of experience among the 57 fire stations and 1677 

members servicing this community.

Phoenix firefighters are fit-tested at least annually, using the OHD Portacount and the 

six-step REDON fit test protocol.6

 In order to pass the fit test, firefighters may cinch the 

respirator more tightly than they would at the fire, sometimes resulting in red marks on 

the face.

Facial features are observed to affect the fit test results. Temples are a problem area, and 

narrow faces, high cheekbones, long chins, and facial scarring can cause difficulties. The 
6

82 female members are overrepresented among those difficult to fit. In 2009, 10 

individuals out of about 1700 failed their fit tests and were refitted with new respirator 

masks. The number represents a significant improvement over the previous year and 

parallels a change in respirator supplier. For one firefighter, a fit was achieved by 

attaching foam to the seal. 

Fit may change with time between fit tests. The shape of the face may change, due to 

dental work and weight gain or loss, for example. The respirator seal may also change. 

Seals may become distorted during improper storage in areas that are not environmentally 

controlled, such as those shown in Figure 3. Changes in temperature (e.g. for respirators 

left in the trunk of a car) affect the elasticity of the seals and straps that secure the mask 

to the face.

Figure 3. Some storage conditions for respirator gear.

The evidence for respirator leaks leading to inhalation of smoke during fire operations is 

not well characterized. Some firefighters have reported experiencing a condition referred 

to as “blow-by” – soot marks on the face or in nose mucus – that suggest that soot has 

penetrated the facemask during the fire. However, it has not been possible to reproduce 

this phenomenon under controlled circumstances, and the obvious conclusion that it is 

due to a leak is confounded by the practices of clicking the regulator into place only when 

smoke is smelled and of removing the SCBA during overhaul. In addition, masks are 

currently stored in black cases that go into the fire area and get dirty themselves. On the 
7

other hand, it is not uncommon for firefighters to bump into things while they are 

working to put out a fire, which can disturb the position of the facemask enough to cause 

a temporary negative pressure and perhaps a transient leak. In addition, heavy sweating 

and hard breathing can adversely affect the fit of the mask to the face and cause the 

internal pressure to become less than ambient.

The idea of real-time monitoring of respiratory phenomena during fire operations raises 

some questions. What should be the response of the firefighter or incident commander to 

a temporary negative pressure that is then fixed, such as that caused by a bumped mask? 

A slow leak in a positive pressure respirator system means that the bottle makes up the 

difference, resulting in less time until the air runs out. Since small leaks around the face 

would cause the air cylinder to dump more air, should the additional flow of air be 

monitored and reported?

2.3 Discussion

The discussion period following this set of talks addressed the nature of leaks for an 

SCBA, the blow-by phenomenon, possible applications for sensors, and concerns about 

real-time monitoring.

An SCBA is a positive pressure device, so that the natural direction of flow in the 

presence of a leak is outward. The demand valve responds to any lowering of pressure 

with increasing release of supply air. However, certification requires pressure testing on 

a breathing machine only up to a maximum demand of 300 L/min. Since peak 

respiratory flow rates can exceed this value (probably reaching values as high as 450 

L/min to 470 L/min, although up to 700 L/min is claimed by some), it is possible that the 

pressure inside of an SCBA may become negative at times when the firefighter is 

breathing heavily due to extreme exertion. This is referred to as overbreathing the 

respirator. Furthermore, it is known that there are spatial variations in the pressure due to 

the complexity of the flow fields created by the interaction between the stream of air 

coming from the cylinder and respiration within the confines of the facemask. Thus, to 

the extent that there are times when the pressure falls below ambient in any region of the 

facemask, the user will be vulnerable to inward leakage of hazardous gases and 

particulates. Leaks are possible through the face seal and valves. For filtering facepiece 

respirators, leaks are also possible through the filter cartridge.

Physical evidence that suggests respirator leaks may be occurring during a fire includes 

the facial markings and black nose mucus known as blow-by. Outward streaks appear to 

represent a positive pressure leak combined with sweating. Streaks along the cheekbone 

have been reported but not confirmed. Blow-by has been observed in Phoenix and by 

firefighters in other locations as well. Claire Austin, a Canadian researcher, noted that 

this is a historical problem that she has not been able to replicate. She hypothesized that 

it may have been due to dirty equipment, such as from oil getting into the cylinder. In 

real situations, the user can usually feel a leak. It was speculated that there should not be 

blow-by unless the firefighter takes off the mask early, and that the soot may be 

transmitted to the face by touching. Workshop participants agreed that this phenomenon 

requires further study.
8

A side issue of safety was brought up regarding body smells that may last up to three 

days after a fire. Personal protective clothing worn by firefighters doesn't seal against 

gases, which may be absorbed by the skin and hair. Some female firefighters have asked 

if it is safe to breastfeed after having participated in a major fire. The risks of these 

absorbed gases and the outgassing odors are unknown.

There was much discussion about the best use of sensors. While a smart mask that 

monitors its own fit and protective capability can be a goal for the future, there are other 

applications worth consideration. Some participants, especially representatives of the fire 

services who have direct experience with the use of SCBAs and other firefighting 

technologies, supported the idea that portable sensing devices could be used to determine 

when it was safe for firefighters to disconnect their air cylinders and remove their 

facemasks. A scientific measure of hazardous substances could help to break down some 

of the cultural resistance to wearing a respirator during overhaul, for example. Sensors 

could collect data on respiratory protection in the workplace, answering the question of 

what gases and particulates get through the protective barrier of the respirator, and the 

results can be compared with controlled laboratory experiments. Sensors can also be 

used to collect data on metabolic effects and to monitor health. How the conditions in the 

respirator affect health remains an unanswered question. In addition to providing 

valuable information, data collection may be a useful way to introduce sensor technology 

into the firefighting culture. As with any new technology, acceptance requires a long 

time, and the wearer needs to be able to trust the sensor if he or she is to make decisions 

based on its output.

Several concerns were raised about the capabilities and appropriate usage of respirator 

sensors. The first question is what should be measured. Particulates are different than 

vapors. Diffusion coefficients differ, as do filtration and permeation characteristics. 

