A Review of In-Flight Thermal Comfort and Air Quality Status in Civil Aircraft Cabin Environments

Abstract

The civil aircraft cabin is enclosed and highly occupied, making it susceptible to a decline in indoor environmental quality. The environmental quality of civil aircraft cabins not only depends on objective factors such as temperature, relative humidity, and the presence of air pollutants such as carbon dioxide (CO₂), carbon monoxide (CO), ozone (O₃), particle matter (PM), and volatile organic compounds (VOCs) but also the subjective factors pertaining to the perceptions and health symptoms of passengers and crew. However, few studies have thoroughly examined the air quality and thermal comfort parameters that are measured during in-flight testing in airplane cabins, as well as the passengers’ subjective perceptions. In order to evaluate the in-flight thermal comfort and air quality status, this study conducted a review of the recent literature to compile data on primary categories, standard limits, and distribution ranges of in-flight environmental factors within civil aircraft cabins. Following a search procedure outlined in this paper, 54 papers were selected for inclusion. Utilizing the Monte Carlo method, the Predicted Mean Vote (PMV) distributions under different exercise intensities and clothing thermal resistance were measured with the in-cabin temperature and humidity from in-flight tests. Recommendations based on first-hand data were made to maintain the relative humidity in the cabin below 40%, ensure wind speed remains within the range of 0–1 m/s, and regulate the temperature between 25–27 °C (for summer) and 22–27 °C (for winter). The current estimated cabin air supply rate generally complies with the requirements of international standards. Additionally, potential carcinogenic and non-carcinogenic risks associated with formaldehyde, benzene, tetrachloroethylene, and naphthalene were calculated. The sorted data of in-flight tests and the evaluation of the subjective perception of the occupants provide an evaluation of current cabin thermal comfort and air quality status, which can serve as a reference for optimizing indoor environmental quality in future generations of civil aircraft cabins.

1. Introduction

The environmental quality of civil cabins significantly influences the safety, health, and comfort of passengers [1,2,3]. Cabins are environments where passengers and crew members are in direct contact with and experience a variety of sensations, including thermal sensations, moisture levels, wind sensations, and odor [4,5]. The objective parameters of civil aircraft cabin environments, such as temperature, humidity, wind speed, ventilation volume, and airborne pollutant concentrations, are often intertwined [6]. This interdependence means that adjusting a single parameter alone cannot provide an optimal environment. Thus, clarifying the controllable parameters of cabin environments and assessing passengers’ subjective perceptions while also quantifying the health effects can enhance the comfort of the cabin environment. Studies have investigated the controllable parameters of cabin environments, the concentration of gaseous pollutants, and their corresponding health hazards, but limited flights were tested due to the high cost of onboard testing [7,8,9]. Determining the present state of the cabin’s environmental factors would be made easier by comprehensively compiling data from the flight test activities that have been carried out. A more understandable representation of the current condition of cabin thermal comfort and air quality can be obtained by combining multivariate and subjective assessments, as opposed to studying environmental characteristics and their distribution patterns.

The thermal environment and ventilation airflow have the primary impact on passengers’ perception of cabin environmental quality [10,11]. However, the air supply outlet in the cabin environment is close to the passenger area, the air supply velocity is fast, and the temperature is low, which makes it easy for passengers to feel the wind [12]. Besides the wind sensation, the low humidity and high occupancy of the cabin environment also led to passengers feeling dry and stuffy [8,13,14]. Due to the variations in clothing and differing sensitivities to the surrounding thermal environment, it’s challenging to maintain a high level of satisfaction in the cabin. A previous study attempted to alleviate members’ discomfort by increasing the relative humidity to 30% and reducing the ventilation flow rate to 1.4 L/s per person, but this exacerbated reported symptoms of headache and dizziness [15,16]. Clearly, the reduction in fresh air volume has made passengers feel more restricted in their breathing. Common thermal comfort methods are utilized to evaluate the temperature and humidity from the measured data of in-flight tests, which are then combined with the characteristics of the flying members’ clothing in winter and summer to give more accurate recommendations.

Typically, the carbon dioxide (CO₂) concentration in the cabin does not adversely affect the human body but can, to some extent, reflect the adequacy of ventilation volume in the cabin [17,18,19]. The mandatory regulations of aviation airworthiness set standards for CO₂ concentrations within the cabin. The Federal Aviation Administration (FAA) has set an exposure limit for CO₂ in aircraft cabins at 5000 ppm [20,21,22]. In indoor buildings, the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) suggests an exposure limit of 700 ppm for CO₂ in buildings. Indoor environments with concentrations exceeding 1000 ppm are considered of low quality [23]. A measurement study on A321 cabin environments revealed that many flights had CO₂ concentrations ranging from approximately 1000–5000 ppm [24]. This results in CO₂ concentrations significantly higher than those found in typical indoor environments, which typically have concentrations below 1000 ppm [25]. Exposure to such levels of CO₂ has been reported to significantly impact the performance of commercial pilots during simulated flight training [26]. In addition, the CO₂ concentration is often used as an indicator to evaluate the quality of cabin air quality as it is related to the volume of cabin ventilation.

