The on-board carbon dioxide
concentrations and ventilation
performance in passenger cabins
of US domestic flights
, Christopher D. Zevitas
John D. Spengler
, Brent Coull
, Eileen McNeely
, Sin Ming Loo
, Piers MacNaughton
Joseph G. Allen
Billions of people travel on airplanes every year, making the aircraft cabin a critical environment to
understand with regard to public health. The main control over indoor environmental quality in the
cabin is ventilation; therefore, maintaining sufficient ventilation rates on-board is essential for creating
healthy and comfortable cabin environments. We measured real-time CO
concentrations, an indicator
of ventilation rates, and cabin pressure in the passenger cabins of 179 US domestic flights from board-
ing through deplaning. The average CO
concentrations were 1353 290 ppmv (mean SD) and the
estimated outside airflow rates were 5.77 2.09 L/s/p across all flights. The results indicated that 96% of
observations met the minimum recommended outside airflow rates for acceptable air quality (3.5 L/s/p),
but only 73% met the rate required in FAA design requirements (4.7 L/s/p), during flying phases. The CO
levels on all flights were well below the occupational exposure limit of 5000 ppmv. Statistical analysis
indicated that the ventilation rates during boarding phases were significantly lower than the others. The
findings are of particular interest because low ventilation in other settings has been associated with
increased rates of disease transmission, increased upper respiratory symptoms, and worse perfor-
mance on cognitive function tests. Verification of ventilation performance rather than reliance on
design estimates for determining compliance with ventilation standards is recommended.
Aircraft cabin, Ventilation, Carbon dioxide, Airflow rate, Air quality
Accepted 20 July 2018
The International Air Transport Association (IATA)
expects 7.2 billion passengers to travel in 2035, a near
doubling of the 3.8 billion air travellers in 2016, with a
3.7% annual Compound Average Growth Rate.
Transformational changes are underway, especially in
Asia and the Middle East where aviation growth is
expected to be even greater. Airplane cabin air quality
has received considerable attention. Several studies
have highlighted the in-ﬂight transmission of infectious
disease, such as SARS, tuberculosis, common cold and
Other studies have focused on chemical
Department of Environmental Health, Harvard T.H. Chan
School of Public Health, Boston, MA, USA
U.S. Department of Transportation, Volpe National
Transportation Systems Center, Cambridge, MA, USA
Department of Biostatistics, Harvard T.H. Chan School of
Public Health, Boston, MA, USA
College of Engineering, Kansas State University, Manhattan,
Department of Electrical and Computer Engineering, Boise
State University, Boise, ID, USA
Joseph G. Allen, Department of Environmental Health, Harvard
T.H. Chan School of Public Health, 401 Park Drive, Boston
02215, MA, USA.
Indoor and Built Environment
!The Author(s) 2018
Article reuse guidelines:
stressors inside the cabin, including ﬂame retardants,
and particulate matter (PM).
The aircraft cabin is a unique environment that
relies on an aircraft environmental control system
(ECS) to move passengers and crew through an exter-
nal environment widely variable in temperature (80 to
>50C), pressure (10.1–101 kPa), and relative humidity
(near dry to saturation).
Air supplied to the aircraft
cabin is a combination of outside air brought in from
engines (bleed air) and air removed from the cabin,
which is ﬁltered and recirculated. At the ECS terminal,
a mixing air distribution system regulates the cabin air
velocity/temperature, relative humidity and provides
sufﬁcient fresh air to dilute gaseous and particulate
contaminants. Therefore, ventilation plays a key role
in maintaining a safe, healthy and comfort cabin envi-
ronment, by conditioning outdoor air, and controlling
in-cabin pollutants’ concentrations.