Health effects for particulates are strongly dependent on size, and there is a wide range of 

sizes of particles generated in a fire. Standard fit tests are not necessarily a good guide 

for sensor choice. Corn oil vapor, sometimes used in fit tests, is not the same as the 

materials in the fire. The Portacount fit test is based on particulates, although it is at least 

as important to protect the first responder against gases. The chemical species of gas to 

measure is not clear. For example, carbon monoxide is not a good surrogate for other fire 

gases. It may be most appropriate to use multiple sensors to monitor multiple gases. 

Other issues include how to determine whether the sensor is sensitive enough and fast 

enough for the purpose of monitoring protection, and how much information to provide 

the wearer. Some wearers find the heads-up display in some new respirators to provide 

too much data and turn it off.
9

3 SENSOR TECHNOLOGIES

The modification of SCBA facemasks to include sensors capable of detecting hazardous 

gases or unusual pressure changes, and thus detecting leaks and monitoring the fit factor, 

would provide assurance that a SCBA is working properly and that the user is protected. 

Although this is a challenging technical problem, recent advances in microelectronics 

have enabled the development of a new generation of novel, compact sensors that appear 

to possess many of the properties required for this application. The current state of the art 

in sensor technology was reviewed in a series of presentations by Brian King, a Ph.D. 

student at the University of California at San Diego (UCSD), Kurt Benkstein, a research 

chemist working in the Chemical Science and Technology Laboratory (CSTL) at NIST, 

Nathan Lazarus, a Ph.D. student at Carnegie Mellon University (CMU), William King at 

NIOSH/NPPTL, and Gary Hunter, a researcher at the National Aeronautics and Space 

Administration (NASA). 

3.1 Gas Sensing with Porous Silicon Photonic Crystals – Brian King 

(University of California at San Diego)

Professor Michael Sailor and co-workers at UCSD are working with NIOSH/NPPTL on 

the development of microsensors for the detection of organic vapors as they break 

through a filter bed, signaling the end of life for a respirator filter. These devices must be 

small, sensitive, and tunable to the type of gas whose passage the filter is designed to 

prevent. To accomplish this, these investigators have fabricated thin films made from 

porous silicon crystals.7

 Electrochemical etching generates rugate pores, characterized by 

wrinkles whose periodic structure determines the refractive index of the crystalline film.8 

The structure is well-controlled and is designed to correspond with the gas to be detected. 

When irradiated with white light, the crystals reflect only light within a narrow band of a 

specific wavelength determined by the refractive index. If the air that occupies these 

pores is displaced through capillary condensation by an organic compound, it results in a 

characteristic spectral shift that can be measured. An example from the presentation 

shows air reflecting as green while toluene vapor reflects as orange.

Figure 4 shows how this sensor is used to signal breakthrough of organic vapors in a 

filter. The sensor is mounted at the tip of an optical fiber probe that is embedded into the 

filter at the desired monitoring position. The probe is attached to both a light source and 

to a spectrometer that detects a shift in reflected wavelength. The magnitude of the 

spectral shift indicates the type of vapor detected and the concentration, and the 

specificity can be improved by modifying the surface chemistry of the film. Sensor 

response time is on the order of 10 s. The researchers are also looking into ways to 

decrease the power requirements of the light source and the detection circuitry with an 

LED and photodiode detection system.
10

Figure 4. Spectral shift caused by displacement of air by organic compound.

The characteristics and advantages of this approach are:

• Flexible, thin fiber optic sensors for gases and vapors

• Simple fabrication

• High sensitivity to small quantities of a gas due to the large surface area

• Good selectivity due to easy modification of the surface and tunable optics –

allows targeting of classes of vapors

• A multiple sensor array may be used to detect multiple vapors

• Size – miniaturization of fiber optic sensors and source/detection hardware

• Low power

Some issues are:

• How long is the response time? The response depends upon the diffusion rate of 

the gas into the porous film and ranges from milliseconds to minutes depending 

on the physical properties and concentration of the gas and the thickness, surface 

chemistry, and morphology of the sensor film.

• What happens if the device sits in diesel fumes for a while? The device can be 

reset with heat at a temperature over 100 °C.

• The system must be calibrated with temperature.

3.2 NIST Chemiresistive Microarray Technology – Kurt Benkstein 

(NIST/CSTL)

Scientists in the Chemical Science and Technology Laboratory (CSTL) at NIST are 

working on another sensor technology that has potential for application to respirators. 

Dr. Steven Semancik and co-workers have pioneered the development of microhotplate 

arrays with metal oxide sensing films, with the objective of developing tunable 
11

chemical/biochemical microsensors that are reliable even in complex dynamic 

environments.9 The microhotplate is a matrix of sensing elements, each of which is 

approximately 100 μm x 100 μm in size. As shown in Figure 5, each element consists of 

three functional components: a polysilicon resistor, which generates heat by application 

of a current, platinum interdigitated electrical contacts, and a metal oxide sensing film. 

An insulating layer of silicon dioxide (SiO2) separates the functional regions. The 

adsorption of gaseous agents onto the metal oxide film changes the electrical conductance 

of the film. A combination of sensing materials and temperatures results in a 

characteristic electrical signature that is different for each gas. Due to their extremely low 

mass (~0.2 μg), these assemblies can be temperature-ramped very rapidly to about 

500 oC, with heating rates approaching 106 oC/s. This ability to program the temperature 

of the sensor in time, and thereby to change the kinetics of the chemisorption process, 

provides an additional dimension for distinguishing between analytes. It is 

straightforward to replicate this structure to produce multi-element arrays, as shown in 

the bottom of Figure 5. Signal processing methods train the device to discriminate 

between multiple target chemicals in real time.

In his presentation, Dr. Benkstein presented data demonstrating the use of microhotplates 

with metal oxide sensing films to detect dangerous industrial chemicals, such as ammonia 

and hydrogen cyanide, in an environment containing non-targeted chemicals such as 

paint fumes and window cleaner and further complicated by humidity. These results are 

particularly promising, since many of these gases are known to be present in fire 

atmospheres. Current work on the monitoring of exhaled breath for the presence of 

acetone, a biomarker for diabetes, is directly applicable to the respirator application of 

interest at this workshop.

