The Air Out There

How can a portable Co2 monitor help keep us safe from Covid-19 when we travel?

Travel is risky during a pandemic.  In the summer of 2022, more than two years after the arrival of Covid-19, community spread is still high while hardly any public protections remain.  Travel by its very nature puts us in contact with people from many different regions and countries.  Often that contact occurs in crowded indoor spaces and vehicles.


Air circulation can have a big impact on our safety when we travel, especially as scientific consensus emerges about the risks of aerosol transmission of Covid-19.  Outdoors, where the air is constantly moving around, there is much less exposure to airborne illnesses, and much less risk.  Indoors, in spaces that are small, crowded, or poorly ventilated, contaminated air can linger and build over time.  Some places have high quality filtration systems that keep interior air fresh and moving.  Other places, not so much.


If we can measure air quality and circulation as we travel, we can get a snapshot of our relative risk of exposure in various interior spaces.  One way to do that is with a portable Co2 monitor.  There are many monitors to choose from, but we selected an Aranet4 Home monitor and have been very happy with it.  Among other features, the Co2 monitor tracks the concentration of Co2 in the surrounding air.  Generally, these Co2 readings tell us how healthy the air is in a space.  Since people produce Co2 when we exhale, a higher concentration of Co2 can indicate poor ventilation.  Co2 can also be a proxy for other particles lingering in the air, including Covid-19.  Several scientific studies have looked at Co2 as a proxy measure for Covid-19 infection risk in different indoor environments.  In summary, higher concentrations of Co2 = less air circulation = higher covid risk.

Our Co2 monitor gives us real time data on air quality, including Co2, temperature, pressure, and humidity. And it's small enough to fit in a pocket.

How much Co2 is too much?  In fresh outdoor air, Co2 measures around 420 parts per million.  Indoor, concentrations of Co2 tend to be a little bit higher.  Some studies indicate that our cognitive performance begins to see negative impacts when Co2 concentrations are between 1000 and 1400 parts per million.  Above 1400 parts per million, these decreases in cognitive function become much more pronounced.  Things get a little bit more complicated when we use Co2 as a proxy for covid risk.  This study indicates that the change in covid risk associated with changes in Co2 concentrations can vary by several orders of magnitude depending on the environment and activities.   On a subway trip where most people are sitting silently, it may take a Co2 increase of 1,000ppm to increase infection risk by .01%.  But if people are moving around and talking loudly, a Co2 increase of only 100ppm may produce the same risk. 


We took our Co2 monitor with us on our recent travels to get an idea of our relative risk in different places.  Are all interior places similar?  Are there differences between vehicles and buildings?  Where is it safe to slip off our mask for a quick bite or drink of water, and where should we probably avoid doing that?  Our measures included public transportation vehicles like planes, trains, and buses.  We also measured air quality in airports and train stations.  These measurements are not intended to be definitive.  Factors like open windows and the density of a crowd can result in a very different air quality in a particular space.  Still, the Co2 monitor is a great way to gauge risk in the moment.  Here is what we discovered:

Air Quality in Different Travel Environments

(measured by me)

Train Stations

The air inside the Évian-les-Bains train station was almost as clean as the fresh air out here on the platform.

We used our Co2 meter at several train stations in France and Switzerland.  Overall, train stations had the cleanest air of any of the buildings and vehicles we measured.  That was true for large and busy stations like Cornavin in Geneva, medium-sized stations like Annecy, and smaller stations like Évian-les-Bains.  Generally, train station interiors were spacious, with doors that opened frequently or stayed open, allowing excellent circulation of fresh air. 

Open doors and wide open spaces helped keep air quality good in Geneva's busy Cornavin station.

The train station in Évian-les-Bains had the best air quality, with a measured Co2 at 500ppm.  That’s nearly the same as outdoor air.  Busy Cornavin was a little bit higher at 670ppm, but this figure is still very low compared to other interior spaces. 


Trains were generally more spacious and less crowded than other public transportation.

Air quality in train stations was excellent, but air quality in the trains was also quite good, with Co2 measurements between 600ppm and 900ppm.  Perhaps this is because train cars are spacious relative to other vehicles, and the seating is often more dispersed.  For most trains, there are no assigned seats, and when passengers enter the train they intuitively space themselves apart from other people.  The activity of train passengers may also be a factor.  Silent activities like sleeping, reading, or looking at phones are the norm, and loud shouting and singing is rare.  Regular stops also mean that doors and opening often and circulating fresh air from outside.