Ventilation standards for the aircraft cabin condi-
tions vary by country. In the USA, the airline industry
is regulated by the Federal Aviation Administration
(FAA). The FAA has established Federal Aviation
Regulations (FAR) to guide the operation of commer-
cial airliners. FAR 14 CFR 25.831
states that the
cabin ventilation system must provide at least 0.55 lb
(0.25 kg) of fresh air for each passenger per minute,
equivalent to 4.7 L/s/p at 8000 feet (2438 m) altitude
and cabin temperature of 22C (72F). This ventilation
rate is also speciﬁed by the joint design regulation
FAR/JAR Part 25
for crew members to perform
their duties without undue discomfort or fatigue and
to provide reasonable passenger comfort. It should be
noted that FAA only certiﬁes performance based on
the design of aircraft ventilation system, not the
actual ventilation performance. US airlines are not
required to certify compliance through routine direct
monitoring or audits. As noted in a National Research
Council (NRC) report,
established FARs may be
inadequate to protect the health of the ﬂight crew
In addition to federal regulations for minimum ven-
tilation rates, ASHRAE issued the ANSI/ASHRAE
for specifying requirements for
air quality within commercial aircraft. This standard
speciﬁes a minimum outside air ventilation rate of 3.5
L/s/p. Further, a total air supply (mixture of outside air
and recirculated air) rate of 7.1 L/s/p should be the
minimum, and a rate of 9.4 L/s/p is recommended.
When the supply air is from the on-board systems
(i.e. the auxiliary power unit (APU) or engine bleed
air), the standard requires a minimum of 3.5 L/s/p of
outdoor air. However, if the source of supply air is
from the airport terminal or ground carts, then the
ventilation rate of 9.4 L/s/p of total air is required.
concentration has long been used as a proxy for
exhaled by passengers as a
tracer gas, and as an indicator of environments that
occupants will ﬁnd unsatisfactory. ASHRAE standard
with respect to odour per-
ception. It states that indoor CO
greater than 700 parts per million by volume (ppmv)
above outdoor CO
levels in typical ofﬁce buildings is
an indicator that visitors entering the space will be sat-
isﬁed with respect to human bioefﬂuents (body odour).
In addition to its use as a proxy for ventilation and
satisfaction, recent evidence suggests that CO
also be a direct pollutant. Hypercapnia (CO
stimulates vasodilation of cerebral blood vessels,
increasing cerebral blood ﬂow and elevating intracra-
nial pressure, which can cause dyspnea on exertion,
headaches, visual disturbance, impaired cognitive func-
tion, and other central nervous system symptoms.
A recent study
in the space station indicated that ele-
can lead to symptoms such as headache and
lethargy. Other studies found that the exposure to CO
at concentrations typically encountered in indoor envi-
ronments (950–2500 ppm) is associated with decre-
ments in human cognitive function.
impacts were observed at concentrations well below
the 5000 ppmv exposure limit set by Occupational
Safety and Health Administration (OSHA).
Investigation of ventilation performance in aircraft
cabin can use simulated environment chamber. Cao
used a large-scale particle image velocimetry
(PIV) system to study the air distributions in a Boeing
737 cabin mockup occupied by 42 heating manikins.
They found that the cabin airﬂows were at low velocity
and high turbulence levels. The low-speed air motion
may lead to poor ventilation effectiveness in the pas-
senger zone, even at a total ventilation rate of 9.4 L/s/p,
as recommended by ASHRAE. Du et al.
tally studied the jet ﬂow characteristics from a ventila-
tion nozzle in an aircraft cabin mockup. They
concluded that the effect of nozzle jet on passenger’s
thermal comfort was mostly affected by air velocity
distribution. Wang et al.
studied the ventilation per-
formance in a Boeing 767 cabin mockup containing 35
manikins. The CO
released by manikins was used as a
tracer gas to evaluate the air exchange rate and venti-
lation effectiveness in the breathing zone of passengers.
They found that the local mean age of air decreased
almost linearly with an increase in fresh air supply
rates. Strøm-Tejsen et al.
used a 21-seat cabin
mockup with realistic pollution sources to investigate
the balance between fresh air and humidity during a
simulated 7-h ﬂight. The study covered four conditions
of outside airﬂow rate ranged from 1.4 to 9.4 L/s per
person. Sixty-eight subjects acting as passengers were
exposed to the four simulated ﬂight conditions.
2Indoor and Built Environment 0(0)
The results showed that the reduced outside air could
intensify the symptoms commonly associated with air
travel (headache, dizziness and claustrophobia). These
studies provided comprehensive evaluation of the ven-
tilation performance in controlled cabin environments
and simulated conditions.
Some researchers have further performed on-board
measurements of ventilation on actual ﬂights. Spengler
conducted environmental monitoring in the pas-
senger cabin of 83 commercial ﬂights, including six air-
craft types (2 Airbus and 4 Boeing), ﬂying US domestic
and international routes. Various environmental
parameters were monitored including CO
ozone, volatile organic compounds (VOCs) and noise.
