A Study of Protection Provided 

by General Aviation Oxygen 

Masks With Open Ambient Ports 

in Toxic Environments

Lawrence N. Paskoff

Joseph G. Mandella 

David A. Self

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

August 2015

Final Report

DOT/FAA/AM-15/15

Office of Aerospace Medicine

Washington, DC 20591
NOTICE

This document is disseminated under the sponsorship 

of the U.S. Department of Transportation in the interest 

of information exchange. The United States Government 

assumes no liability for the contents thereof.

___________

This publication and all Office of Aerospace Medicine 

technical reports are available in full-text from the 

Federal Aviation Administration website.
i

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-15/15

4. Title and Subtitle 5. Report Date

A Study of Protection Provided by General Aviation Oxygen Masks 

With Open Ambient Ports in Toxic Environments

Auugust 2015

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Paskoff LN, Mandella JG, Self DA

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

FAA Civil Aerospace Medical Institute

P.O. Box 25082

Oklahoma City, OK 73125

11. Contract or Grant No.

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aerospace Medicine

Federal Aviation Administration

800 Independence Ave., S.W.

Washington, DC 20591 14. Sponsoring Agency Code

15. Supplemental Notes

16. Abstract

Air transport of medical specimens aboard general aviation aircraft frequently utilizes dry ice for 

preservation. The sublimation of carbon dioxide (CO2) within the confined space of the aircraft cabin, 

without sufficient outside air turnover, presents a potential hazard. Aircraft equipped with oxygen systems 

utilizing modified clinical masks that allow oxygen to be diluted with air drawn in through side ports may 

not offer adequate protection from CO2 contamination. 

Methods. Experiment 1. A simulated aircraft cockpit and cabin area was constructed. Total volume was 

approximately 3.956 m3

. Ten insulated biological specimen bags (12 x 17 x 20 in) that conformed to 

Department of Transportation regulations were filled with 5 lb of dry ice chips. This ratio of specimen bags 

to cabin volume reflected common industry practices. Air flow through the mock-up was set at a turnover 

rate of 9.73. CO2 levels were monitored with mass spectrometry.

Experiment 2. A face mask with re-breather bag delivered aviator’s oxygen at a flow rate of 3 Lpm and 

was fitted to a test mannequin head connected to two breathing machines that produced alternately an 

inhalation of mask contents, and then an exhalation of either ground level alveolar air (78.5% N2, 16% O2, 

5.5 % CO2), or alveolar air resulting from breathing 100 % O2 (N2 removed). The breathing machines 

delivered a physiological breath pattern with a tidal volume of 0.92 L at a rate of 20 breaths per minute 

(bpm). The head was inside a 0.76 m3 partially sealed box. CO2 content in the box was gradually increased 

and the inhaled and end tidal gas compositions were measured.

Results. Experiment 1. Carbon dioxide levels reached a mean average of 2.02% after 12 min, and then 

stabilized. Experiment 2. Inhaled partial pressures of CO2 inside the mask were 5.496 mm Hg (ambient= 

0.1%); 20.93 mm Hg (ambient = 2.44%); and 34.36 mm Hg (ambient= 4.75%).

Discussion. These results suggest that general aviation carriers may be creating levels of CO2 in small 

airframe general aviation cabins with high densities of biological specimen bags that exceed the 

Occupational Safety and Health Administration and Federal Aviation Administration standards (0.5%). 

Further, commercially available general aviation oxygen equipment may not provide aircrews with adequate 

protection in environments with high CO2 content.

17. Key Words 18. Distribution Statement

Oxygen Mask, Carbon Dioxide, General Aviation Document is available to the public 

through the Internet:

www.faa.gov/go/oamtechreports

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 13

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

iii

Contents

A Study of Protection Provided by General Aviation Oxygen Masks 

With Open Ambient Ports in Toxic Environments

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Mock-Up Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Mask Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Mock-Up Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Mask Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1

A Study of Protection Provided by General Aviation Oxyge

With Open Ambient Ports in Toxic Envi

INTRODUCTION 

Air transport of medical specimens aboard general aviation 

aircraft frequently requires the use of dry ice to preserve specimens. Dry ice is the solid form of carbon dioxide (CO