Figure 5. An insulating layer of SiO2 separates functional regions.

packaged

device

2 mm × 2 mm, 

16-element 

array

layers

schematic

Sensing Film

Film Contacts

Insulating SiO2

Doped Polysilicon

Heater

Insulating SiO2
12

The advantages of this approach are:

• Miniaturization on a chip

• Ease of expansion – A single chip can house a matrix of sensors to separate 

several specific chemicals

• Use of signal processing to train the device to recognize and classify chemicals

• Monitoring of breathing is currently being studied

Some issues are:

• Higher sensitivity to trace concentrations of μmol/mol and below, shorter 

response time, and longer stability to drift, which may be improved by the use of 

nanostructured materials

• Training the device to ignore dynamic changes in the background environment, 

such as fluctuating humidity and innocuous species (interferents)

• Dealing with temperature cycling, which changes the conductivity and shape of 

the response. This requires good understanding of the effects of heating within 

the respirator facepiece

3.3 MEMS Sensor Development for End-of-Service-Life Indicators – 

Nathan Lazarus (Carnegie Mellon University)

A research effort directed by Professor Gary Fedder at CMU, in collaboration with 

NIOSH/NPPTL, has demonstrated the efficacy of gold nanoparticle chemiresistor sensors 

as End-of-Service-Life Indicators (ESLIs) for respirator filter cartridges.10 This 

application uses MEMS (microelectromechanical systems) technology to integrate an 

array of sensors, potentially including chemiresistors, mass-sensitive cantilevers, and 

humidity and temperature sensors, with signal processing electronics to create an 

"electric nose". MEMS technology is currently used in automobile airbag 

accelerometers.

To create a chemiresistor sensor, gold nanoparticles dissolved in trichlorobenzene are 

deposited by an ink jet printer onto a spiral gold electrode etched on a silicon substrate. 

The sensor has a diameter of about 200 μm. As shown in Figure 6, the presence of 

volatile organic compounds can be detected by monitoring the resistance of the electrode, 

since chemisorption increases the distance between the gold nanoparticles. A second 

method measures chemisorption by monitoring changes to the frequency of a cantilever 

gravimetric sensor, as shown in Figure 7.11 These sensors are sensitive enough to 

measure very small concentrations in the nmol/mol range. They can be reset by heating. 

In addition to the chemical sensors, sensors for humidity and temperature can be 

fabricated to measure changes in dielectric capacitance and metal resistance respectively.

The complete MEMS device may consist of an array of sensors to measure chemicals of 

interest, temperature and humidity. The integrated electronics analyzes the sensor data, 

using the temperature and humidity sensors to compensate for these factors. The device 

is then embedded into the respirator filter cartridge at a distance from one end that 

provides for adequate warning of impending chemical breakthrough.
13

Figure 6. Chemiresistor sensor, in which changes in electrical resistance indicate 

the presence of volatile organic compounds.10

Figure 7. Cantilever sensor, in which changes in resonant frequency indicate the 

presence of volatile organic compounds.11

The advantages of this approach are:

• Size – Miniaturized mechanical structures, integrated electronics for signal 

processing

• Sensitive to a range of chemicals in the pmol/mol range

• Ease of expansion – Array of sensors

• Stable – long shelf life, insensitive to temperature and humidity

• Low cost
14

Some issues are:

• The presence of particulates, which may interfere with the mechanical cantilevers

• Handling the temperature variations resulting from heating within the respirator 

facepiece

3.4 Ultrasound in Respirators: Concepts and Preliminary Results –

William King (NIOSH/NPPTL)

Dr. William King from NPPTL/NIOSH reported on preliminary results for a different 

approach for monitoring inward leakage in respirators. Rather than sensing the presence 

of an unwanted compound, this work seeks to detect the leak by measuring ultrasonic 

acoustic emissions associated with the flow field in the mask. The work was motivated 

by the much lower value and large scatter in measurements of Workplace Protection 

Factors (WPF) compared to Quantitative Fit Factors (QNFF). To identify the most 

important factors that account for this difference in WPF, there needs to be a method to 

monitor fit factor of the actual respirator in real time during workplace operations.

Ultrasound is a frequency range of sound pressure that is outside of the human audible 

range and is not identified with a health risk. It can be used to detect a leak in one of two 

ways. An ultrasound generator on one side of the leak may send out a signal that is 

detected and analyzed by a receiver on the other side, or, if the flow through the leak is 

turbulent, the leak itself may generate detectable ultrasound radiation. Estimates of the 

flow through a presumed respirator leak and from ordinary respiration show that only 

nasal breathing is expected to be a source of ultrasound. Initial investigations show that 

the strength of the ultrasonic signal through a leak from either a controlled source or from 

nasal breathing provides a measure of the leak size. As shown in Figure 8, a correlation 

was found between acoustic intensity and protection factor measured in a traditional 

manner for several half-facepiece respirators, indicating that this approach may be 

feasible. The timescale for leak detection is on the order of seconds.

Figure 8. Correlation between acoustic intensity and fit factor.
15

The advantages of this approach are:

• Small size

• Low power

• Low cost

• Uses well-developed acoustic technology

• Highly insensitive to temperature and humidity

Some issues are:

• Where should the ultrasound sensor be located? How many? For example, 

would five sensors around the seal be sufficient to detect all seal leaks?

3.5 Chemical Sensors for Aerospace Applications: From Sensor 

Platforms to System Application – Gary Hunter (NASA Glenn 

Research Center) 

Scientists and engineers in the Instrumentation and Controls Division at the NASA Glenn 

Research Center have designed many microsensors packaged in chips and integrated into 

printed circuit boards for aerospace applications including detecting fuel tank leaks, 

dangerous emissions, and fires. At the workshop, Dr. Gary Hunter presented a range of 

gas sensor technologies that have broad potential use. Some versatile, user-friendly, and 

durable microsensors that have been tested and deployed by NASA in recent years are 

illustrated in Figure 9.