They found that the ventilation rates on high-passenger
load ﬂights could be below the required 3.5 L/s/p, as
indicated by high CO
concentrations. The concentra-
tions of some passenger-related VOCs were also higher
for ﬂights with reduced ventilation rates. The air qual-
ity inside cabins of Boeing 747, A330 and A340 air-
crafts were investigated by Lee et al.
on 16 ﬂights.
The measured CO
concentrations were well below
the OSHA exposure limit but exceeded the recom-
mended value by ASHRAE standard 62.1 (about
1000 ppmv), ranging from 2000 to 2500 ppmv.
Twenty one percent of the crew considered that the
air quality was poor during the ﬂight. Similarly, a
study of 43 ﬂights on commercial airlines showed that
the levels of CO
on most ﬂights were higher than that
recommended by the ASHRAE Standard 62.
and Guan et al.
conducted on-board measure-
ments in the passenger cabin of Boeing 737–800 aircraft
on a few domestic ﬂights. CO
concentrations were also
recorded to calculate the outside air ventilation rates,
which were then used to identify contributions of air-
borne particles and VOCs from outside and inside the
cabin. The results showed that the outside airﬂows on
all tested ﬂights were within relevant aircraft cabin ven-
tilation standards limits. For PM, contributions from
the bleed air and cabin interior were both important.
On the contrary, around 90% of VOCs came from
emissions inside the cabin.
Considering the vast numbers of ﬂights that occur
annually on a global basis, the extent of actual meas-
urements on airplanes is quite limited. Further, com-
parison of on-board ventilation performance to
regulations and standards has not been fully explored.
Lastly, little is known about ventilation and the vari-
ability of carbon dioxide throughout the entire ﬂight
experience, and whether there may be critical periods
with inadequate ventilation. Therefore, the overall
objectives of this study were to: (1) characterize the
variability of carbon dioxide throughout all phases of
ﬂight, and (2) estimate ventilation during the whole
ﬂight using CO
as a proxy for ventilation.
On-board monitoring was conducted on a total of 179
US domestic ﬂights from 10 July 2007 to 13 September
2009. The ﬂights covered 24 aircraft types, 7 aircraft
series, 10 airlines and 58 ﬂight routes. The largest pro-
portion of aircraft types were the Boeing 757 and MD-
88 models, accounting for 37% and 30% of all tested
ﬂights, respectively. Most of the investigated aircraft
models are short- to medium-range narrow-body
twin-engine jet airliners (see Table 1), which are com-
monly used for domestic airline services. In these air-
liners, the conditioned air was delivered by means of
diffusers placed high in the cabin, and returned
through grilles at bottom sides of the cabin. The
median sampling duration was 1 h 14 min.
We measured CO
concentrations and cabin pressure
using a modular and portable sensing package devel-
oped by Boise State University, known as the
The ASCENT 1000 package was
ﬁxed to the seatback pocket or was placed on the
tray table to measure CO
concentrations in re-
circulated air, which could represent the average CO
level in the cabin under the fully mixing condition. The
seat locations varied from ﬂight to ﬂight, mostly in
economy cabins (only two tests performed in ﬁrst-
class cabins). The cabin pressure and CO
tions were recorded at intervals of either 10, 30 or 60 s.
The ASCENT 1000 passed tests for electromagnetic
interference by the Boeing Company and was approved
for use on commercial ﬂights. The ASCENT 1000
package was pre-calibrated and tested in six ﬂights by
Loo et al.
Inside the package, pressure sensing
was accomplished by a VTI Technologies SCP1000
sensor module with a measuring range of 30–120 kPa
(200 Pa). CO
concentrations were determined using
a Telaire 6004 CO
OEM sensor (General Electricity
Company). The measured CO
automatically corrected to compensate for air pressure
changes. This adjustment was done in real-time by the
sensor once it received the pressure measurement. The
sensor had a calibrated detection range of 0 to 2000
ppmv (40 ppmv or 3% of readings, whichever is
greater). The narrower range could provide a better
accuracy for lower levels more likely to be encountered
in aircraft cabins. In this study, 98% of measured CO
concentrations were within this range. The sensor could
detect concentrations above 2000 ppmv but with lower
Cao et al. 3
The whole ﬂight was divided into ﬁve phases: board-
ing, ascent, cruise, descent and deplaning. The ﬂight
phases could be clearly distinguished by changes of
cabin pressure. Boarding phase was deﬁned as a
period of constant cabin pressure prior to take-off.