The CO₂ concentration within a cabin can be reduced by introducing fresh air. Simultaneously, high-efficiency particulate air (HEPA), ozone converters, and activated carbon (AC) filters can decrease the concentration of other air pollutants in the cabin [27]. Of particular concern are the potential health risks from volatile organic compounds, such as formaldehyde and benzene, which are classified as a class of carcinogens by the U.S. Environmental Protection Agency (USEPA) [28]. For frequent air travelers and aviation professionals, it’s crucial to be aware of the lifetime exposure and its corresponding health risks. One study evaluated the concentrations of benzene, toluene, ethylbenzene, and xylene in 14 cabin flights, quantifying the carcinogenic and non-carcinogenic risks for adults, children, teenagers, and the elderly, which are significantly below the U.S. Environmental Protection Agency’s limit of 1 × 10⁻⁶ (acceptable level of risk, dimensionless) [29]. Given the occupational exposure, flight attendants face a much higher cancer risk than adult passengers; a study indicates that their exposure to formaldehyde in the cabin has surpassed 1 × 10⁻⁶ in terms of carcinogenic risk. Previous studies have investigated the concentrations of gaseous pollutants and their corresponding health hazards; however, the number of flights surveyed was limited by the cost of on-board measurement campaigns [30,31]. A health risk assessment for the mean and maximum values of measured in-site data of gaseous pollutants in the cabin environment helps to reveal the subjective health perception of the occupants.

In sum, the cabin environmental quality depends on the comprehensive perception of passengers and crew and the coupling effect of subjective and objective factors. Based on the analysis of the aforementioned literature, this study first reviewed the measured data of cabin environment in the published literature since 1987 and summarized the cabin environmental thermal ventilation parameters and air pollutant concentration levels. Based on the Monte Carlo method and predictive mean vote (PMV) [5], recommendations of in-cabin temperature and humidity under different exercise intensities and clothing thermal resistance were made for thermal comfort. The estimated cabin ventilation based on measured CO₂ concentrations was also evaluated. Furthermore, this paper uses the health risk assessment method [28,32] to calculate and analyze the health risk that VOCs in the cabin may cause to passengers and crew, taking formaldehyde, benzene, tetrachloroethylene, and naphthalene as examples.

2. Materials and Methods

2.1. Literature Selection

The purpose of this paper is to review the articles and reports on air quality measurement of commercial aircraft and collect the measured values of cabin environmental parameters of commercial aircraft, including the temperature, relative humidity, ventilation, the concentrations of air contaminants such as CO₂, CO, O₃, PM, and VOCs. The relevant industry journals, meeting records, and books were processed through screening, reading, and organizing the collection of information. Search keywords include aircraft cabin air quality, air pollution, CO₂, CO, O₃, PM, and VOCs. The databases used in this paper include ScienceDirect, Web of Science, and PubMed. The search time ended by March 2023, and articles unrelated to the theme were excluded.

Article selection followed the standard PRISMA processes in Figure 1. The literature search identified 6597 peer-reviewed articles and publicly accessible industry reports. After this, 642 duplicates were removed after comparison. Titles and abstracts were then reviewed, and the articles were excluded if they did not directly measure the values of cabin environmental parameters of commercial aircraft through in-flight tests. From this pool, 210 articles were assessed as meeting full-text eligibility, and of these, 54 articles met the selection criteria. During the literature review, the title, year of publication, and types of flights were recorded. The frequency distribution of the measured temperature, RH, and CO₂ concentrations is provided. For the measured CO₂, CO, O₃, PM, and TVOC concentrations, the average, maximum, and minimum values were given. Since there were significant differences in VOC detection types and concentrations for different flights in the literature (due to variations in sampling methods and locations), only the literature that mentioned the most commonly measured VOC concentrations was analyzed.

2.2. Evaluation Methods of Thermal Comfort and Ventilation Rate

The thermal comfort of the aircraft cabin is evaluated by the PMV method [5]. Considering the temperature, relative humidity, average radiation temperature, air velocity, thermal resistance of human clothing, and metabolic rate in the aircraft cabin, the calculation formula is as follows:

$$PMV=[0.303\exp \left(-0.036M\right)+0.0275]\times L$$

(1)

where: M is the human metabolic rate (W/m²), and L is the intermediate variable:

$$\begin{gathered} L=M-W-3.05\left[5.733-0.07\left(M-W\right)-P_a\right]-0.42\left(M-W-58.2\right)-0.0173M\left(5.867-P_a\right) \\ -0.0014M\left(34-t_a\right)-3.96\times {10}^{-8}\times f_{cl}[{\left(t_{cl}+273\right)}^4-{\left(t_r+273\right)}^4-f_{cl}{h}_c[{(t}_{cl}+t_a\left)\right] \end{gathered}$$

(2)

where W is the mechanical work performed by humans towards the environment (W/m²); P_a is the partial pressure of water vapor in the cabin (kPa); t_a is the cabin temperature (k); t_r is the average radiation temperature in the cabin (k); f_cl is the area of clothes worn by passengers (m²); t_cl is the outer temperature of clothes; h_c is the convective heat transfer coefficient (W/(m²·K)).