). The

sublimation of CO2 within the confined space of the aircraft 

cabin without sufficient outside air turnover presents a potential 

hazard for the aircrew

A frequently employed countermeasure has been to equip 

the aircraft with oxygen equipment using a modified version 

of clinical masks that allow a mixture of 100% oxygen diluted 

with ambient air drawn in through an open side port. It is

unknown if this equipment provides adequate protection from 

CO2 contamination

Although CO2 is limited to a concentration in transport 

category aircraft of 0.5 percent by volume under Title 14 of the

Code of Federal Regulations (CFR) §25.831, general aviation

aircraft flying under Title 14 of the Code of Federal Regulations 

(CFR) Part 91 have no limitations. In fact, they are permitted

to fly with an unlimited quantity of dry ice as cargo regardless 

of air turnover rates or breathing equipment installed in the 

aircraft (Figure 1)

Gibbons (1977) conducted tests using dry ice in general aviation aircraft. In these tests, paper bags were filled with dry ice

and left in a closed aircraft, and the decline in mass of the dry 

ice was documented. Further testing examined the use of CO

fire extinguishers with subjects donning oxygen masks. However,

no testing of the impact of dry ice sublimation on oxygen mask 

in-flight performance was conducted

Maykoski (1968) noted that general aviation pilots had several 

suspected CO2 contamination incidents but were reluctant to 

report them. In one such instance, a pilot had to don an oxygen

mask and use 100% oxygen; but thought the condition was due 

to food poisoning. Two documented instances from this report

mentioned an aircraft that was closed up for several hours and 

caused several crew members to experience carbon dioxide poisoning. Another incident involved a Military Airlift Command

aircraft carrying frozen dinners packed in dry ice

High CO2 levels within aircraft cabins have been previously 

documented. In 1948, two commercial airliner flights required

the activation of carbon dioxide-filled fire extinguishers. The

first one, in May, partially incapacitated the crew. The second,

in June, may have contributed to the plane’s fatal crash (Civil 

Aeronautics Board, 1948)

In another incident concerning closed doors, the National 

Transportation Safety Board (NTSB) reported a DC-8 in Texas 

had possible accumulation of carbon dioxide due to the sublimation of dry ice for a period of time (NTSB, 2001). This flight was

carrying both dry ice and paint. The aircraft had been grounded

for some time due to mechanical issues

The 1998 incident in Texas (NTSB, 2001) prompted the 

Federal Aviation Administration (FAA) to examine sublimation 

rates of dry ice (Caldwell, Lewis, Shaffstall, & Johnson, 2006)

Tests conducted at the FAA Civil Aerospace Medical Institute 

involved the use of boxes used by the toxicology laboratory 

to ship and receive specimens from aircraft fatalities. Dry ice

was placed in these boxes and then placed inside a hypobaric 

chamber for several hours at 8,000 feet to simulate a standard 

cargo flight. Box weights were measured both before and after

each simulated flight, and a rate of sublimation was determined. No measurements of carbon dioxide were taken in the

hypobaric chamber

Figure 1. FAA Hazmat Team Surveillance Team photos of a general aviation aircraft configuration for transporting medical 

specimens in biological specimen bags (NTSB, 2011).
2

In 2009, a fatality occurred at Teterboro airport in New 

Jersey (Fig. 2). The NTSB found the accident’s probable caus

to be pilot error (NTSB, 2011). The company that owned the

aircraft (Quest Diagnostics Inc, Reading, PA) specialized in 

the carriage of dry ice to haul human specimens. In the NTSB

docket management system concerning this accident, a document relating to the company’s carbon dioxide training contains 

meeting minutes that mention two separate incidents involving 

dry ice. One concerns a departure from Gwinnett County –

Briscoe airport, Georgia, and another in an aircraft departing 

Teterboro Airport, New Jersey. The first was possibly due to the

large volume of human specimen bags blocking airflow, along 

with the quantity of dry ice onboard the aircraft. The other was

related to the aircraft sitting on the ramp with the doors closed, 

allowing carbon dioxide to build up inside the aircraft

Our work sought to address whether sublimation of dry ice 

contained in standard biological specimen containers in a simulated cabin environment that mimicked the Teterboro mishap 

aircraft, in terms of dimensions and quantity of containers, 

would produce elevated ambient CO2 levels. Furthermore, we

evaluated the degree to which the Precise Flight mask deployed 

in the Teterboro mishap aircraft (Fig. 3), allowed CO2 to be

drawn in during inhalation

METHODS

Mock-up tests

The first part of the study evaluated the time course for carbon dioxide buildup in a simulated BE-58 Twin Baron aircraft 

cabin. We constructed an aircraft cockpit and cabin area using

3⁄4 inch PVC pipe and covered all sides using 0.006- in thick

plastic sheeting (Figs. 4 & 5). The covering of the sheeting wa

attached to the pipe with industrial-grade gaffer’s tape. Total

volume of the simulated aircraft was 3.956 cubic meter

Figure 2. Wreckage of N167TB after post-crash fire (NTSB, 

2011).