The approach taken at NASA is to develop an intelligent system by distributing 

capabilities to smart components at the local level.12 Important principles include "Lick 

and Stick" stand-alone stamp-size technologies using micro and nano fabrication 

techniques to place tiny sensors, actuators, electronics, battery power, and (wireless) 

communication where needed, reliability, redundancy and cross-correlation of data to 

improve trust, and orthogonality to increase the range of system information. Existing 

gas sensors based on MEMS technology can measure hydrogen, oxygen, CO, CO2, NOx, 

and hydrocarbons, and some sensor platforms have built-in temperature, pressure, and/or 

humidity detectors and heaters. The microfabrication technology permits tailoring of the 

specific sensor array to the application. Attention is given to supporting technologies, 

such as packaging, which can be as much as 70 % of the device cost, signal conditioning 

and processing, software for neural nets or modeling, and power and communication 

networks.

Applications for NASA's chemical sensors that relate to this workshop include fuel and 

oxygen leak detection on spacecraft, detection of fire and fire precursors, detection of 

toxic gases, combustion control, and breath monitoring for human health. Detection may 

take place under harsh temperature conditions or in the presence of interfering gases. 

False alarms are reduced through orthogonal detection and cross-correlation of sensor 

data. Testing is rigorous. The FAA "biscuit", which burns in a repeatable manner and 

releases the same gases each time, may be used. The sensor system must be able to 

continue operating through repeated cycles in harsh environments.
16

NASA's vision of a "smart" suit that monitors both internal and external conditions to 

maintain the health of the wearer and the integrity of the suit can be a model for the first 

responder as well. Breathing gases that can be used to monitor human health include 

water vapor, CO2, O2, NO, CO, volatiles (VOC) excreted during exhalation, and 

semivolatiles in breath condensate. Known biomarkers for various diseases can be 

tracked. NASA has demonstrated the capabilities of a prototype breath analysis system, 

although it is not unobtrusive as would be needed for first responder use. This system is 

on a path to eventual commercialization through the State of Ohio Third Frontier 

Program.

Figure 9. Examples of gas microsensors that have been tested and deployed by 

NASA in a variety of applications.

The advantages of this approach are:

• Small size – miniaturized sensors and electronics

• Response time, sensitivity, selectivity, stability

• Batch fabrication, processing reproducibility, control of structure

• Sensor system tailored for the application

• Low power

• Minimum size, reduced weight, and reduced power consumption are important 

goals

• Consideration for harsh environments, including high temperatures and 

particulates

Some issues and lessons learned are:

• Customer acceptance, ability to trust the data, requires working closely with the 

customer
17

• Need for sensor arrays to carry out multiple measurements at once

• Importance of supporting technologies such as packaging and software

• Breath to breath resolution

• Nanotechnology fabrication challenges

3.6 Discussion

At the conclusion of this set of talks, the workshop participants raised a number of issues 

relating to sensor technology. Of particular concern were the specific quantities that 

should be monitored, the confounding effects of heat, humidity, and particulates, where 

the sensor should be located, and what kind of information is important for the wearer to 

know.

There was no consensus on what gaseous compounds should be targeted, or whether it 

would be sufficient simply to detect the presence of a leak using pressure or acoustic 

sensors. Carbon monoxide is a gas that is found in all fires in which incomplete 

combustion takes place; however, it cannot be considered a tracer gas. In other words, 

monitoring CO alone may not give sufficient information about the hazards to which the 

firefighter is exposed. Other gases that are often encountered in a fire include hydrogen 

cyanide (HCN), nitrogen oxides (NOx), sulfur dioxide (SO2), hydrogen chloride (HCl), 

and various hydrocarbons. In addition to the most toxic gases, it was suggested that it

would be a good idea to also target agents such as benzene and formaldehyde that have 

been identified as contributors to chronic diseases suffered by firefighters. Additional 

concerns could be addressed by a sensor for CO2 to monitor CO2 build-up in the 

respirator and a sensor for oxygen to identify low oxygen conditions. Multi-sensor arrays 

would certainly provide more information on the contents of the "soup" of the fire 

environment, although there is a need for further study on what materials to include and 

exclude from the sensor array.

It was noted that the hot, smoky, high thermal flux conditions that prevail in a fire are 

extremely demanding and would likely degrade the performance of the sensors, possibly 

even damaging them. A particularly challenging test of sensor robustness used by NASA 

is the so-called "Disk of Death", a polymer-based fuel that produces a nasty soup of 

chemicals when ignited, often overwhelming the sensor. The presence of heat, water, 

and particulates were cited as major problems that might confound the sensor or saturate 

it and make it insensitive to more hazardous compounds. The incorporation of humidity 

and temperature sensors would be one approach for compensating for this problem. A 

multiparameter, multisensor approach would be more able to characterize the 

environment from the measurements.

Particulates can interfere with sensors by contaminating surfaces. For a mechanical 

sensor such as the cantilever discussed in Nathan Lazarus' presentation, particulates could 

prevent its operation. In the smoky environment of a fire, this is a major problem that 

suggests the importance of proper sensor mounting and packaging, potentially 

necessitating a filter. Small particulates are also a health hazard, however, and may 

warrant a sensor of their own.
18

Workshop participants again discussed where the sensors would be located. One 

suggestion was to put sensors in the turnout coat, for the purpose of measuring gases that 

may be absorbed into the skin. A sensor measuring the external environment could 

provide data to guide the decision on when the firefighter can remove the respirator. This 

capability would require setting criteria for respirator removal, which in turn requires 

further research into health effects, followed by practical methods for enforcing new 

procedures. Measurements both inside and outside the respirator would provide the best 

record of what the hazard is and how well the firefighter is protected against it. The 

sense of the workshop was that there are a lot of opportunities here – to determine when 

it is safe enough to remove the respirator, to monitor breathing uptake of hazardous 

gases, to track the physiology of the firefighter including heat stress, and to monitor the 

condition of the PPE itself.