A statistical method called Monte Carlo simulation is used to produce a set of results from a predictive model that is based on a probability distribution (such as a normal distribution). The independent variables’ values are randomly generated by continually running the simulation. Until a sufficient number of outcomes were gathered to form a representative sample of almost endless combinations, this process was repeated. The range of cabin temperature and humidity parameters that meet the cabin thermal comfort requirements can be output by the PMV prediction model in conjunction with the fitted normal distribution of in-flight test cabin temperature and humidity data that was discovered from the literature review.

Utilizing the Monte Carlo method, the PMV distributions under different exercise intensities and clothing thermal resistance were measured with the in-cabin temperature and humidity from previous studies. The exercise intensities were simulated as leaning, sitting, and standing with relaxation, with the metabolic rate M valued at 0.8 met, 1.0 met, and 1.2 met. The clothing thermal resistance reflects the dress change in summer and winter, and the I_CL is set at 0.5 clo and 1 clo, respectively. According to the application conditions of PMV, the wind speed inside the cabin is in the range of 0–1 m/s. A total of six different operating conditions were simulated, and the random sample was within the temperature and humidity range, with frequency n = 10,000.

Similarly, the distributions of fresh air volume under different in-cabin CO₂ concentrations were simulated using the Monte Carlo method. The following prediction model allows for the prediction of aircraft cabin ventilation based on the distribution of CO₂ concentration within the aircraft cabin. Based on the measured CO₂ concentration inside the cabin, the fresh air volume inside the cabin can be estimated. The instantaneous concentration of CO₂ inside and outside the cabin, instantaneous ventilation volume, and per capita CO₂ production rate follow,

$$V\frac{{dC}_{bj}}{dt}\times {10}^{-3}=N{Q}_j\left(C_{oj}-C_{bj}\right)\times {10}^{-6}+\frac{Nm}{3600}$$

(3)

where Q_j is the fresh air volume (L/s per person); C_oj is the concentration of CO₂ outside the cabin; C_bj is the CO₂ concentration inside the cabin; V is the volume of the cabin (m³), N is the number of passengers; m is the average CO₂ production rate per person (L/h). In approximate steady state, the fresh air volume is obtained as,

$$Q_j=\frac{{10}^{6}m}{3600(C_{bj}-C_{oj})}$$

(4)

Within the measured range of CO₂ concentration, the random sample frequency is n = 10,000.

2.3. Inhalation Risk Calculation of Selected VOCs

The health risk is calculated from the estimated total exposure of VOCs in the cabin. The exposure dose of VOCs in an aircraft cabin mainly refers to the exposure through breathing in the air of an aircraft cabin:

$$BD=\frac{C\times BR\times EF\times ED}{BW\times AL\times 365}$$

(5)

where BD is an adjusted air concentration (as proposed by USEPA [28,32]) and estimated to represent continuous lifetime excess exposure; C is the mass concentration of a chemical substance in the cabin environment, μg/m³; BR is the breathing rate, m³/h; EF is the exposure frequency, h/year; ED is the exposure duration, year; BW is the weight of a person, kg; AL is average life expectancy of human beings, year. The required exposure parameters (weight, exposure frequency, etc.) were from the results of a human exposure parameter survey based on the actual situation of the Chinese people in the literature [33,34]. The values of each parameter are detailed in

Table 1. Summary of exposure parameters.

Parameter	Passengers		Crew members
	Male	Female	Male	Female
BR, m3/h	0.793	0.59	0.793	0.59
EF, h/a	100	100	1300	1300
ED, a	70	70	70	70
BW, kg	62.7	54.4	62.7	54.4
AL, a	70	70	70	70

.

The non-carcinogenic risk HQ [28,32] is calculated as follows:

HQ = BD/RfC

(6)

where HQ is the Hazard index, which represents the risk of specific non-carcinogenic health hazards and is dimensionless. If HQ is less than or equal to 1, indicating that it is not expected to cause significant damage. If HQ > 1, indicating that the non-carcinogenic risk is high. RfC is the reference concentration of the inhalation route, μg/m³.

The carcinogenic risk is calculated as follows:

CR = BD × IUR

(7)

where CR is the estimated risk of inhalation carcinogenesis, IUR is the estimated unit risk of inhalation of a chemical substance, (μg/m³)⁻¹, representing the estimated lifetime carcinogenic risk caused by continuous inhalation of 1 μg/m³ of VOC, here from US EPA Integrated Risk Information System (IRIS) or California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment (OEHHA) [29,30,31].

Table 2. Toxicity parameters of typical VOCs in the cabin.

Typical VOCs	IARC	IUR (m3/μg)	RfC (μg/m3)
Formaldehyde	1	1.30 × 10−5	9
Benzene	1	2.20 × 10−6	30
Tetrachloroethylene	2A	6.10 × 10−6	40
Naphthalene	2B	3.40 × 10−5	3

shows the toxicity parameters of formaldehyde, benzene, tetrachloroethylene, and naphthalene. The IARC represents the level of carcinogenicity assigned by the International Agency for Research on Cancer.

3. Results

Table 3. Summary of aircraft cabin environmental factors based on published data.