Figure 3. Oxygen bottle and attached mask with open 

ambient ports shown behind pilot seat from the same fleet 

as the Teterboro mishap aircraft (NTSB, 2011).

Figure 4. Skeletal structure of the cabin mockup, along 

with part of the skin surrounding it.

Figure 5. Internal view of the mock-up showing the 

ventilation system.
3

We attempted to simulate realistic air turnover rates in the 

mock-up. Beechcraft documentation reported that an air turnover

rate in an unpressurized aircraft like their BE-58 was unknown 

(Beechcraft, 2011). However, Citation aircraft with similar cabin

volume report they have cabin air turnover rates ranging from 10 

to 26/hr (Shelinbarger, 2015). Given that the most dangerous

situation would occur in a closed aircraft parked on the tarmac 

(zero air turnovers/hr), with the least danger resulting from the 

highest turnover, we selected an intermediate rate of around 10 

to test. To simulate the movement of ram air (forced air) through

vents in an unpressurized aircraft, 3⁄4-inch PVC pipes were installed 

and connected to a Regen Air Pump (Model R2103, Regen Air 

Mfg. Corp, Benton Harbor, MI). The air exited the simulate

cabin through a 3-inch PVC pipe located in the rear bulkhead 

approximately half-way between the floor and ceiling (Fig. 6

Exiting airflow velocity was measured using a Fisher Scientific 

anemometer (Model 01-241; Pittsburgh, PA) placed at the exit 

pipe. These values were converted to volumetric flows based on

the exit pipe diameter, and then to cabin air turnovers/hr based 

on the mock-up dimensions. To create a well-stirred environment, a 6-in fan was placed near the back of the aircraft blowing

air forward. Air temperature, relative humidity, and dew point

were measured using an Omega temperature humidity alarm 

(Model OM-THA2; Omega Engineering, Inc.; Standford, CT

Ten biological specimen bags, used for this research on loan 

from Quest Diagnostics (12 in x 17 in x 20 in; Figs. 7 & 8) were

each filled with 5 lb of dry ice chips and placed in the mock-up 

cabin. The bags had two ventilation ports on the top to satisfy

ventilation requirements of 49 CFR Part 173.217. C levels in 

the cabin were continuously recorded over time. Two air turnover

rates (9.73 and 10.58 turnovers/hr) were used throughout th

34 min that CO2 levels were measured

Figure 6. Location of ram air entrance, exit and placement of the 

anemometer.

Figure 7. Typical bags used for the transport 

of biological specimens.

Figure 8. Biological specimen bags each filled with 5 lb of 

dry ice chips placed inside the mock-up cabin.
4

Mask Tests

To evaluate the extent to which the Precise Flight mask 

deployed in the Teterboro mishap aircraft allows ambient air 

containing a high CO2 content to enter, we constructed a novel 

breathing system that utilized two flow-volume simulators (Series 1120 Hans Rudolph; Shawnee, KS). Connected together

(Fig. 9), they produced alternately an inhalation of either mask

contents or of ambient air and then an exhalation of alveolar air 

(78.5% N, 16% O2, 5.5 % CO) delivered from a custom mixed 

gas cylinder into a reservoir bag that the exhalation breathing 

simulator drew from

Appropriate directional air flow through the system was 

established with one-way valves. The simulators delivered a

physiological breath pattern and allowed tidal volume, respiratory rate, dead air space, and functional residual capacity (FRC) 

to be preset (Fig. 10

The generated air flow was “inhaled” and “exhaled” from a 

research head (Hans Rudolph model DR1299) fitted with an 

oxygen mask (Precise Flight model 020N0002-1; Fig. 11) via a

simulated trachea (total dead air space=170 ml). The mask was a

modified version of the Teleflex-Hudson RCI face mask, model 

1009 (Fig. 12). The changes in this mask made by Precise Fligh

Inc. for general aviation use included the removal of two check

valves: one on the side and one connecting to the rebreather bag

Figure 9. Two flow-volume simulators connected together 

such that one inhales ambient air and the other exhales 

simulated alveolar gas.

Figure 10. The breathing system circuit.

Figure 11. Hans Rudolph 

Research Head.