Measurement campaign typically began when most or
all the passengers have boarded. So, the boarding
phase included a fraction of boarding process and
whole taxiing process. Ascent phase was a period char-
acterized by a steady decline in cabin pressure after
take-off until a constant pressure was achieved at
cruise phase. Descent phase was a period of steady
increase in cabin pressure after the cruise phase. After
landing, the cabin pressure became constant again
during deplaning phase. The pressure and CO
were continuously monitored from ascent to descent
on most ﬂights. However, the measurement could not
be performed during boarding or deplaning phases on
some ﬂights due to operational restrictions.
Estimation of outside airflow rate
The transient in-cabin/ambient CO
outside airﬂow rate and generation rate of CO
person together follow equation (1)
fully mixed approximation
¼outside airﬂow rate at time j(L/s/p);
generation rate by a person (L/hr);
concentration inside the cabin at time
point j(ppmv); C
concentration outside the
cabin at time point j(ppmv); N¼number of passen-
gers; V¼cabin volume (m
); t¼time (s).
Based on this formula, the outside air ventilation
rates can be estimated using measured CO
tions and a steady-state approximation, also known as
the constant concentration method. This method
assumes that the in-cabin CO
concentration is steady
during each measuring period, and the dominant
source of CO
is the exhaled breath by passengers at
an average generation rate. The constant concentration
method has been successfully applied in several studies
on in-ﬂight ventilation performance.
of the high number of air changes per hour on
Table 1. Measured CO
concentrations and estimated outside airflow rates on all the flights.
concentration (ppmv) Outside ventilation rate (L/s/p)
Min Max Mean Median SD Min Max Mean Median SD
Airbus 319/320 A319 2 1003 1635 1228 1211 112 4.05 8.19 6.11 6.13 0.82
A320 3 682 2990 1175 1235 382 1.94 17.08 8.15 5.95 4.04
Boeing 737 B737-300 4 1032 1773 1479 1497 111 3.64 7.83 4.68 4.55 0.54
B737-400 1 1656 1937 1808 1804 65 3.26 3.98 3.56 3.57 0.16
B737-500 1 1000 1220 1124 1142 49 6.06 8.23 6.88 6.69 0.49
B737-700 7 850 2947 1261 1272 188 1.97 10.90 6.05 5.71 1.40
B737-800 6 661 2976 1288 1355 316 1.95 18.38 6.73 5.22 3.62
Boeing 757 B757 67 703 2992 1438 1421 284 1.94 15.95 5.19 4.88 1.52
Boeing 767 B767-300 5 756 1662 1234 1238 160 3.96 13.66 6.25 5.93 1.65
Bombardier CR-7 1 1094 2209 1903 1911 160 2.77 7.14 3.38 3.32 0.48
CR-9 5 658 2616 1451 1427 265 2.27 18.59 5.04 4.86 1.32
CRJ-100 2 620 1620 889 878 160 4.10 21.60 10.61 10.25 2.91
CRJ-140 1 1264 1566 1398 1384 79 4.28 5.76 5.02 5.07 0.39
CRJ-150 1 793 1233 969 976 127 5.97 12.42 9.07 8.58 1.88
CRJ-200 2 721 1442 1070 1049 173 4.79 15.09 7.90 7.63 2.14
Embraer E-135 1 1143 2077 1417 1381 185 2.99 6.68 5.04 5.08 0.79
E-145 1 682 2054 1201 1070 403 3.03 17.08 7.86 7.40 3.85
E-170 1 855 1352 1097 1083 160 5.23 10.78 7.49 7.25 1.73
E-175 8 686 2137 1162 1120 200 2.89 16.85 6.91 6.89 1.69
E-190 1 1392 2340 1771 1717 233 2.59 5.03 3.75 3.80 0.60
MD DC-9 3 934 1951 1340 1159 320 3.23 9.23 5.82 6.54 1.59
MD-80 1 659 1094 897 891 111 7.14 18.52 10.41 10.01 2.45
MD-88 53 514 2979 1321 1318 264 1.95 39.50 5.89 5.42 1.98
MD-90 2 656 2993 1251 1265 416 1.94 18.72 6.97 5.75 2.83
Summary 179 514 2993 1353 1333 290 1.94 39.50 5.77 5.34 2.09
4Indoor and Built Environment 0(0)
airplanes, steady-state conditions are reached more
rapidly than other indoor environments. Under the
steady-state condition, the left side of equation (1)
becomes 0, so the outside airﬂow rate can be estimated
by equation (2)
If the CO
concentration is changing rapidly, equa-
tion (2) will not be accurate due to the transient term in
equation (1). A simple order of magnitude assessment
showed that rates of change greater than about
1000 ppm/h may adversely impact the accuracy of the
ventilation rate calculated using the steady-state
approximation. This requirement typically will not be
met in the beginning of the boarding phase when the
concentration rises rapidly as large numbers of people
enter the aircraft. Even though equation (2) may not
give an accurate measure of the actual instantaneous
value of outside air being supplied to the cabin during
rapid transients in concentrations, the levels of CO
the air are indicative of contaminant levels in the air.