Environmental Factor	Number of Articles	References	Flights		Mean (SD)	Max	Min
			Number	Type
T	19	[12,17,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]	252	13	23.7 °C (1.15 °C)	25.2 °C	20.6 °C
RH	20	[12,17,35,36,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53]	273	15	18.4% (7.2%)	43.6%	11.6%
CO2	24	[17,24,26,36,37,38,39,40,43,44,45,46,47,48,49,50,51,52,54,55,56,57,58,59]	416	17	1254.7 (308.6) ppm	5173.9 ppm	713.7 ppm
CO	8	[24,39,45,48,49,50,51,56]	224	>10	1.1 (0.47) ppm	1.6 ppm	0.01 ppm
O3	10	[24,41,45,50,51,56,60,61,62,63]	534	>10	50.2 (6.55) ppb	205 ppb	<3 ppb
PM	12	[17,39,46,50,55,59,60,64,65,66,67,68]	398	>10	0.04–4.83 μg/m3
TVOC	24	[17,24,29,30,31,36,40,45,46,48,51,56,62,69,70,71,72,73,74,75,76,77,78,79]	1045	>15	0.12–1.8 mg/m3

lists a summary of the temperature, the relative humidity (RH), the CO₂ concentrations, and the average concentrations of in-cabin contaminants, together with their maximum and minimum levels. From 19 studies with 252 flights [12,17,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51], the minimum measured temperature is 19.7 °C. The maximum temperature is 25.2 °C. The average and standard deviation (SD) of the measured temperatures is 23.7 ± 1.15 °C. There were 20 studies with 273 flights that reported the RH values [12,17,35,36,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53]; the minimum and the maximum measured RH levels are 9% and 40.6%, and the average and SD of the measured RH values were 18.4% ± 7.2%. 24 studies [17,24,26,36,37,38,39,40,43,44,45,46,47,48,49,50,51,52,54,55,56,57,58,59] encompassing 416 flights show the CO₂ concentration in the cabin is 1254.7 ± 308.6 ppm, and the average minimum and the maximum measured CO₂ concentration is 809 ppm and 2379 ppm. The average CO concentration is 1.1 ppm, and the maximum is 1.6 ppm from 8 studies with 224 flights [24,39,45,48,49,50,51,56]. The ozone (O₃) concentrations were reported in the range of <3 ppb~205 ppb with an average of 50.2 ppb from 10 studies with 534 flights [24,41,45,50,51,56,60,61,62,63]. The PM concentrations measured using mass concentration sampling ranged from 0.04 to 4.83 μg/m³ from 12 studies covering 398 flights [17,39,46,50,55,59,60,64,65,66,67,68], and the particle count concentration was not listed. In most of the published papers included in the present review, measured concentrations of TVOC are in the range of 0.12–1.8 mg/m³ in 24 studies and 1045 flights [17,24,29,30,31,36,40,45,46,48,51,56,62,69,70,71,72,73,74,75,76,77,78,79] excluding flights that allow smoking. It should be noted that the same paper will involve multiple cabin environmental factors, so the total number of papers is 54, but the number of papers for each environmental factor could be repeated.

Table 4. Summary of indoor environmental quality standards for civil aircraft cabin and built environment.

Environmental Factor	FAR-25 [20]	ASHRAE 161-2013 [1]	JAR-25 [21]	CS-25 [80]	BS-EN4618 [81]	CCAR-25 [82]	AP-25 [83]	GB/T18883-2022 [84]
T		18.3–23.9 °C ≤26.7 °C						22–28 °C summer 16–24 °C winter
RH								40–80% Summer 30–60% winter
CO2	5000 ppm		30,000 ppm	5000 ppm	20,000 ppm 15 min 5000 ppm peak 2000 ppm	5000 ppm	5000 ppm	≤1000 ppm 24 h in average
O3	100 ppb TWA 3 h 250 ppb any time	100 ppb TWA 3 h 250 ppb any time	100 ppb TWA 3 h 250 ppb any time	100 ppb TWA 3 h 250 ppb any time	100 ppb TWA 3 h 250 ppb any time 60 ppb TWA 8 h	100 ppb TWA 3 h 250 ppb any time	100 ppb TWA 3 h 250 ppb any time	≤0.16 mg/m3 1 h in average
PM2.5					100 μg/m3 TWA 1 h (healthy) 40 μg/m3 (constant healthy)			≤0.05 mg/m3 24 h in average
PM10					150 μg/m3 TWA 24 h			≤0.1 mg/m3 24 h in average
CO	50 ppm	9 ppm TWA 10 min 50 ppm 1-min peak	50 ppm	50 ppm	50 ppm peak 25 ppm TWA 1 h 10 ppm TWA 8 h	50 ppm	50 ppm	≤10 mg/m3 1 h in average
TVOC								≤0.6 mg/m3 8 h in average
Formaldehyde					2.47 mg/m3 15 min (safety) 0.9 mg/m3 TWA 8 h (safety)		0.5 mg/m3	≤0.08 mg/m3 1 h in average
Benzene					3.2 mg/m3 TWA 8 h (safety) 12.8 mg/m3 15 min (healthy)		5 mg/m3	≤0.03 mg/m3 1 h in average
Toluene					760 mg/m3 15 min (safety) 190 mg/m3 TWA 8 h (healthy)			≤0.2 mg/m3 1 h in average