Finally, the participants discussed how the information from the sensors should be 

conveyed to the user of the SCBA. A sensor can provide too much information. To be 

useful, it needs to call attention to itself only when the value changes in an important 

way. In order to avoid overload, the firefighter on the scene wants it kept very simple, on 

the level of green light/red light, go/stop binary directions. The logic attached to the 

sensor must therefore be sophisticated enough to decide in real time whether the 

condition is safe or unsafe. On the other hand, a full analysis of the respirator sensor 

data, including the physiology of the firefighter's breathing pattern and the materials he or 

she has been exposed to, will require storage of the entire record of sensor values, in such 

a way that data can be easily correlated.

The importance of developing trust and confidence of the wearers in the usefulness and 

reliability of these devices, as with any new equipment, was emphasized.
19

4 SENSOR INTEGRATION AND ENGINEERING

The presenters for this section were Jay Snyder (NIOSH/NPPTL), Paul Greenberg 

(NASA-Glenn), Arthur Johnson, a professor of Bioengineering at the University of 

Maryland, and Kathryn Butler (NIST). Their presentations focused on engineering issues 

relating to the conditions inside of respirator facemasks and how best to design and 

position sensors to facilitate reliable measurements. 

4.1 Sensor Development for ESLI & Its Application to Chemical Detection 

– Jay Snyder (NIOSH/NPPTL)

The sensor technologies currently under investigation by the End of Service Life 

Indicator (ESLI) program sponsored by NIOSH/NPPTL are highly relevant to the 

problem of monitoring respiratory protection. This area was reviewed by Dr. Jay Snyder, 

who began by raising the issues of what information is useful, what is the best format for 

communicating it, and how much is too much.

The ESLI program is directed at developing microsensors that can be embedded directly 

in filter cartridges used in air purifying respirators (APRs) and eventually in powered air 

purifying respirators (PAPRs) to indicate when target compounds are able to break 

through as a result of saturation of the filter bed. The two technologies currently being 

actively pursued are the MEMS electronic system discussed in Section 3.3, consisting of 

a tiny array of sensors on a chip that could include chemiresistors, cantilevers, and 

humidity and temperature sensors, and the optical system of photonic crystals presented 

in Section 3.1. This presentation focused on the engineering challenges of this work, such 

as the integration of sensors into the filter cartridge, the use of standard electronic devices 

for packaging, and the testing and evaluation of the ensemble.

Figure 10 shows a prototype respirator equipped with a sensor array integrated into the 

filter cartridge. The sensors are packaged in a standard TO5 electronics package, a short 

cylindrical metal case, that is embedded into the cartridge. Data are then communicated 

across an electrical interface from filter to mask to signal the wearer through ESL 

indicators. Eventually it is hoped that the sensors may be wireless, so that they may 

transmit data from anywhere within the filter using radio frequency (RF) signals. Since 

the filter is a conductive medium, this is not a trivial challenge. The optimal location 

within the cartridge (in the center? along the side wall?) is yet to be determined. Static 

electricity is an issue, and grounding straps may be necessary.
20

Figure 10. Prototype respirator equipped with a sensor integrated into the filter 

cartridge.

Several sensor integrated cartridges have been assembled in collaboration with multiple 

respirator manufacturers, and successful tests to detect toluene breakthrough have been 

carried out. It is expected that this environment is complex enough that multiple sensing 

modalities will be needed.

4.2 Engineering Considerations – Paul Greenberg (NASA Glenn Research 

Center)

In his presentation, Dr. Paul Greenberg delineated some of the engineering considerations 

that would have to be addressed in the design of real-time monitoring of first responder 

respiratory protection. Recognizing that the harsh environment of a fire and the nature of 

firefighting would present a severe challenge, he states that "Effective solutions often 

require creative ways of thinking." Figure 11 shows the "Lick and Stick" Multi-species 

Leak Detector discussed by Gary Hunter as an example of engineering decision-making 

in sensor design.

Figure 11. Multi-moment particulate sensor, showing engineering considerations 

that contributed to its design
21

Dr. Greenberg's list of engineering considerations for tracking respiratory protection 

follows:

• Environmental

o Temperature, humidity, shock, vibration, physical orientation, 

corrosive, reactive, or flammable surroundings

• Packaging considerations/Physical attributes

o Size/volume, mass, power consumption, durability

• Application-specific considerations

o Physical sampling of ambient pressure or flow, avoiding potential 

biases

o Conditional sampling – what are the data rate requirements, 

correlated vs. random sampling

o Form, fit and function: user compatibility for specific field 

situation

o Differences in measurement with location inside mask

• Operational Considerations

o Data logging and/or wireless transmission; data transfer

o Internal processing: providing the answer vs. providing raw data

o Duration of event(s) of interest

o Overall anticipated service life

o Reliability

o Visibility and operability (i.e. user interface)

o Cost and number of units required

o Calibration and calibration interval

o Requirement for internal health status monitoring – "Is my sensor 

working?"

• Scaling and Fabrication Considerations

o Physics of scaling – simply making an existing sensor smaller 

generally doesn't work

o An alternative or completely new measurement approach may be 

required

o This may introduce issues such as materials compatibility or subelement inter-operability

o Different or possibly novel methods of fabrication may be required

• Testing considerations

o Nuisance backgrounds

o Metrics

o How good does the sensor need to be?

4.3 Measuring and Visualizing Flows Inside Respiratory Masks – Arthur 

Johnson (University of Maryland at College Park)

Professor Arthur Johnson informed the workshop about several studies to visualize flow 

and measure leakage volumes experimentally under laboratory conditions. Although this 

work was done using Powered Air-Purifying Respirators (PAPRs), which are negative 

pressure devices, aspects of the results are applicable to firefighter SCBAs.
22

Figure 12 illustrates the visualization of flow from a headform covered by a loose-fitting 

PAPR and connected to a breathing machine. The flow paths are visualized by digital 

video image capture of a glycerol fog that covers nearly the entire face area during all 

breathing phases. A thread fixed at the mouth identifies whether the phase is inhalation 

or exhalation, and the flashing of a light emitting diode (LED) helps to time the video 

frames. The flow field for the loose-fitting PAPR is downward over the face from a fan 

in back, but the flow is not steady. The delineated flow pathways are contorted, twisted, 

and multilayered, indicating that measurements of gas concentration and other variables 

that are being considered for sensor arrays may also not be steady at any given location. 