Figure 12. Precise Flight 

Mask.
5

The removal of these valves allowed the individual’s breath to 

enter the rebreather bag, mix with 100% oxygen, and permitted 

ambient air to enter the mask during inhalation

Between the oxygen mask and source of 100% aviator’s oxygen 

was an In-Line Dual Scale Flowmeter (Model A-5, Precise Flight 

Inc; Bend, OR; Fig. 13). To create an enclosed environment tha

simulated a small aircraft cabin area, the research head was placed 

inside a well-stirred 27 cubic foot clear plastic box into which 

CO2 was injected (Fig. 14). The box vented outward such tha

barometric pressure was maintained at ground level

The Precise Flight general aviation oxygen mask was attached 

to the research head, as seen in Figure 11, and a capillary tube 

from a mass spectrometer was inserted into one of the small 

port holes on the side of the mask. A pneumotachometer (Hans

Rudolph, 3700 series, 0-160 Lpm) was placed proximal to the 

trachea (Fig. 14) to measure gas flow (L/min). A Perkin-Elme

Medical Gas Analyzer, MGA-1100 (Perkin-Elmer Life and 

Analytical Sciences, Inc.; Waltham, MA), was utilized for realtime measurement of the percentages of atmospheric gases (N

, 

O2, CO2 , and H2O) passing into and out of the mask, and the 

ambient air composition within the box. Voltage output linearity

was 0.5% of full scale for all gasses. Two-point calibrations wer

performed before each experiment using room air and a certified 

calibration gas (3% CO2, 0% O2, 97% N2; ± 1%)

The Society of Automotive Engineers Aerospace Standard 

1224B (SAE AS1224B) requires CO2 exclusion performance 

testing of continuous flow aviation oxygen masks to have a 

breathing rate of 20 breaths per minute (BPM) with a displacement of 750 mL/cycle. The breathing machine was set to 20

(BPM) and a tidal volume of 920 mL to allow for the added 

dead air space of our research head-trachea circuit

Figure 13. Precise Flight Flowmeter.

Figure 14. Configuration of the oxygen mask testing apparatus.
6

Baseline performance of the system in room air with a groundlevel alveolar gas mixture was characterized without the mask in 

place. For this, the mass spectrometer sampling line was placed

just inside the mouth. Values for O and CO2 are presented in 

Table 1

Next, the mask was attached to the head, and the margins in 

contact with the face were sealed with tacky wax. The oxygen

system flow regulator was set at 3 Lpm. An in-line pneumotachometer (Hans Rudolph, 8411 series, 0-10 Lpm) reported that

this created a flow of 3.43 Lpm. The alveolar gas compositio

used was switched from a ground level ambient air composition 

to one that simulated a pilot breathing on a mask suppling 100% 

oxygen (N2 component removed)

A simulated scenario of dry ice sublimation was created by 

slowly pumping carbon dioxide into the cabin box. As carbon

dioxide levels within the box rose, inhaled and exhaled gas composition was measured through the mass spectrometer sampling 

line connected to the mask

Analog signals from all monitoring equipment were digitized 

at 20 samples/s and recorded with a custom-built LabView 

data acquisition instrument (National Instruments Corp.;

Austin, TX)

RESULTS

Mock-Up Tests

We evaluated the degree to which CO2 would build up over 

time in a ventilated space that mimicked the Teterboro mishap 

aircraft cabin. We investigated a scenario where the mock-up

contained 10 specimen bags, each containing 5 lb of dry ice 

chips. CO2 levels were measured under two cabin air turnover

rates (Table 2)

Carbon dioxide levels climbed to 2.2% over 16 min, with a

9.73 turnover rate. The CO2 levels appeared to stabilize following an increase in air turnover rate to 10.58, suggesting that th

high turnover rate was required to prevent CO2 accumulations

Table 1. Values for O2 and CO2 tensions during baseline 

performance of the breathing simulator.

CO2 O2

Inhaled gas 2.084 mm Hg 152.437 mm Hg

End tidal gas 29.374 mm Hg 128.938 mm Hg

Table 2. Sequential measurements of gas composition, temperature, dew point, and relative humidity in the mock-up 

under 2 air turnover values.

Time

(Min)

O2

(%)

CO2

(%)

N2

(%)

Temp

(°C)

Dew Pt.