lists the environmental quality standards for civil cabins and built environments. In brief, the average temperatures were almost within the range recommended by ASHRAE recommended value. However, the average temperature inside the cabin is only slightly higher than the minimum temperature in summer air conditioning rooms (22 °C) in GB/T18883-2022. That is, passengers generally report that the temperature inside the cabin is relatively low, and extra blankets are required. The average measured RH level is 18.4%, which is far below the minimum indoor environmental standard limit (30%) of the building, easily causing discomfort to passengers. CO₂ exposure levels higher than 1000 ppm may affect cognitive function and psycho-physiological responses. The average CO₂ concentration has exceeded the indoor air quality standard limit (1000 ppm) but is lower than the aviation standard limit (5000 ppm). The main source of cabin CO is emissions from aircraft engines. The concentration of CO in the cabin is strictly controlled due to its serious harm to humans. The average CO concentration is 1.1 ppm, and the maximum is 1.6 ppm, which is much lower than the limits in buildings (8.7 ppm) and in cabins (50 ppm). The ozone enters aircraft cabins through the ventilation system and decomposes on surfaces. It may also undergo reactions with other pollutants on surfaces and in the air. The reported O₃ concentrations did not exceed the limits regarding air quality in aircraft (100–250 ppb). For the remaining pollutants, the concentration level in the passenger compartment is far less than the specified limit.

3.1. Temperature Data Distribution

Table 5. Measured cabin temperature data by aircraft types.

Aircraft Type	Number of Flights	Ave (°C)	Min (°C)	Max (°C)
A319/320	25	22.4	21.1	23.9
A321	5	25.5	-	-
A330	4	23.6	20.0	24.7
A340	7	22.2	19.9	25.7
A380	5	23.0	-	-
B737	83	25.0	24.9	26.4
B747	24	23.7	20.8	25.8
B757	1	24.6	-	-
B767	58	22.7	18.4	26.3
B777	15	23.0	-	-
BAe146	8	23.5	19.0	25.6
DC 9	15	22.0	21.0	23.6
MD 80	2	24.7	-	-

gives the mean temperature values as well as the extreme levels for the 15 aircraft types and the corresponding number of flights, with the mean cabin temperatures for each type ranging from 22 °C to 25.5 °C. The mean temperature of the cabin for each type of aircraft was 25 °C. Among all aircraft types, the B737 series flights had the largest number of samples, totaling 83 flights, with an average temperature of 25 °C and temperature fluctuations of no more than 1.5 °C. The A321 cabin had the highest average temperature of 25.5 °C. The DC 9, the A319&320, and the A340 cabin had low average temperatures of less than 22.5 °C. The above results reflect the overall low cabin temperatures, although there are large differences in the measurement methods and sample sizes of each aircraft types.

Figure 2 gives the probability distribution of cabin temperatures for all flights in the literature studied, which approximately follows a normal distribution. Approximately, over 60% of the flights have cabin temperatures in the range of 23–24 °C. It is worth noting that the thermal sensation of the passengers is not uniform, and for the existing cabin air conditioning systems, the air supply vents located above the passenger’s head result in a colder sensation.

3.2. RH Data Distribution

Table 6. Measured cabin RH data by aircraft types.

Aircraft Type	Number of Flights	Ave (%)	Min (%)	Max (%)
A319	16	21.8	13.9	44.6
A320	24	31.4	7.0	66.7
A321	6	22.2	18.3	-
A330	5	30.3	21.2	42.5
A340	7	19.3	6.9	53.1
A380	5	10.5	-	-
B737	84	16.4	15.6	30.6
B747	24	16.8	10.7	45.2
B757	1	10.6	-	-
B767	58	7.1	1.5	26.0
B777	16	12.1	-	-
B787	2	29.4	-	-
BAe146	8	21.0	8.3	40.2
DC 9	15	-	12.3	-
MD 80	2	10.1	-	-

shows the mean and extreme values of cabin RH are counted for a total of 15 types of aircraft, with relative humidity distribution in the range of 10–30%. Similar to the temperature statistics, the B737 series flights also had the maximum sample numbers with a mean humidity of 16.4%, and the maximum and minimum humidity were 30.6% and 15.6%, respectively. Although the B747 series flights have been withdrawn from the CAAC flight echelon, they had the lowest cabin humidity level of 7.1% and a minimum of only 1.5% of the 58 flights tested, with lower cabin humidity levels leading to passenger discomfort. Similar to the B737, as the mainstream aircraft in the current civil aviation flight echelon, the total sample size of A319/320 series flights was 40, with the relative humidity of the cabin environment maintained at 20–30%. The A320 flight had the highest average humidity value of 31.4%. Excessive cabin humidity can cause passengers to experience suffocation, resulting in a feeling of poor ventilation, as well as potential flight safety hazards. According to the statistics, the humidity in the existing cabin environment is below 30%, and most remain below 20%, which can lead to symptoms such as dry nose, throat, and eyes for passengers and crew.

As shown in Figure 3, the cabin humidity level of all flights in the statistical literature is between 5% and 50%, and about 50% of the flights’ cabin humidity is between 20% and 25%. It is worth noting that about 35% of flights have cabin humidity levels below 15%, which means that passengers and crew members will experience discomfort caused by low humidity levels. About 99% of the statistical flight cabin humidity level is lower than the recommended humidity level of the indoor environment in buildings (30–80%).