This makes it difficult to make correct measurements and highlights the importance of 

positioning the sensor in a location that represents the actual uptake of firefighter 

breathing.

Figure 12. Flow visualization in a loose-fitting PAPR – diagram of flow pathways

The second study looked at inward leakage in tight-fitting PAPRs. Since a significant 

portion of these facepieces is opaque, flow visualization is difficult. The flow rate from 

the blower flow rates during inhalation is not constant, and during exhalation flow into 

the blower has been observed. For this study, flow meters were added to the 

instrumentation to measure the leakage flow rates and flow volumes, and a bronchoscope 

failed to detect any inhaled fog at the mouth of the headform.

The third study used glycerol fog and digital video imaging to visualize the flow for ten 

human subjects in loose-fitting PAPRs. A pneumotach was used to measure volume flow 

rate through the mouth. As for the headform study, the flow paths were found to be 

twisted and curled. This can actually add to the respirator protection, since only the 

contaminants that reach the mouth are important, not simply the contaminants within the 

mask. Even if contaminants are leaking into the respirator, the longer the flow pathway,

the less likely that these contaminants will be inhaled. Given that contaminant 

concentrations are not uniform, obtaining a representative measure of concentrations is a 

challenge.

In the fourth study, protection factors were calculated for a headform attached to a 

breathing machine inside an environment chamber, using CO2 as a tracer gas, such that 

the protection factor was based on the amount of CO2 inhaled. If during exhalation 

blowers can clean the dead volume within the loose fitting PAPR of contaminants, the 

protection factor could be improved. The blower effectiveness, or how much of the air 
23

within the PAPR is coming from the blower rather than from the outside environment, is 

the important factor.

4.4 Simulating Flows Inside (and Outside) Respiratory Masks – Kathryn 

Butler (NIST/BFRL)

Dr. Kathryn Butler demonstrated that computational fluid dynamics (CFD) software tools 

can be used to simulate the flow inside of the respirator. Given the geometry of the face 

and the respirator, it is possible to define the three-dimensional space bounded by these 

surfaces. The flow dynamics are controlled by the breathing pattern applied at the mouth 

or nose, the locations of entry and exit valves, and the location and size of a defined leak. 

CFD has the capability of providing information on flow velocity, pressure, gas 

concentration, particle motion, and other variables throughout the computational space, 

that can then be visualized in still images and videos.

Figure 13 demonstrates the capability of CFD to describe the flow and pressure fields 

within a half-facepiece respirator mask during the exhalation phase. The velocity vectors 

and streamlines show the exhaled breath striking the wall of the respirator across from the 

mouth. The exhaled gases then flow downward or upward and around to leave through 

the center valve. Contours of pressure show a maximum on the respirator across from the 

mouth. The analysis determines the values of all problem variables for the entire volume 

as a function of time, and therefore gives a much more complete picture of what is 

occurring than is possible in experiments with discrete sensor measurements. 

Experiments are critical, however, to validate the analysis and ensure that the results are 

realistic.

Figure 13. Computational analysis of flow and pressure during exhalation in a half 

mask.

It is possible to use computational methods to investigate the flow fields and pressure 

inside a specific respirator for a number of breathing patterns, geometries, and potential 

leaks, with the goal of determining where within the respirator would be a good location 

for a sensor to monitor conditions.
24

Other examples of computational analysis presented in this talk included a study of the 

effect of an external leak of oxygen from a Closed Circuit SCBA (CC-SCBA) into a near 

flammable environment13 and the measurement of fit and discomfort by gradients in 

contact pressure for a respirator pulled onto a head.

4.5 Discussion

It is clear from the workshop presentations that the basic components of a sensor system 

for real-time monitoring of respiratory protection for first responders exist. As a 

minimum, respirator sensors must be compact and self-contained. These requirements 

have been met by functioning technologies described at this workshop, including 

miniaturized sensors, multivariate analysis techniques, wireless communication, and 

power supplies. After this final set of presentations, the discussion centered around the 

engineering challenges to accomplish the goal of tracking respiratory protection on the 

fireground.

An important step is to narrow the expectations and scope of the problem to make it more 

specific, especially for the first stage of implementation of sensor technology for field 

measurements. What is it that we want to measure? Speakers at this workshop have 

shown that a number of sensor technologies are available, but the problem space, 

including species and concentrations, needs to be defined. A list of components of the 

fire environment includes CO, CO2, O2, hydrogen cyanide, hydrogen chloride, chlorine, 

formaldehyde, SO2, NO2, hydrocarbons, water vapor, and particulates. A well-defined 

initial problem would be to measure some minimum number of gas species, perhaps a 

single tracer gas or two to three gases of primary importance. For example, CO2 and O2

were suggested as good choices. If a laboratory demonstration of the technology is 

needed with a real person, sulfur hexafluoride, helium, and fluorescent aerosol were 

mentioned as tracers. Humidity and particulates pose challenges to the measurements –

humidity can saturate a sensor and particulates can prevent proper operation. The effects 

of these confounding factors cannot simply be subtracted out.

The quantities to be measured are affected by the purpose of the measurement. The 

detection of a leak is a simpler problem than tracking firefighter physiology, for example. 

Even this goal is non-trivial, however. Multiple sensors or careful sensor placement will 

be required, since conditions within the respirator, including the pressure, are not 

uniform. The threshold values that define a leak need to be determined.

The respiratory environment that is acceptable for breathing needs to be defined. OSHA 

has defined limits for short-term as well as long-term exposure. Is this an appropriate 

starting place for the protection of the firefighter in his/her "workplace"? Do we want to 

tell the firefighter when he should get out? Or when he is safe? The capability of the 

technology must be balanced with the need.