Consequently, the ventilation rates calculated with
equation (2) during transient periods may be consid-
ered as equivalent steady-state ventilation rates during
each measuring period and should be viewed in that
concentration in the atmosphere generally
ranges from 365 ppmv to 390 ppmv, summarized by Li
Here, we used a mean outdoor CO
tion of 386 ppmv, averaged from the global monthly
mean atmospheric CO
concentration data reported by
National Oceanic and Atmospheric Administration
during the whole test period. The genera-
tion rate of CO
per person can be derived by
where RQ ¼respiratory quotient: 0.83 for an adult of
average size and engaged in sedentary activities;
M¼metabolic rate per unit surface area (METs);
¼DuBois surface area (m
), which can be calculat-
ed by equation (4) as
where H¼height (m) and W¼weight (kg).
The generation rates will vary for different people
depending on inputs in equations (3) and (4). Thus, a
Monte Carlo simulation was used to determine the
distribution of the CO
generation rate per person
from 100 air travellers.
The metabolic rate for each
passenger was selected using a uniform distribution in
the range of 0.9–1.3 METs, representing different phys-
ical active types from sleeping to sit/watch TV.
average generation rate of CO
per person from 10,000
simulation runs was 18.2 L/h.
Descriptive analysis was performed to show the mea-
concentrations and estimated outside air-
ﬂow rates by ﬂight phases and aircraft types. Box
plots and probability distributions were used to com-
pare the data distributions by ﬂight phases. Post-hoc
multiple comparisons using the Fisher’s least signiﬁ-
cant difference (LSD) method
were performed to
test the inﬂuence of ﬂight phases/duration on the ven-
tilation performance. A p-value <0.05 indicates a sta-
tistically signiﬁcant difference for the mean value.
Figure 1 depicts an example of the measured cabin
pressure and CO
concentrations during a typical
ﬂight. As shown in Figure 1, the ﬂight phases could
be clearly identiﬁed by changes in cabin pressure. The
FAR 14 CFR 25.831
states that the minimum cabin
pressure under normal operating conditions should not
be less than the pressure found at an altitude of 2438 m
(75.3 kPa), and all the tested ﬂights met this standard.
This example highlights the typical variations in CO
concentration over the course of a ﬂight. During the
boarding phase, the CO
concentrations increased rap-
idly, indicating low outside airﬂow rates. CO
trations decline steadily after taking-off and are lower,
but above levels typically acceptable in buildings (1000
ppmv) during the cruise and descent phases. Finally,
concentration increased again after landing.
The descriptive statistics of all measured CO
centrations and corresponding outside air ventilation
rates by aircraft types are listed in Table 1. The CO
levels on all ﬂights were well below the occupational
exposure limit of 5000 ppmv. Overall, across all aircraft
types and ﬂight phases, the CO
from 514 to 2993 ppmv, with an average value of 1353
290 ppmv (mean SD) and a median of 1333 ppmv.
Correspondingly, the outside airﬂow rates ranged from
1.94 to 39.50 L/s/p, and the average and median values
were 5.77 2.09 L/s/p and 5.34 L/s/p, respectively.