3.3. Monte Carlo Simulation Results of PMV

Figure 4 shows the Monte Carlo simulation results of PMV calculated based on cabin temperature and humidity, representing the comfort of passengers in different environmental scenarios. The three coordinate axes in Figure 4 represent the temperature, humidity, and PMV values inside the cabin. The red dots indicate that the PMV is between ±0.5, which is the range of values for human thermal comfort under ISO standards.

In order to achieve human thermal comfort, the temperature, relative humidity, and wind speed inside the cabin should be controlled within the range shown in

Table 7. Recommended range of thermal comfort parameters based on simulated PMV.

M (met)	ICL (clo)	PMV	T (°C)	RH (%)	v (m/s)
0.8	0.5	×	n/a	n/a	n/a
0.8	1	√	25–28	0–40	0–1
1	0.5	√	25–27.3	0–34	0–0.5
1	1	√	22–28	0–40	0–1
1.2	0.5	√	23–28	0–44	0–1
1.2	1	√	20.3–26.7	0–46	0–1

. When meeting the thermal comfort requirements, in-cabin humidity should be kept below 40%; the wind speed should be maintained in the range of 0–1 m/s, and the temperature should range from 25–27 °C (summer) 22–27 °C (winter). Obviously, due to the clothing differences, the in-cabin temperature of winter could be appropriately lowered compared to summer, while humidity and wind speed are maintained at the same level.

In comparison to the actual measurements of aircraft temperature, the temperature recommendation based on PMV should result in an increase of approximately 5 °C in the summer operating temperature of the aircraft cockpit environment, while a reduction of approximately 3 °C should be observed in winter. This is in close alignment with the recommended thermal comfort operating values as outlined in the literature [8,12]. With regard to humidity, although the PMV-predicted value for humidity is 0–40%, it is recommended that the moderation of the cabin environment be maintained at 10–20% in order to ensure flight safety.

3.4. CO₂ Concentration Distribution

As shown in

Table 8. Measured cabin CO₂ concentrations by aircraft types.

Aircraft Type	Number of Flights	Ave (ppm)	Min (ppm)	Max (ppm)
A319	16	1197	967	1744
A320	27	1286	1035	2286
A321	17	1883	969	5177
A330	1	967	760	1491
A340	5	759	540	1636
A380	5	1253	-	-
B737	77	1325	791	2273
B747	23	1142	635	2340
B757	67	1421	703	2992
B767	63	1030	1573	3245
B777	15	1499	-	-
B787	8	1242	968	2019
BAe146	8	1073	457	2019
CR 7&9	6	1508	731	2548
CRJ	6	1036	790	1487
DC 9	16	915	688	1480
MD80&88&90	56	1308	522	2946

, the cabin CO₂ concentrations of the 17 aircraft types tested were distributed in the range of 1000–2000 ppm. Although the B757, B767, and MD series airliners have been withdrawn from the civil passenger market in China, there were 186 flights in the previous studies of cabin CO₂ concentrations of civil aircraft, which accounted for 45% of the total samples. For the common narrow-body civil airliners B737 and A320/321, the average cabin CO₂ concentration was 1500 ppm. For the common wide-body civil airliner types of A330, A380, and B777, the average value of the measured cabin CO₂ concentration was about 1300 ppm. Since the measured cabin CO₂ concentrations generally exceeded 1000 ppm, prolonged exposure to such an environment would inevitably cause some damage to the cognitive ability and health of the crew.

As shown in Figure 5, about 20% of the CO₂ concentration levels in the statistical flights are lower than 1000 ppm, which could meet the indoor recommended value for buildings. About 15% show that the CO₂ concentration level in the flight cabin exceeds 1500 ppm, which may have a negative impact on the efficacy level of pilots and crew members. About 65% of the literature’s statistics for flights show that cabin CO₂ concentration levels range from 1000–1500 ppm and that the fluctuation of concentration levels is largely affected by flight occupancy and measurement locations.

Figure 6 illustrates the estimated ventilation rates determined by the measured CO₂ concentrations, which could basically meet the requirement of the current FAR (4.7 L/s per person). The cabin ventilation volume and Monte Carlo simulation are computed based on Equations (3) and (4) and the CO₂ concentration found in the literature. Approximately 95% of flights meet the ASHRAE-recommended cabin ventilation volume, while approximately 85% of flights either meet or significantly exceed the FAA-mandated ventilation volume. It is important to note that the ventilation volume mentioned above is computed using measured data from the literature, disregarding inaccuracies brought about by the real cabin ventilation and passenger occupancy rate.