The placement of the sensors is a fundamental question to be resolved. A number of 

participants suggested placing a sensor outside of the respirator as an initial project. The 

purpose would be to monitor the firefighter's immediate surroundings and indicate 
25

whether it is safe to remove the respirator mask. This doesn't need to be near the face – it 

could be somewhere on the turnout gear, such as in a pocket to keep it from excessive 

heat. It was Dawn Bolstad-Johnson's opinion that firefighters would be very interested in 

such a device. Integrating a sensor into the respirator itself has been demonstrated for the 

ESLI in a filter cartridge (Figure 10) and for a clear mask in a NASA project carried out 

seven years ago. For a sensor within the respirator, the most logical position for 

measuring the contents of a breath would be in front of the mouth or nose. However, this 

is probably not the most practical location. Another potential placement is on the 

regulator. The viability of this and other possible solutions could be investigated using 

computational or experimental means. The most difficult set of measurements would be 

comparing interior and exterior environments on the fireground. A minimal sensor array 

within the respirator and a more complete array outside was one of the proposals

The question was raised as to which is the bigger problem for the firefighter (and thus 

perhaps the problem that should be tackled first): inward leakage or removing the 

respirator early. Although the positive pressure SCBA tries to compensate for a leak by 

increasing the flow to the respirator, inward leakage is a concern. The amount of inward 

leakage occurring during fire operations is unknown. It is observed that in order to pass 

the fit test firefighters may tighten the mask more than they would in actual practice, 

making the fit test an uncertain measure of how well the respirators really operate.

In addition to the engineering challenges, the firefighter culture adds another set of 

considerations. Firefighters are not willing to leave the fireground unless they are out of 

air. The only way to change this is to install a new standard operating procedure (SOP) 

and/or Safety Officer oversight, along with consequences for failure to comply. If the 

intention that the new technology will protect firefighter health is not fully appreciated, 

awareness that their equipment is recording data may raise concerns that the information 

may be used against them. It may be seen as introducing a Big Brother aspect to the 

operations that will not be welcomed.

Finally, the cost of this new technology will certainly be a factor in whether or not it will 

be adopted.

A suggestion to add a hood to the SCBA to increase dead space that can be protective in 

nature was made, but it was argued that it would add mass and an extra heat load to the 

heavily burdened firefighter.
26

5 CONCLUSIONS

Information presented and discussed in this workshop has highlighted the challenges 

involved in ensuring that SCBA respirators fit the firefighters who depend on them for 

protection from hazardous gases and particulates known to be present on the fireground. 

Because of the difficulties involved in maintaining a good fit, it is likely that the seal 

between the mask and the face will fail on occasion, especially during times of extreme 

exertion. If this occurs, the consensus view is that the firefighter is still protected because 

SCBAs are designed such that the pressure is greater inside the mask than outside so that 

the net direction of flow is outward. However, as communicated by Professor Johnson 

and other workshop participants, the flow field inside of a respirator mask is not 

homogeneous, and the direction of flow changes as a function of location and time. Thus, 

the possibility that an intermittent leak could develop in a region of the mask where the 

pressure gradient is small (or even negative), thereby allowing hazardous substances to 

penetrate the facemask, cannot be completely eliminated. In the context of the sporadic 

claims (albeit undocumented) of soot penetration (“blow-by”) and the expectation that 

rapid breathing during exertion will reduce the pressure inside the mask, this possibility 

prompts the conclusion that developing a technology capable of monitoring the 

respiratory intake of SCBA users, or at the least the fit of their facemasks, should be 

seriously considered. In addition, as pointed out by workshop participants, microsensors 

could be used in handheld devices or mounted on turnout gear to enable monitoring of 

external conditions for the purpose of determining when it is safe for the firefighter to 

remove his/her mask. This application would be extremely beneficial because SCBAs are 

cumbersome, so that users tend to remove them as soon as they think it is safe to do so. 

This behavior can obviously result in serious injury in the absence of the capability of 

detecting the presence of toxic gases and particulates. 

Although the sensor technologies currently being investigated at NIOSH, NASA, NIST, 

and in the NIOSH ESLI program appear to be sufficiently compact and versatile to be 

implemented in an SCBA, there are formidable engineering challenges that must be 

overcome before these technologies can be implemented on a routine basis. These 

challenges include accounting for the adverse effects that interfering compounds, 

particulates, temperature extremes, and humidity have on the accuracy and reliability of 

the measurements. In addition, recognizing the complexity of the flow field within the 

mask, it is also necessary to investigate where to position the sensors in order to obtain 

measurements that reflect the respiratory intake of the person using the SCBA. 

On the basis of these considerations, the discussion during the workshop led to the 

recommendation that a program of research directed at providing answers to the 

following questions should be instituted:

• Does pressure inside of a SCBA face mask ever become less than ambient? 

o Where?

o Under what conditions?

• Is “blow-by” real and does it indicate that there is a leak?
27

• What subset of environmental species or particulates actually lead to adverse 

health effects in firefighters? 

o Gases or particulates?

o Pressure, sound, or other physical indications of leakage?

o Is it possible to reduce the number of targets to a small number while still 

providing the necessary information?

• How sensitive, accurate, and fast do the measurements need to be?

o What is the nature of a leak?

o Is there a threshold size?

• How do we develop occupational hygiene (or safe operating) procedures from 

health effect information?

o What criteria should be used to decide when the risk to the firefighter is 

minor? 

o What criteria should be used to decide when to use respirators during 

overhaul?

o What gases are absorbed by the hair and skin, and are there any adverse 

health effects associated with the absorption of these gases?

• Where should the sensors be positioned?

o Are multiple sensors required to detect leaks?

Once these engineering issues have been resolved, performance evaluations should be 

conducted to assure potential users of the usefulness and reliability of this technology.
28

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591, 2001.

10 Fedder GK, Barkand DT, Bedair SS, Garg N, Greenblatt J, Jin R, Lambeth DN, Lazarus N, Rozzi TR, 

Santhanam S, Schultz L, Snyder JL, Weiss LE, Wu J, "Jetted Nanoparticle Chemical Sensor Circuits for 

Respirator End-of-Service-Life Detection," 12th International Meeting on Chemical Sensors, July, 2008, 

Columbus, Ohio.

11 Bedair S and Fedder G, "CMOS-MEMS Microgroove Cantilever Oscillator Gas Vapor Sensor," 12th 

International Meeting on Chemical Sensors, July, 2008, Columbus, Ohio.