Figure 2 shows distributions of CO
and corresponding outside airﬂow rates calculated
using equation (2) during each ﬂight phase. Figure 2
(a) shows that CO
concentrations were the highest
during the boarding phase, with a median of 1539
Cao et al. 5
ppmv and an interquartile range (IQR) of 407 ppmv.
levels during climbing were lower compared to
that during the boarding phase. The median CO
centration was 1304 ppmv during cruise, the lowest
among all phases. During descent to deplaning, a grad-
ual increase in CO
levels was observed. The high CO
levels during the boarding phase indicated inadequate
ground ventilation by APU or gate-based ventilation
systems. As shown in Figure 2 (b), calculated ventila-
tion rate was signiﬁcantly lower in the boarding phase
compared to all the other ﬂight phases (p<0.0001),
with a median of 4.38 L/s/p (below the FAA regula-
tions) and an IQR of 1.84 L/s/p. The ventilation per-
formance gradually improved during ascent, and
reached a peak ventilation at the cruising phase, with
a median of 5.51 L/s/p and an IQR of 1.91 L/s/p.
Figure 3 depicts the percentages of in-cabin outside
airﬂow rates that met the two aircraft ventilation reg-
ulations. The ﬂights met ASHRAE standard (3.5 L/s/p)
for 80% during boarding and the vast majority of time
Figure 2. Box plots of (a) CO
concentrations and (b) outside airflow rates by flight phases.
Figure 1. Measured CO
concentration and pressure during a typical flight.
6Indoor and Built Environment 0(0)
(96%) during ﬂying, but this is a minimum for accept-
able air quality. They only met the FAR design require-
ment of 4.7 L/s/p, 42% of time during boarding and
73% of time during ﬂying. Figure 4 shows the proba-
bility distributions of outside airﬂow rates during
boarding and cruising with an interval of 0.5 L/s/p.
The values higher than 20 L/s/p during boarding
(0.2%) are not shown for a clear comparison. The
black curve shown in each plot represents the corre-
sponding normal distribution with the same mean
value and standard deviation. As compared to the
black curve, the distribution of outside airﬂow rates
during boarding is obviously more positively skewed
(skewness ¼3.119) to the low end than during cruising
(skewness ¼1.964). Therefore, the boarding phase was
characterized by worse ventilation performance com-
pared to the cruising phase. A part of the very low
ventilation rates calculated for the boarding phase
may be not accurate due to the fact that the high
concentration was changing rapidly in the begin-
ning of boarding. The effect of increased metabolic
activity during boarding may also account, in part,
for the increased CO
the high CO
concentrations seen during boarding are
indicative of contaminants not being removed effective-
ly from the air by the ventilation.
Cabin outside air ventilation rates were lower than
those typically found in other indoor environments.
Low ventilation and high physical activity levels possi-
bly increase the risk for airborne infectious disease
transmission, discomfort and other complaints. Low
ventilation rates in buildings are strongly associated
with higher risks of infectious airborne disease trans-
For instance, a recent study indicated that
crowded dormitories with low outdoor airﬂow rates
were associated with more respiratory infections
among college students.
Similarly, risk of disease
transmission within the aircraft cabin also seems to
be affected by ventilation.
A study of in-ﬂight tuber-
culosis revealed that doubling ventilation rate within
the cabin could halve the infection risk.
tested ﬂights, passengers spent 20% of their boarding
time with equivalent ventilation rates below 3.5 L/s/p
and 58% of their time with rates below 4.7 L/s/p. Our
results showing low ventilation rates during boarding
also suggest that characterizing in-cabin exposures
based only on cruising conditions may overlook the
critical exposures likely occurring during ground oper-
ations. A recent Airport Cooperative Research
Program (ACRP) report
has highlighted the increas-
ing risks of transmission of infectious aerosols while the
aircraft is parked at the gate and APUs are shutdown.
Figure 3. Percentages of in-cabin outside airflow rates
met the aircraft regulations.
Figure 4. Probability distributions of outside airflow rates during boarding and cruise.