3.5. Health Risk Estimates of In-Cabin VOCs

The VOCs in the cabin come primarily from cabin trim materials, the flying members themselves, and engine bleed [31]. Figure 7 shows the commonly measured VOCs in the cabin environment for literature retrieval. From different flights, the measured types and concentrations of VOCs are diverse. The data show that the average concentration of aldehydes in the cabin is in the range of 5~12 μg/m³, and the formaldehyde concentration in the cabin is 5.3 μg/m³, which is lower than that in an indoor environment. The average concentration of benzene series in the cabin is less than 15 μg/m³, and the average concentrations of benzene and toluene are 8.4 and 13.9 μg/m³, respectively. The average concentration of acetone in the cabin is about 15.7 μg/m³, which is much lower than the 40~60 μg/m³ of smoking flights in the 1990s. Previous studies have shown that limonene is considered as a representative odor VOC in the cabin of short-haul flights, and its average concentration is about 12.1 μg/m³, much higher than the average level measured in buildings. Similarly, related studies show that naphthalene and tetrachloroethylene also have high carcinogenic risk levels, and the average concentrations listed in published data are 2.5 and 13.4 μg/m³, respectively.

To evaluate the exposure health risks in the cabin, the carcinogenic and non-carcinogenic risks of formaldehyde, benzene, tetrachloroethylene, and naphthalene were evaluated. By using the EPA IRIS, the lifetime inhalation carcinogenic risk of passengers and crew members of different genders was estimated, as shown in Figure 8. In Figure 8a, the lifetime inhalation carcinogenic risk for all passengers is less than 1 × 10⁻⁶, and the hazard to passengers can be ignored. The highest estimated carcinogenic risk of formaldehyde for passengers of males and females exceed the EPA recommended value 1 × 10⁻⁶, with exceeding standard rates of 20% and 10%, respectively. In Figure 8b, for crew members, the mean carcinogenic risk of formaldehyde, tetrachloroethylene, and naphthalene is all greater than the EPA recommended value of 1 × 10⁻⁶ but lower than the EPA high-risk recommended value of 1 × 10⁻⁴. That is, the crew members have a certain risk of cancer in their professional careers. The above conclusion is similar to the results of Yin et al. [31].

Figure 9 shows the assessment of non-carcinogenic inhalation risks for passengers and crews of different genders. The non-carcinogenic inhalation in adult males has a slightly higher risk compared to females due to differences in weight and respiratory rate. The HQ values of formaldehyde, benzene, tetrachloroethylene, and naphthalene for passengers and crew members are lower than 0.015 and 0.15, respectively, which are far below the evaluation standard HQ < 1, indicating that there is no corresponding chronic non-carcinogenic risk.

4. Discussion

In this paper, we have reviewed and statistically analyzed 54 studies dating from 1987 to the present. We comprehensively summarized the temperature, humidity, and gas pollutant concentrations within the civil aircraft cabin environment. We quantified the health effects on cabin occupants from exposure to specific VOC concentrations. Compared with previous studies on thermal comfort, ventilation, and gaseous pollution levels in civil aircraft cabins, this study places greater emphasis on evaluating and optimizing the subjective perceptions of personnel based on objective cabin environment parameters. The PMV distributions under different exercise intensities and clothing thermal resistance were simulated using the Monte Carlo method. We provided suggested parameters for the cabin’s thermal environment in both winter and summer seasons and estimated the ventilation rate for civil aircraft cabins based on the measured CO₂ concentrations. The risks of carcinogenesis and non-carcinogenesis from exposure to specific VOC concentrations offer a clear and direct reference for individuals who engage in frequent air travel. During the analysis, we tallied data from various sources, including years, flight numbers, and different aircraft types, and established a comprehensive data inventory. The following section discusses thermal comfort, air quality, occupant perception, and future recommendations.

4.1. Cabin Thermal Comfort

The thermal environmental quality of civil cabins has a significant influence on the safety, health, and comfort of passengers. Narrow, highly occupied, low relative humidity, and limited ventilation conditions in the cabin may cause discomfort. Most passengers report feeling dry eyes, sore throat, chest tightness, and irritability during flight, and a few passengers even have serious reactions such as drowsiness, headache, and nausea. In addition to individual differences, this discomfort is mainly due to the mismatch between the cabin environment parameters of passenger aircraft and the comfort requirements of the passenger and crew. At the present stage, the thermal comfort regulation of the cabin environment of civil aircraft lacks mobility, and the thermal comfort parameters for the season, flight duration, and passenger occupancy rate are very limited. For example, passengers are more sensitive to the thermal comfort of the cabin environment because of the lower thermal resistance of their clothing when they fly in summer. In addition, the low humidity of the cabin environment is a major factor that has been troubling passengers. In this article, the clothing thermal resistance at 0.5 clo and 1 clo in the Monte Carlo simulation of the thermal environment significantly simplifies the clothing diversity among passengers. This simplification may lead to the comfort temperature, humidity, and wind speed being brought from PMV, making it difficult to achieve the desired effect in actual cabin conditions. More detailed and realistic simulations should be considered in future research to provide references for thermal comfort control of civil cabin environments.

4.2. Cabin Air Quality

In addition to the thermal parameters of the cabin environment, the air quality of the cabin will also cause discomfort to the passengers. Especially during the epidemic of infectious diseases, civil aircraft are considered as the infection carriers that lead to rapid spread. The fresh air supply in the cabin of civil aircraft comes from bleed air. Thus, smell events caused by engine oil leakage may occur, resulting in dizziness, nausea, and complaints from passengers. Since most of the existing aviation filters are for particulate, and few VOC catalytic decomposition devices have been installed, the odor VOCs also need attention. The negative effects of high CO₂ levels in cabin environments on pilots, as well as occupants, are also of concern when seat occupancy increases and flight times are long.

