The Air We Breathe at 30,000 Feet: Measuring Cabin CO₂, Humidity, and Pressure

Project Motivation

I set out to measure CO₂ levels, humidity, temperature, and pressure over the course of four short-haul flights aboard a DHC-8 Dash 8 turboprop. My goal was to determine the quality of air during a commercial flight.

The data revealed that at cruising altitude, CO₂ levels consistently exceeded recommended indoor thresholds. During boarding and deplaning — when ventilation is reduced — levels spiked to nearly six times the recommended limits. Humidity remained low, reinforcing the dry-mouth, dehydrated sensation I always experience.

Beyond raw numbers, this project clarified why flying often leaves me fatigued, headachy, and bloated.

Airplane HVAC Background

If you’re already familiar with how airplane HVAC systems manage airflow, exchange rates, and humidity, skip this section and jump straight to my experiments and results.

Airflow in an Aircraft Cabin

Most commercial aircraft utilize a top-to-bottom airflow pattern: fresh, conditioned air is supplied through overhead vents, flows downward through the cabin, and exits via return-air grilles near the floor. This design helps limit the spread of airborne contaminants between passengers.

The air circulation is shown in Figure 1:

1. Air Intake & Compression — Fresh air enters through the aircraft’s turbine engines, where it is compressed (since outside air pressure is too low for cabin use).
2. Temperature Control — The air then passes through a heat exchanger.
3. Mixing with Cabin Air — Conditioned fresh air is blended with cabin air, typically at a 50/50 ratio.
4. Filtration — Before being recirculated, cabin air is passed through HEPA filters, which remove ~99.9% of bacteria and viruses.
5. Distribution & Exhaust — The mixed air is supplied through overhead vents, moves downward, and exits through an outflow valve near the aircraft’s base.

In theory, every passenger breathes a blend of filtered and fresh outside air. This air enters through overhead vents, is directed downward through the cabin, and is either recirculated through filters or expelled from the aircraft. In reality, however, some mixing inevitably occurs (Figure 2). To reduce contamination and transmission risks, airplane cabins are typically divided into distinct air zones, each equipped with its own ventilation system. These zones (separated by fancy curtains) restrict airflow, ensuring shared air is largely confined to passengers in close proximity rather than circulating widely.

CO₂ Levels in an Aircraft Cabin

At rest, an average person exhales approximately 15 liters of CO₂ per hour (Sundell, 1982). Without adequate air exchange with the outdoors, CO₂ levels in an aircraft cabin would steadily increase, eventually causing adverse health effects.

Health Canada’s 24-hour exposure guideline recommends keeping CO₂ levels under 1,000 ppm (Health Canada, 2024). The Health Canada 2024 report cites studies showing that levels above this threshold can cause cognitive impairment, dry throat, eye irritation, and breathing difficulties.

While no single study conclusively establishes the 1,000 ppm threshold, the collective body of evidence indicates that levels below 1,000 ppm enhance perceived air quality and mitigate discomfort. This aligns with ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) best practices which stipulate indoor CO₂ concentrations should not exceed 700 ppm above outdoor levels.

Though I am not an aerospace expert, I would be surprised if the commercial aviation industry did not strive to maintain CO₂ levels within the 1000 PPM ranges to prioritize passenger comfort and safety.

For reference: Outdoor CO₂ levels are around 427 ppm in 2025 and gradually increasing (NASA Climate Data).

Humidity in an Aircraft Cabin

According to ASHRAE Standard 55–2017 (Thermal Environmental Conditions for Human Occupancy), relative humidity (RH) should be maintained between 30% and 60% to ensure occupant comfort and health.

Below 30% RH, dry air irritates mucous membranes (eyes, nose, throat), causes discomfort, and amplifies static electricity — posing risks ranging from minor shocks to interference with electronics. Low humidity also creates a perception of stale air, even when ventilation rates are adequate.

Conversely, humidity above 60% RH introduces moisture-prone conditions for mold growth and microbial proliferation, compromising indoor air quality and exacerbating allergens. Excess moisture can also lead to condensation, raising concerns of mold and musty odors from microbial activity.

My Flight Data

Over two weeks, I took four consecutive short-haul flights aboard a DHC-8 Dash 8 turboprop. Seizing the chance to gather data, I carried an Aranet 4 air quality monitor and placed it in the seatback pocket at knee height — mid-cabin on every flight — to ensure consistent positioning.

The device recorded data continuously, except when I checked it during a tarmac delay and saw CO₂ levels reach over 3,100 ppm.

The air monitor measures CO₂, temperature, relative humidity, and atmospheric pressure. It has a convenient export feature that produces a CSV file with the data, which I used for analysis.

One of the easiest ways to pinpoint my exact flight duration was by tracking atmospheric pressure. During each flight, cabin pressure dropped from approximately 1000 hPa at ground level to around 750 hPa at cruising altitude. For context, that’s roughly a 3 PSI drop — equivalent to the pressure change you’d feel diving to the bottom of a 7-foot pool. In the cabin, my body experienced the opposite pressure change as if I were surfacing from a dive.

This drop in pressure was unexpected. Perhaps I was naive, but I assumed cabins would be more pressurized. The change explains some common flight sensations:

1. Lower air pressure reduces oxygen absorption, contributing to fatigue and headaches.
2. Rapid pressure shifts cause ear and sinus discomfort if they don’t equalize quickly.
3. Expanding trapped air leads to bloating and sinus pressure as external pressure decreases.

At cruising altitude, CO₂ levels averaged 2000 ppm — twice the long term recommended indoor threshold. The highest spikes occurred on the ground: during boarding and deplaning (when ventilation was off or minimal), CO₂ consistently exceeded 3000 ppm. One unusually long deplaning event peaked at nearly 6000 ppm.

For those shocked by these numbers, I also happened to have recorded 4000 ppm in a crowded bar on March 20 around 19:00, where levels stayed elevated for 30 minutes — proof that enclosed, high-occupancy spaces rapidly accumulate CO₂ and are not unique to airplanes.

Relative humidity remained consistently low across all flights, hovering near 20% RH (below the recommended 30–60% range). No surprise I felt parched and dried out — this aligns with widespread reports of dry eyes, nasal irritation, and dehydration during flights.

This data, to me, suggest aircraft cabin air quality, especially on the ground, could be improved. Better ventilation practices to reduce CO₂ buildup before takeoff and after landing would create a fresher environment.