Cao et al. 7
The elevated CO
concentrations in the airplane
cabin are also of interest in light of recent studies on
impacts of CO
and ventilation on cognitive function
of occupants that saw effects at levels seen during all
ﬂight phases. In a study of a controlled ofﬁce environ-
participants were exposed to three CO
trations for 8 h each while they completed their normal
work activities. At the end of each day, they completed
1.5-h cognitive tests. Participants scored 15% and 50%
lower on cognitive tests on days when they were
exposed to 950 ppmv and 1400 ppmv of CO
tively, compared to the reference condition of 550
ppmv. Similar ﬁndings were shown by Satish et al.
during 2.5-h exposure windows. Detrimental impacts
on cognitive function may inﬂuence the performance
of ﬂight crew. In particular, the poor ventilation per-
formance during boarding warrant more attention. The
ﬂight deck door is usually open during boarding, which
concentrations on the ﬂight deck could be
inﬂuenced by the high CO
concentrations in the cabin.
concentrations on the ﬂight deck may
impact pilot performance during the subsequent taxiing
and take-off. A study by Allen et al. found that pilots
had 1.52 times higher odds of passing ﬂight maneuvers
at 1500 ppm and 1.69 times higher odds at 700 ppm
compared to 2500 ppm, as rated by a FAA Designated
Pilot Examiner during simulated ﬂight sessions. Pilots
were particularly impacted during maneuvers following
boarding; passing rates were twice as high at 700 ppm
and 1500 ppm during a takeoff with an engine ﬁre than
at 2500 ppm, and four times as high at 700 ppm during
a rejected takeoff than at 2500.
In addition to cog-
nitive function, studies have found impacts on acute
health symptoms and physiological indicators such as
heart rate variability and peripheral blood circulation
from high indoor CO
higher than 800 ppm were associated with an increase
in workers’ SBS symptoms, such as eye irritation and
upper respiratory symptoms.
The measured ranges of CO
airﬂow rates were also comparable to other studies.
Spengler et al.
studied the cabin air quality during
the cruise phase of 83 ﬂights ﬂying US domestic and
international routes. In their study, the CO
tions were 1404 297 ppmv with a range of 863–2065
ppmv, correspondingly the outside airﬂow rate was 5.5
1.8 L/s/p with a range of 3.0 to 10.9 L/s/p. Giaconia
measured the CO
concentrations on 14 short-
haul ﬂights with A319 in Italy. The concentrations of
varied between 734 and 2252 ppmv from boarding
to disembarking, which corresponded to outside air-
ﬂows of 14.60 and 2.76 L/s/p, respectively. The mean
concentrations on each ﬂight ranged from 925 to
1449 ppmv, with the corresponding outside airﬂow
rates from 5.18 to 10.56 L/s/p. Similarly, Li et al.
reported that concentrations of CO
varied from 792
to 2253 ppmv during ﬁve domestic ﬂights with Boeing
737–800 in China. The mean CO
each ﬂight ranged from 976 to 1135 ppmv, with the
corresponding outside airﬂow rates from 7.18 to 9.79
calculated the ventilation capacity for
several representative aircraft types (Airbus, Boeing,
MD, USA) using their designed air exchange rates,
and reported that the outside air ventilation capacity
was within the range of 3.2 to 11.2 L/s/p. Our mean
outside ventilation rates were well within this range for
all 24 aircraft types.
The ﬁnding of higher CO
boarding was consistent with previous studies. In a
study of 16 ﬂights, Lee et al.
concentrations during boarding and
deplaning than during cruising. They noted that CO
concentrations were higher during boarding and de-
boarding with typical levels of 2000 to 2500 ppmv.
Likewise, in a study of 26 intercontinental ﬂights with
Boeing 767–300, Lindgren and Norb€
concentrations during non-cruise conditions due
to low air exchange rates. The CO
1656 ppmv under non-cruise conditions compared to
734 ppmv during cruise.
According to previous studies, the average bleed air
fraction of the air supplied the cabin is typically about
0.5 during the whole ﬂight.