Although some civil aviation regulations give the limits of cabin air quality parameters of passenger aircraft, the settings of test scenarios and exposure time are vague, which also leads to inconsistent test results of cabin air quality parameters. The measurement of gaseous pollutant concentration in civil aircraft cabin environments strongly correlates with the test scenarios. However, the sampling points for gaseous pollutant concentrations during actual onboard measurements are relatively fixed, making it challenging to describe the overall cabin concentration levels. For instance, measuring PM concentrations in the cabin is heavily influenced by the sampling location. The particle concentration near the ceiling air supply outlet of the civil aircraft cabin is approximately 70 particles/L [85]. However, the particle concentrations in the supplementary air at the grille on the lavatory door can swiftly rise to 200–400 particles/L after passengers or dining carts passing by [85]. While the data from onboard measurements are more authentic, the reproducibility and universality of these measurements remain questionable. Moreover, variations in measurement instruments have resulted in inconsistency of the units for gaseous pollutants in the literature, complicating further analysis.

4.3. Future Research Suggestions

Establishing a link between cabin environmental parameters and the subjective perception of passengers is an important means of accurately assessing the environmental quality of civil aircraft cabins. Individuals of different ages, genders, and weights perceive the cabin thermal environment differently. For instance, the psychological condition of individuals influences their perception of overall environmental quality [86]. A previous research indicates that women have a higher expectation for indoor temperature and a lower satisfaction rate compared to men [87]. Surveys of cabin crew members’ subjective feelings indicate that rising noise levels significantly affect their mood, leading to negative evaluations of the cabin environment [88,89]. This indicates that variations in the cabin environment’s pressure, light, and noise will also have an impact on passengers’ subjective perceptions [89], which is why future aircraft designs for thermal comfort should take this into consideration. The database search should be used to identify the topics or themes with evident subjective feelings of passengers in addition to the aforementioned parameters and the corresponding literature review. This will help to provide potential directions for future cabin environment optimization.

The outbreak of COVID-19 has drawn attention to the spread of infectious bacteria, viruses, fungi, and other microorganisms in the confined space of the aircraft cabin environment. However, similar to the assessment of the health effects of other pollutants, the dose of health threats to passengers has not been well quantified [90]. This uncertainty makes it challenging to precisely quantify and evaluate the health risks these pollutants pose. Moreover, current environmental standards for civil aircraft cabins lack theoretical parameter guidance. Consequently, the measured concentrations of gaseous pollutants from the available literature can only serve as a reference for a comprehensive understanding of these pollutant levels in aircraft cabins [91,92]. The analysis of gaseous pollutant concentrations and their corresponding health effects in civil aircraft cabin environments requires extensive supporting literature from medical, pathological, and toxicological disciplines.

This review underscores the interconnected relationship between environmental parameters within civil aircraft cabins and the subjective experiences of personnel. It concludes that regulating thermal environment-ventilation parameters and managing the distribution of gaseous pollutants significantly impact the comfort and physical health of individuals onboard. Future research endeavors could focus on introducing more flexible and personalized ventilation systems in aircraft cabins to enhance occupant engagement in regulation and improve overall cabin comfort. For instance, personalized ventilation ducts could be strategically designed near seats without compromising flight safety. Additionally, efforts should be made to address the non-uniform temperature distribution in the cabin under existing ventilation airflow organization by optimizing airflow mixing through changes in ventilation arrangements. In light of recent pandemics, designing civil aircraft cabin environments should prioritize enhancing resilience against infectious diseases. This could involve implementing special ventilation measures and modifying airflow organization designs to alter the transmission pathways of infectious agents, thereby reducing the risk of crew infection. Furthermore, it is recommended that the civil aviation industry continuously update the database of passengers’ in-flight environmental perceptions and comfort levels. This could be leveraged to regulate the quality of civil aircraft cabin environments, ensuring continuous improvement and alignment with passenger preferences and safety standards.

5. Conclusions

This paper presents a systematic review of onboard measured data from 54 studies, encompassing key objective parameters of the civil aircraft cabin environment, such as temperature, humidity, CO₂, CO, O₃, PM, and concentrations of common VOCs. Drawing upon the reviewed literature and flight data, this paper provides probability distributions and statistics for temperature, RH, and CO₂ concentrations with various aircraft types and numbers of in-flight tests. Furthermore, it outlines the concentrations of gaseous pollutants in the cabin, offering mean, maximum, and minimum values. Based on the Monte Carlo simulation of PMV, this study recommends maintaining relative humidity in the cabin below 40%, keeping wind speed within the range of 0–1 m/s, and ensuring temperatures fall between 25–27 °C (during the summer) and 22–27 °C (during the winter). The current estimated cabin air supply rate generally aligns with relevant standard requirements. Additionally, this paper assesses health-related risks associated with main VOCs, such as formaldehyde, benzene, tetrachloroethylene, and naphthalene. The findings suggest that flight crew members face a certain risk of cancer during their careers, while the risk to passengers is considered negligible. Neither passengers nor flight crew members are at risk for chronic non-carcinogenic effects. The reviewed results could serve as a reference for initiatives aimed at enhancing aircraft cabin environmental quality.