12 Hunter GW, Liu, CC, Makel D, "Microfabricated Chemical Sensors For Aerospace Applications,"

MEMS Handbook Second edn, Design and Fabrication, CRC 2006, Press LLC, Boca Raton, Ch. 11.

13 Butler KM, "A Computational Model of an Outward Leak from a Closed-Circuit Breathing Device," 

Journal of the International Society for Respiratory Protection 25: 53-65, 2008.
29

Appendix A Workshop Agenda

Workshop on

Real-Time Monitoring of Total Inward Leakage of Respiratory Equipment Used by 

Emergency Responders

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, Maryland

Friday, May 1, 2009

Meeting Location: Building 101 / Lecture Room B

8:00 Opening Remarks and Agenda – Kathryn Butler

8:30 Session 1: First Responder Needs

, NIST/BFRL

NIOSH Fit Test Research – Ziqing Zhuang

Fit Testing First Responders –

, NIOSH/NPPTL

Dawn Bolstad-Johnson

9:15 Discussion 

, Phoenix Fire Department

10:15 Break

10:30 Session 2: Sensor Technologies

Gas Sensing with Porous Silicon Photonic Crystals – Brian King

UC San Diego

and Michael J. Sailor, 

NIST Chemiresistive Microarray Technology – Kurt Benkstein

NIST/Physics Laboratory

and Steve Semancik, 

MEMS Sensor Development for End-of-Service-Life Indicators (ESLI) –

Nathan Lazarus

Ultrasound in Respirators: Concepts and Preliminary Results –

and Gary Fedder, Carnegie Mellon University

and J. V. Szalajda, NIOSH/NPPTL

William P. King

Chemical Sensors for Aerospace Applications: From Sensor Platforms to System

Application – Gary Hunter

11:30 Discussion 

, J.C. Xi, P. Greenberg, and P.G. Neudeck, NASA 

Glenn Research Center, C.C. Liu, Case Western Reserve University, D.B. Makel 

and B. Ward, Makel Engineering Inc., P. Dutta, Ohio State University, R. 

VanderWal, USRA at NASA Glenn Research Center, L. Dungan, NASA 

Johnson Space Center

12:30 Lunch

1:30 Session 3: Sensor Integration and Engineering

Sensor Development for ESLI & Its Application to Chemical Detection – Jay Snyder

NIOSH/NPPTL

, 

Engineering Considerations – Paul Greenberg

Measuring and Visualizing Flows Inside Respiratory Masks –

, NASA-Glenn

Arthur T. Johnson

University of Maryland

,

Simulating Flows Inside (and Outside) Respiratory Masks – Kathryn Butler

2:30 Discussion

, NIST/BFRL

3:30 Break

3:45 Wrap-up and Conclusions – Kathryn Butler

4:30 Close

, NIST/BFRL
30

Appendix B Workshop Registrants and Attendees

Claire Austin NRC Canada

Nathan Beck SAIC

Kurt Benkstein NIST

Dawn Bolstad-Johnson Phoenix Fire Department

Les Boord NIOSH/NPPTL

Adam Boussouf

Djamel Boussouf Rve Inc.

Keith Brower Loudoun County Fire, Rescue and Emergency Mgmt.

Rodney Bryant NIST

Nelson Bryner NIST

Kathryn Butler NIST

Karen Coyne U.S. Army ECBC

Dennis Ertel SOMA

Ken Farmer National Fire Academy

Kenneth (Beau) Farmer TSI Inc.

Kenneth Gaiser City of Jackson

Paul Greenberg NASA-Glenn

Gary Hunter NASA

Shaya Jamshidi SAIC

Arthur Johnson UMd College Park

Brian King UC San Diego

William P. King NIOSH/NPPTL

Adam Kochanski University of Utah

Nathan Lazarus CMU

Nathan Marsh NIST

Jennifer Marshall NIST

Jack Mawhinney Hughes Associates, Inc.

Stephan B. Miller University of Houston

Mitch Molenof D.C. Fire Department

Carlo Alberto Monti ICS SRL

Marc Nyden NIST

William Reinhard Fire Service Instructor / Course Development

Peter Rutkowski Mine Safety Appliances Company

Dongil Shin

Lei Song University of Science and Technology of China

Jay Snyder NIOSH/NPPTL

Natalia Stakhiv OSHA

John Steelnack OSHA

James Stewart NIST-OLES

John Szalajda NIOSH/NPPTL

Qiyuan Xie University of Science and Technology of China

Ziqing Zhuang NIOSH/NPPTL
31

Appendix C NIOSH Fit Test Research – Ziqing Zhuang (NIOSH/NPPTL)
32
33
34
35
36
37
38

Appendix D Fit Testing First Responders – Dawn Bolstad-Johnson (Phoenix Fire Department)
39
40
41
42
43
44
45
46

Appendix E Gas Sensing with Porous Silicon Photonic Crystals – Brian King (University of California San Diego)
47
48
49
50
51
52
53

Appendix F NIST Chemiresistive Microarray Technology – Kurt Benkstein (NIST)
54
55
56
57
58

Appendix G MEMS Sensor Development for End-of-Service-Life Indicators – Nathan Lazarus (Carnegie Mellon 

University)
59
60
61
62
63

Appendix H Ultrasound in Respirators: Concepts and Preliminary Results – William King and Jonathan Szalajda 

(NIOSH/NPPTL)
64
65
66
67
68

Appendix I Chemical Sensors for Aerospace Applications: From Sensor Platforms to System Application – Gary 

Hunter (NASA Glenn Research Center)
69
70
71
72
73
74
75
76

Appendix J Sensor Development for ESLI & Its Application to Chemical Detection – Jay Snyder 

(NIOSH/NPPTL)
77
78
79
80
81

Appendix K Engineering Considerations – Paul Greenberg (NASA)
82
83
84

Appendix L Measuring and Visualizing flows Inside Respirator Masks – Arthur Johnson (University of Maryland 

at College Park
85
86
87
88
89
90
91
92
93
94
95
96

Appendix M Simulating Flows Inside (and Outside) Respirator Masks – Kathryn Butler (NIST)
97
98
99
100
101
102
103