Thus, the mean total
ventilation rate may be approximately twice the mean
outside airﬂow rate (11.54 L/s/p). We used sampling
duration as a proxy to test the inﬂuence of ﬂight dura-
tion. The tested ﬂights included mostly short-haul
ﬂights (<2 h) and the rest were medium-haul ﬂights
(2–6 h). No signiﬁcant difference for the mean outside
airﬂow rate can be found between the short-haul ﬂights
with duration t<1 h and 1 h <t<2h (p¼0.944). The
ventilation rates were slightly higher on the medium-
haul ﬂights (p<0.0001). The results of this study may
not be representative of average conditions on long-
haul international ﬂights which have a longer cruise
phase and where high activity during boarding and
deplaning constitute a shorter fraction of the overall
The estimation of outside airﬂow rate has some
uncertainties. Through Monte Carlo simulations, we
estimated that the range of CO
generation rate by a
passenger is 17–19.3 L/h,
thus resulting a maximum
deviation of 6% for the estimated outside ventilation
rates due to uncertainty in metabolic rate. Our gener-
ation rate is consistent with that used by ASHRAE
(18 L/h), which corresponds to an average size adult
engaged in typical ofﬁce work (1.2 METs). However,
other values have been suggested depending on sub-
jects. For example, Lee and Siconolﬁ
generation rate of 23 astronauts in a seated position
8Indoor and Built Environment 0(0)
and obtained a value of 13.2 1.2 L/h. A recent study
suggested that CO
generation rates of young Chinese
people were overestimated by the empirical equation
used in our research, which should be corrected by
factors of 0.75 and 0.85 for females and males under
sitting/standing conditions, respectively. The genera-
tion rates of passengers may be increased during a frac-
tion of boarding phase when physical activity levels
were elevated. However, it is worth noting that a pas-
senger is seated during most of the boarding and taxi-
ing process before taking-off. For a sensitive test, we
assumed that on average the highest 10% of CO
centrations was recorded during a passenger walking
inside the cabin and lifting up the luggage with a met-
abolic rate of 3 METs (implied walking, putting away
household items, moderate effort).
assumption, the median of ventilation rates during
boarding would increase to 4.77 L/s/p with an IQR
of 1.49 L/s/p, still signiﬁcantly lower compared to
other phases (p<0.0001).
The application of constant concentration method
was based on the assumption that the in-cabin air
was well mixed by ventilation and the only source of
was the respiration by passengers. The well-mixed
assumption has been validated by several experimental
However, no information was available
concerning whether there was any other source of CO
on tested ﬂights (e.g. dry ice used for chilling food and
beverages in the galley systems). As discussed previous-
ly, rapid changes in CO
concentration will introduce
errors in the calculated rate. However, these errors
should have minimal effect on the average rates calcu-
lated over the period of a ﬂight phase. These uncertain-
ties may lead to over- or underestimating ventilation
rates, but do not affect ﬁndings related to the absolute
In this study of 179 US domestic ﬂights, we found that
96% of the observations met the minimum recom-
mended outside airﬂow rates for acceptable air quality
during the ﬂying phases (3.5 L/s/p), but only 73% met
the rate required by the FAR (4.7 L/s/p). The CO
levels on all ﬂights were well below the occupational
exposure limit of 5000 ppmv. The ground operation
phases, especially the boarding phase, had higher
concentrations than the ﬂying phases, likely due
to the inadequate ground ventilation on many ﬂights
and higher metabolic rates during boarding.
Ventilation rates were highest during the cruising
phase. Current ventilation requirements are often not
met, particularly during the boarding phase, which is of
concern because low ventilation in other settings has
been associated with increased rates of disease
transmission, increased upper respiratory symptoms,
and worse performance on cognitive function tests.
Veriﬁcation of ventilation performance rather than reli-
ance on design estimates for determining compliance
with regulatory mandated minimum outdoor air venti-
lation rates is recommended.
All authors contributed to the data analysis and manuscript
preparation. BJ and SML led the ﬁeld sampling effort. CZ,
JGA, JDS, BC, and EM were involved in the initial data
analysis and manuscript preparation. XC and PM led the
ﬁnal data analysis and manuscript preparation in coordina-
tion with JGA, BC and JDS. All authors contributed to and
approved the ﬁnal manuscript.
Although the FAA has sponsored this project, it neither
endorses nor rejects the ﬁndings of this research. We thank
the anonymous peer-reviewers for their excellent comments
that have enhanced the manuscript.
Declaration of conflicting interests
The author(s) declared no potential conﬂicts of interest with
respect to the research, authorship, and/or publication of
The author(s) disclosed receipt of the following ﬁnancial sup-
port for the research, authorship, and/or publication of this
article: This study was funded partially by the US Federal
Aviation Administration (FAA) Ofﬁce of Aerospace
Medicine through the National Air Transportation Center
of Excellence for Airliner Cabin Environment Research
(ACER)/Research in the Intermodal Transport
Environment (RITE), Cooperative Agreements 10-C-RITE-
HU, 07-C-RITE-HU and 04-C-ACE-HU.
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