(°C)

Rel. Humidity

(%)

Air Velocity

(m/s)

Complete Cabin Air 

Turnover/hour

Barometric Pressure 

(mm Hg)

0 20.6 1.45 77.1 23.3 8.1 52.3 2.2 9.31 735.33

2 20.6 1.6 76.9 23.3 8.3 52.6 2.3 9.73 735.39

4 20.6 1.69 76.8 23.5 8.2 52.7 2.3 9.73 735.33

6 20.6 1.78 76.8 23.5 8.1 52.6 2.3 9.73 735.33

8 20.6 1.83 76.7 23.5 8.0 52.6 2.3 9.73 735.33

10 20.5 1.88 76.7 23.6 7.9 52.5 2.3 9.73 735.33

12 20.6 2.02 76.6 23.6 7.8 52.5 2.3 9.73 735.33

14 20.5 2.2 76.5 23.6 7.8 52.5 2.3 9.73 735.33

16 20.6 2.1 76.6 23.6 8.0 52.7 2.3 9.73 735.39

18 20.5 2.25 76.4 23.6 8.0 52.6 2.5 10.58 735.39

20 20.5 2.15 76.5 23.6 8.2 53.1 2.5 10.58 735.39

22 20.5 2.07 76.5 23.7 8.2 53 2.5 10.58 735.39

24 20.6 2.1 76.5 23.7 8.2 53 2.5 10.58 735.39
7

Mask Test

We tested the degree to which the Precise Flight mask was able 

to exclude ambient CO2. Ventilation and gas composition measurements were made with both mask OFF (Figs. 15 & 17) an

mask ON configurations on the mannequin head (Figs. 16 & 17

-80

-60

-40

-20

0

20

40

60

80

0

20

40

60

80

100

120

140

160

0 1 2 3 4 5

Flow (L/min)

PO2 & PCO2 (mm Hg)

Time (Seconds)

CO2

O2

Flow

Figure 15. Mask OFF breathing ambient air at 730.694 mm Hg. Composition of the exhaled 

gas was 5%CO2, 16.4%O2, and 79%N2.

These results show that increases in ambient CO2 were 

transmitted to the gas mixture inside the mask with an open 

ambient side port

Figure 16. Comparison of rising CO2 levels with and without a mask. As ambient [CO2] 

approached that of end tidal CO2, the directional changes in PCO2 caused by ventilation 

approached nil.
8

DISCUSSION

This research investigated the impact of open-to-ambient ports 

in general aviation oxygen masks within a CO2 contaminated 

cockpit environment. With the use of a breathing simulator

and research head that could exhale a predetermined alveolar 

gas, it appears that an individual attempting to use these masks 

will inhale only slightly less ambient CO2 than without a mask

This suggests that the use of this style of mask will probably offer 

little protection to the user

Hunter and Olson (1988) tested the Hudson Multi-Vent 

oxygen mask, which was designed for clinical use. This mask

was similar to the one used in the present study. They noted that

location of the mask on the face was important in determining 

the amount of oxygen delivered to the user. Although the oxygen may be set to a maximum flow, the location of the mask on

the user’s face may allow oxygen flow to exit through the vents

Brantigan (1975) tested several types of general aviation 

oxygen masks at varying altitudes by measuring arterial blood 

gases. One brand of mask, the Hudson-type mask, is the same

type used in this present study. Although Brantigan measured

end tidal PCO2 at varying altitudes and oxygen flow rates, any 

ambient air inhaled consisted of the standard air at that altitude

High concentrations of carbon dioxide in the flight deck were 

not considered for his research, but Brantigan noted that oxygen 

tends to flow into the mask and out of the holes of the Hudson 

mask during the passive moments of respiration

A.

B.

Figure 17. Inhalation and exhalation O2 (top) and CO2 (bottom) tensions under varying 

ambient concentrations of CO2 . Pb = 730.5 mm Hg under both conditions. End tidal 

O2 tensions that exceeded inhaled tensions seen with the mask on are believed to be 

created by turbulent air flow patterns in the mask.
9

While describing performance characteristics of general aviation oxygen masks, McFadden, Harrison, and Simpson (1967) 

noted a disadvantage to constant-flow phase-dilution general 

aviation masks. This limitation was the inability to compensate

for to the pilot’s physical or emotional state. It maintained a

preset flow rate regardless of demand. Testing mask performance

under toxic ambient air composition conditions was not part 

of the testing criteria

We have demonstrated that the masks marketed by Precise 

Flight that are currently deployed in some general aviation aircraft 

do not exclude ambient air from entering during inhalation. This

dynamic, although providing adequate oxygen delivery, is unlikely 

to provide protection to the user in a toxic cabin environment

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Engineering Specialist, ASE Textron Aviation (2015)