Air quality is pretty good on this Leman Express train from Annecy.

One caveat is that these measurements were generally taken on intercity passenger trains.  There was no time or space to take measurement in the standing-room-only subway cars beneath Lyon, for example.  Certainly, the Co2 measurements would be much higher in that environment.


Passengers sit close together on buses like this one in Geneva, but frequent stops and open windows keep air circulating.

We didn’t ride many buses on our recent trip to France, but we did take a regional coach from Annecy to Geneva.  On this trip, the Co2 measured up to 1329ppm.  This isn’t terrible compared to some interior spaces, but it is quite a bit higher than trains and train stations.  I think the reasons are pretty straightforward.  The interior space of the bus is more compact, while passengers are seated much closer together.  A point-to-point trip means that the doors are not opening regularly to circulate the air.

No one is shouting or singing on this pre-dawn bus ride but close proximity and limited stops reduce air quality.

One advantage of the bus trip over a plane trip is that it is often possible to open windows next to your seat and circulate fresh air.  The ability to easily bring in fresh air from outside seems to make buses slightly safer than planes in terms of air quality.


Geneva's terminal is spacious, and the gates are uncrowded.

Of all the spaces and vehicles we measured, airports had the widest variety of air quality.  Geneva’s airport performed the best in terms of air quality with Co2 measurements of 623ppm.  The terminal was spacious, the gates were not overcrowded, and the ventilation system was efficient.  The air quality at Geneva’s airport was similar to train stations we visited, with air circulation levels almost as good as being outside.  At the other end of the spectrum, our gate at Los Angeles International Airport measured Co2 concentrations at 1224ppm.  Ceilings were much lower, so there was just less air inside for particles to circulate.  Gates were spaced very closely together with inadequate seating for the passenger traffic.  This made the terminal quite crowded. 

Overcrowded gates and cramped spaces reduce the air quality at LAX.

Unlike the generally calm behavior of passengers on trains or planes, people do a lot of talking, shouting, and exercise (people sprinting to their gate) in an airport terminal.  This also contributes to an increase in Co2 concentrations and a reduction in air quality.  Co2 measurements in other airports we visited were somewhere between the lows of Geneva and highs of LAX.  Usually, air was cleaner in the airports than on planes and buses, but not as good as trains and train stations.


Surprisingly, our highest risk of covid exposure during our travels was when we were flying.

During our travels, the worst air quality we experienced was unquestionably on airplanes.  This surprised us because of numerous articles about the safety of airline travel.  We expected that because of the air filtration system on the planes and the ceiling vents at each plane seat, the circulation would be quite good.  That is not what our Co2 meter showed us.  The circulation on our flights was poorer than any other vehicle or building we visited during our travels.  Our flight with the best air quality measured Co2 at 1424ppm on the ground and 1360ppm in the air.  Other flights were as bad as 2013ppm on the ground and 1560ppm in the air.  Intuitively, this makes sense.  The number of passengers on a plane is absolutely maximized, with the minimum feasible spacing between passengers.  No doors or windows are opened during the flight and the air quality is completely dependent on the efficiency of the mechanical filtration system on the plane.  Even the best filters can’t do much when a dozen people are within just a few feet of you for many hours at a time.

The reality of air quality on planes seems to be a little bit different than official claims.

Airline mask requirements were lifted shortly before we travelled, and we wonder how much different that might make.  In addition to assumptions about masking, early studies on airline safety also included precautions like empty middle seats, which would have reduced the number of passengers and increased spacing.  Regardless, it’s clear to us that the air quality safety features of planes in reality is very different than what airlines and health officials advertise.


Overall, our portable Co2 meter has proven to be a very useful and convenient tool to manage our airborne illness risk during travel.  Sometimes we cannot choose if or when we travel.  Usually, we cannot screen who we might encounter during our travels.  Having the ability to make informed decisions in real time about personal safety is a necessity for anyone like me with elevated risk and consequences of infection.  Understanding our relative risk in different travel environments can be liberating, because it allows us to fully experience places where we know the risk is low.  While we will continue to wear masks indoors, we became increasingly comfortable over the course of our travels in the trains and train stations of the cities we visited.  But we will probably limit flying to absolute necessities.  For now.

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